Field Guide to the Reptiles of the Northern Territory

By Chris Jolly, Brendan Schembri and Stewart Macdonald

A land of extremes, the Northern Territory’s arid deserts and monsoonal forests harbour some of Australia’s smallest and the world’s largest reptiles, as well as some of the world’s most venomous snakes.

Field Guide to the Reptiles of the Northern Territory is the first regional guide to the crocodiles, turtles, lizards and snakes of this megadiverse region. It presents introductions to order, family and genus; keys to family, genus and species; and species profiles, including descriptions, photos, distribution maps and notes on natural history. It features profiles for the 390 species that occur or may occur on the land and in the sea of the Northern Territory.

Extensively illustrated, this is an essential resource for wildlife enthusiasts and professional and amateur herpetologists.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Jun 1, 2023
• ISBN: 9781486312702

Chris Jolly

Dr Chris Jolly is an ecologist with a broad interest in natural history, ecology, evolution and conservation. An inordinate fondness for Australia's reptiles has seen him traverse much of the country in search of the country’s most elusive and poorly understood species. Chris is a Postdoctoral Research Fellow at Macquarie University and an associate of the Museum and Art Gallery of the Northern Territory and Australian Museum. Chris has published over 40 scientific reports, books, popular science stories and journal articles.

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Preface

We created this Field Guide to the Reptiles of the Northern Territory to give users information about, and to aid in the identification of, the reptiles of the Northern Territory. We present introductions to each taxonomic grouping (e.g. order, family, genus); dichotomous keys to family, genus and species; and species profiles, including morphological descriptions, photos, distribution maps and notes regarding natural history. This book features profiles for the 390 species that are known to occur or may occur on the land and in the sea of the Northern Territory and its associated offshore islands as of June 2022. Due to the rapid rate of species descriptions and re-descriptions, this book will undoubtedly be out of date by the time it reaches you. While the information is not exhaustive, we hope to provide sufficient information so that you can identify the reptile you are looking at in the field. Frustratingly, because some species are simply not reliably distinguishable based on morphological characteristics, accurate identification is not always possible. Where this is the case, we have highlighted which species may prove difficult to identify and why. The distribution maps will be helpful in many of these situations, but sometimes species may only be definitively identified by using genetics.

Taxonomic classification

Humans love putting things into groups. It helps us bring order to the chaos of the world around us. In the 1700s, a Swedish botanist named Carl Linnaeus developed the system of classification that is, in essence, the same system in use today. Building on the work of those who came before him, Linnaeus standardised the way we group organisms into a hierarchy of ranks. Taking the northern blue-tongue as an example, the major ranks are:

Of particular note is Linnaeus’s implementation of the binomial (‘two name’) system, involving a genus (Tiliqua) and a species (scincoides) name. The genus (plural: genera) name always starts with an uppercase letter, while the species name is in lowercase. Some species are further divided into subspecies, a rank typically used for geographically separated populations that may differ in small ways. Subspecies are represented by a third name (to make a trinomial system): Tiliqua scincoides intermedia. All three names are always italicised or underlined when written. This hierarchical and branching structure is often thought of as the Tree of Life, with the trunk and larger branches representing the higher level ranks and the smaller branches and leaves representing genera and species. The decision as to which branch these names are applied to is often arbitrary, giving rise to intermediate rankings, such as superfamilies, infraorders and subgenera.

The ranks that are most relevant to this book are class (the book only deals with class Reptilia), order, family (each chapter deals with a different reptile family), genus and species (within each family chapter, profiles are organised alphabetically by genus and species). Relevant subspecies are mentioned even if those subspecies don’t occur in the Northern Territory.

Historically, organisms were grouped together based on shared traits (e.g. two animals that both give birth to live young were put in the same group). Nowadays, however, most scientists agree that species should be grouped based on a shared evolutionary history – two individuals that are closely related are classified as the same species; species that are each other’s closest relatives are put in the same genus. While closely related organisms will indeed share many traits, convergent evolution – the process by which similar evolutionary pressures result in similar body forms or behaviours arising in otherwise unrelated species – means that using morphology alone can result in inaccurate groupings. Recent advances in genetic analysis have allowed us to build much more accurate family trees of species. Instead of using only a handful of morphological characteristics, scientists can now use hundreds, or thousands, or tens of thousands of genetic characteristics to assess relatedness and determine what we should call a species and where on the Tree of Life that species should be placed.

What is a species?

It’s very easy to define a species in theory (so easy, in fact, that there are numerous species definitions), but surprisingly hard to apply that definition in practice. One of the most popular concepts, the biological species concept, defines a species as a group of individuals that can freely breed with one another and produce fertile offspring – a simple concept that breaks down when you try to use it. For example, it can only be applied to species that reproduce sexually – it’s of no use for bacteria. Another popular definition views species as the end points on the Tree of Life. You can look at an individual, and all its children, and all its children’s children, and so on. Eventually, you will end up with a group (called a clade) comprised of an ancestral organism and all of its descendants. This definition fits nicely with our concept of the branching nature of life, but an arbitrary decision still needs to be made about how far back up the family tree to start the group. If you go back far enough, you’ll end up with everything on Earth classified as a single species.

Despite the difficulties of creating and applying a useful definition, the species is the fundamental unit of taxonomy. In turn, species are used to measure conservation outcomes. We talk about population declines when individuals of a species die without replacing themselves. We talk about biodiversity loss when species go extinct. We talk about biodiversity hotspots in areas that have lots of species, especially if those species are found nowhere else. As such, being able to identify species is vital when documenting the planet’s biodiversity. We hope this guide will make that job easier for users in the Northern Territory, while simultaneously recognising that it can be hard or impossible to assign an individual to a species. This may be because individuals can exhibit variation not covered by a species definition or the profile in this guide. That variation might be genetic (a random mutation that might be passed on to any progeny) or it might be non-genetic (maybe the embryo experienced extreme temperatures while developing in its egg, which caused its scales to form differently). That variation might be a one-off situation, or it might be because the individual you found actually represents an undescribed species. This guide covers the known variation within each species. If you find a reptile in the Northern Territory that does not fit any of the descriptions within this guide, please let the Museum and Art Gallery of the Northern Territory (MAGNT) know and include as much information as possible, such as the date, location (use your phone’s GPS to get accurate coordinates), and photos of the animal (highlighting the features that are making identification tricky). Despite all this talk of variation, chances are that the majority of the reptiles you find in the Northern Territory will be known species and you’ll be able to confidently identify them.

Common names

Common names are given to taxa to improve non-technical communication, but are non-standardised. For example, the tree snake species Dendrelaphis punctulatus has only one scientific name. However, this snake is known by several, regionally specific common names, such as Australian tree snake, common tree snake, green tree snake and golden tree snake. In Sydney, this species is green in colour and is colloquially known as the ‘green tree snake’. In Darwin, this same species is golden–yellow in colour and is colloquially known as the ‘golden tree snake’. Although common names are not standardised in Australian reptiles, we have provided the most commonly used common name for each species, as well as some alternative names where they are in common usage in the Northern Territory.

As a general rule, common names are not capitalised. However, any proper nouns (e.g. the names of people or places) in the name are capitalised (e.g. Cann’s long-necked turtle, Groote Eylandt dwarf blind snake). Following the rules of English grammar, the first word in a common name is also capitalised if it appears at the start of a sentence.

Scientific names

It’s often said that scientific names are better than common names because a species only ever has one scientific name that never changes, and everyone knows to use that name. This, however, is not true. Scientific names often change more than common names do. This is because scientific names are tied to a species concept. The advantage of this is that a scientific name will immediately tell you something about the organism’s close relatives. Based on its scientific name, we can tell that the eastern brown snake (Pseudonaja textilis) is closely related to all the other species in the Pseudonaja genus. The downside of this system is that, as our understanding of how organisms are related to each other changes, so too can the names we call them. The great desert skink was originally described with the name Egernia kintorei, but recent work has shown that it’s more closely related to a group of burrowing skinks in the genus Liopholis, so it has been moved into that genus and had its name changed to Liopholis kintorei. While frustrating, name changes are the inevitable consequence of an increased understanding of the evolutionary histories of the organisms we apply them to.

Name changes can also come about due to what is essentially the heavily opinion-based nature of taxonomy. Scientists have different opinions about what constitutes a species, as reflected in the multiple species definitions that are in use. Even when using one definition, scientists may differ in how they apply that concept. And even for a widely accepted species, people may argue about which name to use.

The International Commission on Zoological Nomenclature (ICZN) has developed a series of rules (‘The Code’) for how to name taxa and what constitutes a valid name. Typically, the first name given to a species has priority. When Eric Worrell described Varanus bulliwallah in 1956, he didn’t realise that Ludwig Glauert had already described the species in 1951. Given that Glauert’s description was published 5 years earlier, the name he used, Varanus mertensi, has priority and is the name we use today for Mertens’ water monitor. This concept even applies within the same publication or article, where the name that first appears, even if it’s misspelt, is the name that has priority. While we refer to them as rules, they act more like guidelines. When Gerard Krefft first described the freshwater crocodile and named it after the discoverer, Mr Robert Johnstone, he unfortunately misspelt the discoverer’s name as Mr Johnson and repeatedly referred to the new species as Crocodylus johnsoni. When he realised his mistake, Krefft acted to point out his error and the name was corrected to Crocodylus johnstoni. While johnsoni was published first and should therefore have priority, ‘The Code’ does have provisions for emending names where mistakes have been made. As such, the name Crocodylus johnstoni is generally accepted as the correct, valid name and is the name used by most publications today.

The situation becomes murkier when names are published without adequate descriptions, because what constitutes an ‘adequate description’ is up for debate. Where multiple names are available for a species, we typically use the names recognised by the Australian Society of Herpetologists’ Official List of Australian Species (2022) (available online at http://www.australiansocietyofherpetologists.org/ash-official-list-of-australian-species). Ultimately, usage by the wider community will determine which names stick.

As well as information on relationships, scientific names can give you other information about the animal: where it comes from (e.g. Rhynchoedura eyrensis, found in the Eyre Basin), who it was named after (e.g. Strophurus horneri, named after Dr Paul Horner of MAGNT), or what it looks like (e.g. the long-nosed dragon, Gowidon longirostris, named for its long rostrum).

If you want to refer to a member of a genus without specifying the species (because you don’t know the species, or because you want to refer to multiple species), you can use the abbreviations ‘sp.’ (singular) and ‘spp.’ (plural). For example, if you can’t identify an animal to species, but you know it’s a dtella gecko, you could refer to it as Gehyra sp. (no need to italise the sp., but make sure you include the trailing full stop to show it’s an abbreviation). Alternatively, if you want to refer to multiple dtella geckos, you could talk about Gehyra spp. In fact, you might often refer to Gehyra sp. or Gehyra spp., because they are notoriously problematic to identify.

Identification

Reptile identification can be tricky, even for experts. Some reptiles are easy to identify. For example, a 5 m crocodile observed on the Adelaide River is undoubtedly a saltwater crocodile (Crocodylus porosus) because no other species of crocodile grows this large in the Northern Territory. Most reptiles, however, are considerably smaller and require close examination to identify. Some reptiles are remarkably variable within a species, while others are extremely conservative within a genus. Sometimes, the arrangement or presence of a single scale can be the only means of differentiating two similar species. Increasingly, species are being identified via genetic differences and taxonomists struggle to find morphological features to readily distinguish closely related species from one another. With experience, however, many similar reptile species become easy to identify based on subtle differences in colour pattern, body proportions, behaviour and habitat associations. When using this guide to identify a reptile, the starting point will be the appropriate dichotomous key.

Dichotomous keys

The dichotomous keys in this guide are like a ‘Choose Your Own Adventure’ story that will help you identify any reptile you find in the Northern Territory. Starting with broad questions, the key will ask you more and more specific questions until you end up with a species name for your mystery beast. The keys in this guide cover all taxonomic rankings. If you already know you’re looking at a skink, skip to the Skink chapter and use the key to genera to determine its genus, then move to that genus and use the key to species. An example key for some common inhabitants of a fruit bowl might look something like this:

1. Not orange in colour . . . . . . . . . . 2

Orange in colour . . . . . . . . . . orange

2. Yellow in colour . . . . . . . . . . 3

Not yellow in colour . . . . . . . . . . 4

3. Elongate and curved in shape . . . . . . . . . . banana

Round to oval in shape . . . . . . . . . . lemon

4. Red in colour . . . . . . . . . . red apple

Green in colour . . . . . . . . . . green apple

You’ll notice that each step has only two, mutually exclusive options. In the first step of our example, you need to determine if your mystery fruit’s colour is orange or not orange. At each step of the key, the group of potential options is divided in two. After continuing through the key, you will eventually be left with just one option and you will have identified your reptile (or piece of fruit).

Most of the options in the keys in this guide will relate to morphology or colour pattern. Where morphological differences may be hard to describe in text, we have provided illustrations of the feature discussed. Sometimes, distribution will be used to narrow down the identification. Frustratingly, there will be some situations where the key will be incapable of discriminating between species. Where features to distinguish species are unreliable or not apparent, we have explained these uncertainties below each key.

Note that because this is a field guide to the reptiles of the Northern Territory, the keys will only allow you to confidently identify reptiles you find in the Northern Territory. If you use these keys on species from elsewhere in Australia, you might end up with an incorrect identification.

Species profiles

Each profile starts with the species’ common and scientific names, followed by a reference for who described it (the author) and when. All species profiles include an adult body size, which tends to be the largest recorded individual. The morphological and colour pattern features of each species are described, with key features useful for identification highlighted in bold. If the species is divided into subspecies, information is provided about how to distinguish the subspecies. Notes are then provided on the species’ known global, national and Northern Territory distribution. Where known, habitat and microhabitat preferences are presented, as is notable information on behaviour, breeding and diet. However, if this information is common to all species in the genus, it tends to be presented only in the genus introduction. For all species that are listed as threatened, their conservation status and the jurisdiction of that status are presented. Similar species, from which the species may prove difficult to distinguish, are noted. Any relevant taxonomic notes that require discussion are presented. Each profile includes photos of typical specimens of the species and, where possible, includes photos to document variation within the species.

Measurements

Typically, we have provided reptile sizes using maximum recorded measurement for the species, usually from the original description or subsequent publications. Occasionally, when a species is known to get very large but rarely does, more details are given (e.g. king brown snakes [TL 290 cm (commonly 150–200 cm)] and saltwater crocodile [TL 5 m (but with reports of up to 7 m]). Body size measurements are given in group-specific units: carapace length (CL) for turtles; total length (TL) for crocodiles, varanids and snakes; and snout–vent length (SVL) for all other lizards. Tail length will be variable for the many lizards that can drop and regrow their tails, whereas the distance from the snout to the vent will be relatively standard.

Maps

Range maps are an extremely important tool for the identification of reptiles. Some species are identified most easily or are disqualified from a list of possibilities by their location. The distribution maps in this book are based on records from the Atlas of Living Australia and other sources, plus our own observations. We have confirmed or disqualified outlying specimen records by consulting with experts and inspecting museum specimens. Arrows are used to highlight small or isolated populations that might not be immediately discernible. Question marks are used in areas where distributions are uncertain. Our maps include squares that refer to some geographically important towns (see the map above). It is worth noting, however, that reptiles seldom read field guides and do occasionally appear in unexpected places, especially in remote and poorly surveyed regions.

Map of geographically important towns of the Northern Territory.

Photographs

To show diagnostic characteristics and intraspecific variations in colour and pattern, we have included up to six photos in each species account. Where possible, photos of individual animals found in the Northern Territory have been used. However, several species have yet to be photographed within the Northern Territory or have only poor-quality photos available. In these cases, we have sourced photos as close to the Northern Territory as possible using individuals that look as we would expect them to in the Northern Territory. Some Northern Territory reptiles have never (e.g. Anilios fossor and A. yirrikalae) or have rarely been photographed in life. In these cases, we have included photos of museum specimens.

Introduction

Reptiles of the Northern Territory

Australia is a globally recognised hotspot for reptile diversity and endemism and the Northern Territory (‘The Territory’, or simply the ‘NT’) accounts for more than its fair share of this global reptilian acclaim. Although the Northern Territory makes up less than 20% of the Australian landmass, despite having relatively uniform topography, it boasts nearly 40% of the country’s reptile species and is home to representatives of every Australian reptile family. A land of extremes, its arid deserts and monsoonal forests harbour some of Australia’s smallest (the dwarf skinks, Menetia spp.) and the world’s largest (the saltwater crocodile) reptiles. It is also home to some of the world’s most venomous snakes, such as the eastern brown snake, coastal taipan and Dubois’ sea snake. While Territorians often find morbid pleasure in boasting about the extreme danger of their reptile fauna, it pales in comparison to the most mundane of everyday tasks, such as driving a car or interacting with horses, cattle or dogs. In fact, the vast majority of the Territory’s iconic reptiles, such as frill-necked dragons, thorny devils and central bearded dragons, are perfectly harmless. Reptiles such as ‘ta-ta dragons’ (Gowidon, Lophognathus and Tropicagama spp.), rainbow skinks, snake-eyed skinks and dtella geckos are so commonplace in Territorians’ lives, even in urban areas and backyards, that they barely warrant a passing thought as we go about our daily routines. Reptiles have also been important in the lives of the people indigenous to the Northern Territory for tens of thousands of years and feature heavily in their creation stories and rock art.

The number of currently recognised reptile species described (at 5-year increments; in blue) and the cumulative number of currently recognised species (in orange) that occur or are suspected to occur in the Northern Territory. A rapid increase in the number of species recognised to occur in the Northern Territory can be observed from the 1960s coincident with increasing accessibility of the Territory, potentially via the advent of four-wheel drive vehicles, as well as Dr Glenn Storr’s prolific tenure at the Western Australian Museum.

Currently, there are 368 described species of reptile known to occur on the land and in the sea of the Northern Territory. There are an additional 22 species that, because they occur very close to the Northern Territory border, we suspect may occur in the Northern Territory but have yet to be recorded. The Northern Territory is a vast, rugged and remote expanse of land with an often harsh and brutal climate. Potentially because of this, it boasts the lowest human population density in the country (0.2 people/km²), with fewer than 250 000 people (about 1% of Australia’s population) calling the Northern Territory home. Despite the Territory’s ancient history of human habitation, many areas remain under surveyed and poorly known by Western scientists and the true diversity of the Territory’s reptile fauna continues to remain a mystery and a treasure trove of discovery.

Climate, bioregions and habitats of the Northern Territory

The Northern Territory has a landmass of over 1.3 million km², including almost 400 offshore islands, and is Australia’s third largest jurisdiction. The Northern Territory is approximately 1700 km from north to south and 950 km from east to west. Despite its vast size, the Northern Territory is relatively topographically flat (average elevation: 190 m ASL) and lacks any major mountain ranges (highest elevation: Mount Zeil at 1531 m ASL); however, it does have some areas of rugged topography that are rich in endemic species. It can be divided into two broad climatic zones: the wet–dry tropics, and the semi-arid and arid zones. The climatic zones are differentiated primarily on a north–south rainfall gradient, with high annual rainfall in the coastal north and low rainfall in the inland south.

Wet–dry tropics

The northern climatic region of the Northern Territory is referred to as the wet–dry, or monsoonal, tropics. This region has a tropical climate with consistently high mean temperatures, variable but typically high humidity, and two distinct seasons – the wet (October to April) and the dry season (May to September). Throughout the year, daytime temperatures average above 30°C every month. Overnight temperatures are highest in the wet season and lowest in the dry season; however, they rarely drop below 14°C even in the coolest months (June and July). Through the dry season, the days are typically warm and sunny, and humidity is relatively low. During this period there is little to no rainfall. As the seasons transition from dry to wet, the temperature and humidity build – a period known as ‘the build-up’ – before relief is provided by monsoonal rains. The wet season is associated with high humidity and rainfall, most of which falls between December and March. During this period, thunderstorms, monsoon systems and cyclones are common. Rainfall during the wet season is high, with an average of over 1500 mm falling during this period, declining as you move from the coast to the arid interior.

Common habitats of the Northern Territory’s wet–dry tropics: (A) tropical savanna woodland, Darwin area (Etienne Littlefair); (B) monsoon forest, Litchfield National Park (Jules Farquhar); (C) sandstone escarpment, Arnhem Plateau (Brendan Schembri); (D) tropical floodplains, Fogg Dam Conservation Reserve (Jules Farquhar).

The dominant habitat throughout the wet–dry tropics is tropical savanna, generally structured with a Eucalyptus canopy and a grassy understorey. The region is also known for its large, tropical rivers, which are fringed by riparian vegetation, such as mangroves, monsoon forest and paperbark swamps. During the wet season, large rivers regularly break their banks and expand into the surrounding floodplains. Rock escarpments, often sandstone, granite or limestone, are a common feature throughout the wet–dry tropics and their topographical relief provides climatic refuges that diversify the composition of habitats across the landscape. Deeply dissected rock escarpments provide gorges vegetated with moist monsoon forest. Exposed escarpments with shallow, sandy soils provide purchase for spinifex hummocks (Triodia spp.) and heathlands to dominate, protected from fires that might break out on the surrounding savannas.

The savanna of the wet–dry tropics is the most fire-prone biome on the planet and fire is an integral and unavoidable feature of the landscape. During the dry season, the grassy understorey dries and cures and, without intentional ignition, often ignites via lightning strike during severe fire conditions towards the end of the dry season. Fire regimes have been managed by Indigenous people for tens of thousands of years, but have been altered by changing patterns of burning and the introduction of invasive grasses and herbivores since the arrival of Europeans in Australia. While the Territory’s species have adapted to these historical conditions, altered fire regimes are likely to be responsible for the decline of a number of species.

Semi-arid and arid zones

The southern climatic region of the Northern Territory, situated towards and at the centre of the Australian landmass, is composed of semi-arid and arid zones. This vast region has much more variable daily temperatures and more distinct seasons, with very hot summers and cool winters. Humidity is generally relatively low and the little rain that does fall, usually less than 300 mm per year, predominantly falls during the hottest months (October to March). The Northern Territory arid zones have a more tropical influence in the north, with slightly higher rainfall, higher humidity and higher winter temperatures.

The dominant habitat throughout arid regions of the Northern Territory is desert and xeric shrubland, usually composed of sparse Acacia and Eucalyptus shrubland or woodland, often with a sparse groundcover of hummock grasses. Dominant vegetation types through the semi-arid and arid zones tend to be driven by soil types, with Mitchell grasses (Astrebla spp.) dominating vast blacksoil plains, sparse Acacia shrubs on hard sandplains, and spinifex (Triodia spp.) hummocks dominating gravely, rocky or loose sandy soils.

As it is in the tropical north, fire is incredibly important in structuring the Northern Territory’s arid habitats. The fine-scale application of fire to the landscape generates a mosaic of habitats with differing degrees of openness at various post-fire ages. Such fire regimes had been maintained by Indigenous people for tens of thousands of years, but have been disrupted recently. Unfortunately, large swathes of arid hummock grasslands and riparian strips have been converted to invasive buffel grasslands with a vastly different burning profile.

Common habitats of the Northern Territory’s semi-arid and arid zones: (A) Arid escarpment, Ormiston Gorge, West MacDonnell (Tjoritja) National Park (Jules Farquhar); (B) desert sand dune, near Yulara, Great Sandy Desert (Stephen Zozaya); (C) blacksoil plains, Barkly Tablelands, Mitchell Grass Downs (Brendan Schembri); (D) stony, spinifex-dominated plain, Tanami Desert (Brendan Schembri).

The Northern Territory’s arid zone is host to an incredible diversity of reptiles, particularly lizards. Although they look fairly homogenous, the spinifex hummock grasslands that dominate this vast region provide perfect habitat for reptiles by creating a range of cool, humid, structurally complex microhabitats protected by an impenetrable matrix of spines. A footy field sized area of Northern Territory arid grassland can be home to many more species of reptile than some entire countries.

Bioregions

Within these broad climatic zones, the Territory can be further subdivided into bioregions, which can inform our understanding of the biota of a given area. Bioregions are a landscape-scale approach to classifying the environment using geology and vegetation. Twenty-five bioregions have been identified in the Northern Territory, many of which host unique compositions of reptile fauna and some host regionally endemic reptile species. Where informative, we have referred to the bioregions that individual reptile species occupy.

Reptile biogeography in the Northern Territory

Understanding the structural and functional features of the landscape at different spatial scales is key to understanding the composition and distribution of Australian fauna. The structural and function composition of the landscape is affected by temperature, rainfall, geology, soil and elevation, which determine vegetation composition, thermal niches and local reptile abundance, species richness and composition. Some reptiles are extremely widespread, and are habitat and climatic generalists that have very few specific requirements that need to be met to occur in an area (e.g. Children’s python, Antaresia childreni; Burton’s snake-lizard, Lialis burtonis). As a result, these species can be encountered in most habitats throughout the Northern Territory. Others, however, have very specific habitat and climatic requirements (e.g. dappled snake-eyed skink, Cryptoblepharus daedalos; Arnhem phasmid gecko, Strophurus horneri) and have exceptionally small distributions. Biogeography, however, is incredibly complex and the current habitat and climate cannot entirely explain why and where species occur without a deeper appreciation for species interactions, and geological and evolutionary history.

Threats to reptiles in the Northern Territory

While Australian reptiles appear to have avoided the precipitous population declines experienced by our native birds and mammals following the arrival of Europeans, there are a number of threatened reptiles and numerous species have suffered declines in recent decades. Unfortunately, potentially because people have tended to view reptiles as at less risk than other groups of native animals, our understanding of their conservation status and the threats to them in the Northern Territory is undermined by a lack of monitoring and research. We do, however, have some evidence that reptiles are vulnerable to many of the same threats facing other Australian animals, such as mammals and birds, that have enjoyed significantly more research interest and investment. These threats include invasive plants and animals, habitat clearance, changed land practices, wild harvesting, altered fire regimes and climate change.

Invasive plants and animals

When we think of the impact of invasive species on reptiles in northern Australia, there is undoubtedly a rough-skinned, hopping invader at the forefront of everyone’s mind. Cane toads (Rhinella marina), native to South America, invaded the Northern Territory via Queensland in the early 1980s and have since proceeded to colonise all of the Territory’s mainland wet–dry tropics. As the toads advanced across the Territory, many of our hapless native reptile predators mistook these amphibian invaders for an easy meal – which they were, but at a huge cost. Because Australia has no native toads, most Australian reptiles have no evolutionary history with true toads and their chemical defences and are extremely vulnerable to their toxic secretions. In fact, many are so sensitive that merely biting or mouthing a toad proves fatal. The fallout from the arrival of cane toads in the Northern Territory appeared stark and severe. Some of our most beloved and conspicuous large reptilian predators, such as northern blue-tongues (Tiliqua scincoides intermedia), yellow-spotted monitors (Varanus panoptes), king brown snakes (Pseudechis australis), plains death adders (Acanthophis hawkei) and some populations of freshwater crocodiles (Crocodylus johnstoni) suffered precipitous population declines. The impact of cane toads on reptiles has probably attracted more research than just about any other threat to Australian reptiles, and the – sometimes nuanced – nature of their impact is becoming clearer. Some reptiles are severely impacted, while others indirectly benefit (reviewed in Shine 2010). However, the vehement hatred of toads is such that they are regularly blamed for any and all perceived population changes in Northern Territory reptiles. There are, of course, numerous other potential threats to the reptiles of the Northern Territory, some of which risk being overlooked due to the impact of the highly visible toad.

Invasive predators, such a feral cats and red foxes, are thought to be responsible for most Australian mammal extirpations and extinctions since European arrival, and feral cats have been implicated in northern Australia’s recent and severe mammal declines. In the Northern Territory, cats occur throughout the mainland and on many offshore islands, while foxes are restricted to more arid areas south of about Elliott. Reptiles are known to be important prey items for cats and foxes, particularly in the arid zone, and cats alone are estimated to eat over 450 million Australian reptiles a year (Woinarski et al. 2018). Yet, little is known about the impacts of these feral predators on reptiles in the Northern Territory. Clearly, elevated mortality rates due to invasive predators could result in significant population declines, and research and monitoring of this potential impact are warranted.

Feral herbivores, such as buffalo, camels, cattle, donkeys, horses and pigs, are abundant across the Northern Territory. Through trampling, wallowing, herbivory and defecation, feral herbivores impact on the environment by altering the structure, composition and functioning of ecosystems. Feral herbivores have recently been implicated in northern mammal declines and, although information is lacking, they are also probably having negative impacts on populations of reptiles. The impact of feral herbivores on reptiles in the Northern Territory is likely to be felt most severely by species that rely on fragile habitat components, such a spinifex, burrows and fallen hollow logs, which are often destroyed where herbivores are abundant. Because they are opportunistic omnivores, feral pigs may also be impacting reptile populations by digging up and eating their eggs – a particular threat to sea and freshwater turtles.

An overlooked potential impact of invaders on reptiles in the Northern Territory is the impact of invasive vegetation, such as gamba grass (Andropogon gayanus) and buffel grass (Cenchrus ciliaris). These introduced pasture grasses are a major threat to native grasslands and woodlands because they transform plant communities, change vegetation structure and alter fire regimes. For example, the tropical savanna woodlands of the Northern Territory’s Top End are progressively being turned into invasive tropical grasslands because of the spread of gamba grass. Gamba grows much taller and denser than native grasses, and burns at far greater heights into the canopy and at far greater temperatures, often killing the native canopy and promoting the growth of more gamba. Similarly, in more arid systems, vast plains of buffel grass are progressively replacing native grasslands, such spinifex sandplain, resulting in simplification and degradation of these complex arid systems. Such changes to vegetation structure and function are bound to have profound impacts on local reptile populations.

Habitat clearing and changed land practices

Much of the Northern Territory is relatively unaltered by land clearing and development. However, urban, rural, industrial, mining and agricultural development all require habitat clearing and changed land practices, which alter the structure and function of ecosystems, sometimes irreversibly. Habitat clearing is likely to have the most immediate, severe and long-lasting impact on local fauna because it typically removes all ecosystem features and functions that sustain native flora and fauna. Habitat clearance imposes the most severe impacts on habitat specialist species with small distributions. For instance, the yellow-snouted gecko (Lucasium occultum), Kurnbudj ctenotus and Stuart’s ctenotus (Ctenotus kurnbudj and C. stuarti) all occupy specific habitats in a small area between the Mary and South Alligator rivers in the western Top End. Although some of this habitat is protected by the north-western edge of Kakadu National Park, conversion of the Point Stuart area to intensive agricultural practices, such as cotton production, could remove a considerable portion of the habitat required by these species to persist.

The majority of the Northern Territory is designated as pastoral land, much of which is managed as cattle properties. However, most cattle stations in the Territory stock cattle on native habitats, which, unlike their southern counterparts, are not entirely cleared. Given the proportion of land designated to pastoral activities, striking a balance between land management for both pastoral and biodiversity outcomes is paramount to the conservation of all native species in the Northern Territory.

Wild harvesting

In the scheme of things, wild harvesting appears to be a fairly minor threat to the reptiles of the Northern Territory. The impact of the harvesting of reptiles for food, such as the eggs and adults of sea turtles and goannas, by Indigenous people almost certainly pales in comparison to the impact caused by car and boat strikes and invasive species. However, some species in some locations may be so imperilled by other threats, such as feral predators, pigs and cane toads, that they may require a reduction in hunting pressure while populations recover.

While some species of reptiles are targeted by illegal collectors for the pet trade, there is currently no evidence that this has caused population-level impacts to any reptile species in the Northern Territory. However, illegal reptile collectors are known for their unscrupulous methods of pillaging reptiles from the landscape, which can cause considerable damage to local habitats by overturning and destroying rocks, breaking open tree hollows and removing loose bark from trees. Such destructive activities are likely to have a far more pervasive and long-term impact on reptiles than the removal of individuals from the area.

One of the larger impacts of wild harvesting that is seemingly overlooked is the impact of fishing trawlers on sea snakes. Annually, thousands of sea snakes are caught in fishing trawler nets throughout waters off the coast of the Northern Territory. While many are released alive, plenty are killed in the nets, and those that are released are likely to be released some distance from where they were collected, potentially injured and into unsuitable habitat. It is currently unknown what impact this may be having on sea snake populations.

Altered fire regimes

The Northern Territory has some of the most fire-prone habitats on the planet, particularly the northern tropical savannas. For tens of thousands of years Indigenous people have burnt the Northern Territory to promote movement through the landscape and fresh vegetation growth, and to improve hunting. Traditional Indigenous fire management typically involves deploying cool, patchy burns early in the dry season that reduce grass fuel loads. Traditional burning creates mosaics of habitats of different ages, to which the fauna of Australia has adapted. It also creates firebreaks in the landscape that help stop larger and far more severe fires late in the dry season. Unfortunately, the arrival of Europeans has altered these fire regimes via the removal of traditional burning practices, the introduction of invasive weeds and changes to landscape structure. Altered fire regimes are known to threaten thousands of species worldwide and are probably having a considerable impact on some reptiles of the Northern Territory, particularly those that require long-unburnt habitats for persistence. Considerably more research should be directed towards investigating the impacts of altered fire regimes on populations of reptiles in the Northern Territory.

Climate change

The climate of the Northern Territory is often brutal. Although renowned for its extreme temperature and humidity, long periods of drought, epic thunderstorms, flooding monsoons, destructive cyclones and little in the way of topography to provide climatic refugia, species native to the Northern Territory are adapted to these climatic conditions. However, many animals living in the Northern Territory are likely to already be at the limits of their climatic tolerance, with little relief to be found by way of range shifts across rainfall, temperature or elevational gradients. Rapid human-induced climate change has begun and will continue to affect the Northern Territory, including by increasingly intense and unpredictable rainfall events, longer droughts, increasing average temperatures, increasing frequency of temperature extremes, sea level rise and saltwater intrusion, and altered fire behaviour and severity. As ectotherms, reptiles are beholden to the prevailing weather conditions for all their biological functions, and although they can adapt to changing conditions, changes that are too rapid or extreme are likely to result in population declines, range contractions, reductions in niches, skewing of sex ratios in species with temperature-dependent sex determination and, in the most severe instances, extinction.

Crocodilians

(Order Crocodilia)

Family Crocodylidae

Crocodiles genus Crocodylus Laurenti, 1768

Crocodilians first appeared 95 million years ago and are an order of predominantly large, semi-aquatic, predatory reptiles divided into three ancient families – Alligatoridae (alligators and caimans), Gavialidae (gharial and false gharial) and Crocodylidae (true crocodiles). In Australia, crocodilians are represented by ‘true crocodiles’ (Crocodylidae) in a single genus (Crocodylus). True crocodiles are composed of three extant genera (Crocodylus, Mecistops and Osteolaemus) with a global distribution from the Western Pacific, through Asia, Africa, and North and South America. Globally, there are 13 species in the genus Crocodylus; two species are found in Australia, one of which is endemic to Australia (Crocodylus johnstoni) and the other of which has a much broader distribution (Crocodylus porosus). Both species occur in the Northern Territory.

Crocodilians include the largest living reptiles on the planet. We now know that crocodilians are the closest living relatives of birds, and are more closely related to birds and dinosaurs than they are to other reptiles. Despite their prehistoric appearance, crocodiles are one of the most biologically advanced reptiles, with a four-chambered heart, cerebral cortex and the functional equivalent of a diaphragm. Crocodiles are unmistakeable from any other group of Australian reptiles. They possess long, streamlined bodies armoured with bony, sometimes keeled, plates (osteoderms); powerful, laterally compressed tails; short, robust limbs with webbed feet; eyes set on top of their head, which can be retracted into their skull; elongate snouts with nostrils set high above the tip to allow them to breathe while their body is submerged; and powerful jaws lined with long, conical teeth.

Australian crocodiles are largely ambush hunters. While they are regularly observed basking and hunting during the day, they are most active at night. Crocodile skull, snout and tooth morphology is driven by diet. Species that predominantly eat smaller, soft-bodied prey (e.g. freshwater crocodiles) have slender jaws that can be swiftly swiped through the water to catch their agile quarry. Species with more varied diets (e.g. saltwater crocodiles) that include large, hard- and/or heavy-bodied prey that require crushing or dismemberment before being consumed have broad snouts with extremely powerful jaws and muscles.

All crocodiles are oviparous, laying eggs in an excavated hole (e.g. freshwater crocodiles) or in a mound constructed from vegetation (e.g. saltwater crocodiles). Despite their fearsome reputation as brutal predators, some crocodiles, including both Australian species, exhibit maternal care. Female saltwater crocodiles in particular are extremely attentive mothers, displaying some of the most sophisticated maternal care among reptiles. After constructing the nest and laying her eggs, a female crocodile will guard the nest from any threats (including people) throughout the entire incubation period. During hatching, the young crocodiles will call out to their mother, who will gently excavate the nest and delicately carry them to the water in her powerful jaws.

Saltwater (Crocodylus porosus) (right) and freshwater crocodiles (C. johnstoni) (left) co-occur in many Northern Territory waterways. Daly River, NT. Brendan Schembri.

Saltwater crocodiles are the world’s largest living reptiles with the strongest bite force of any living animal. They are a formidable predator and are by far the most dangerous reptile in Australia. Adult saltwater crocodiles are sufficiently large that humans fall well within the size of their natural prey, and they have been responsible for a number of human fatalities. Extreme caution should be taken near waterways in saltwater crocodile habitat in the Northern Territory.

Key to Crocodylus of the Northern Territory

1. Relatively slender snout (A); a single row of enlarged nuchal shields, separated from the smooth-skinned parietal region by fewer than eight granular scales (A) . . . C. johnstoni

Relatively broad snout (B); two rows of enlarged nuchal shields, separated from the smooth-skinned parietal region by more than eight granular scales (B) . . . C. porosus

Freshwater crocodile

Crocodylus johnstoni Krefft, 1873

TL 3 m. A moderately sized crocodile with a relatively narrow snout; and enlarged nuchal shields in a single row that is separated from the smooth-skinned parietal region by fewer than eight scales. Dorsal surface olive–green to dark brown with darker brown to black markings often coalescing to form incomplete dorsal cross-bands. Ventral surface paler than dorsum. Notes: Endemic to Australia. Found across much of northern Australia, from the interior of Far North Qld to the Kimberley region, WA. It occurs through much of northern NT. Predominantly found in freshwater rivers, creeks and billabongs. Can be found in smaller, more inland reaches of waterways than saltwater crocodiles, but the two species are regularly found in sympatry. Feeds on crustaceans, fish, frogs, reptiles, birds and small mammals, such as flying-foxes. In some locations, it has been significantly impacted by cane toads (Rhinella marina). While generally not considered dangerous to humans, care should still be taken around it as it can bite and cause serious injuries if threatened or surprised. Similar species: C. porosus

Crocodylus johnstoni. Daly River, NT. Brendan Schembri.

Saltwater crocodile; estuarine crocodile

Crocodylus porosus Schneider, 1801

TL 5 m (but with reports of up to 7 m). An enormous crocodile with a relatively broad snout; and two rows of enlarged nuchal shields, separated from the smooth-skinned parietal region by more than eight granular scales. Dorsal surface grey, green to almost black with darker mottling. Ventral surface paler than dorsum. Females are considerably smaller and more lightly built than males, rarely exceeding 3 m in length. Notes: Found from eastern India, through South-East Asia, New Guinea, northern Australia and Micronesia. Despite their name, saltwater crocodiles inhabit saltwater, brackish and freshwater. While they are most common in major rivers, floodplains and billabongs within 100 km of the coast, they also inhabit some inland waterways more than 200 km from the coast, such as in the Katherine region. Typically inhabit floodplains, creeks, rivers, estuaries, and coastlines, and will cross open ocean to reach offshore islands. Feed on crustaceans, fish (including sharks and rays), birds, mammals and reptiles (including other crocodiles). Known to kill and eat people. Extreme caution should be exercised around any waterways that are potentially inhabited by saltwater crocodiles. VERY DANGEROUS. Similar species: C. johnstoni

Crocodylus porosus. Daly River, NT. Brendan Schembri.

Turtles and tortoises

(Order Testudines)

Turtles and tortoises (testudines) are an iconic, unmistakeable group of reptiles. While the entirely terrestrial species are known as tortoises and the entirely marine species are known as turtles, the freshwater species have been referred to as both turtles and tortoises. Nowadays, most Australian sources (including this guide) refer to them as freshwater turtles. Australia has no terrestrial tortoises, but we do have a diverse array of marine and freshwater turtles. Australian coastal waters are home to six of the world’s seven marine turtle species, with representatives from both families (Cheloniidae and Dermochelyidae). Our native freshwater turtle species are divided into two families: Chelidae, with 24 species; and Carettochelydidae, containing only the Northern Territory endemic pig-nosed turtle (Carettochelys insculpta). While their movements on land are somewhat ungainly, both marine and freshwater turtles are graceful and efficient swimmers when in the water. All turtles possess a hard, bony shell (made up of many small bones covered in soft flesh in the case of the leatherback turtle), composed of a carapace (upper half) and a plastron (lower half).

Carapace and plastron of a chelid turtle (Chelodina canni) showing the position and name of scutes from a dorsal and ventral perspective.

Carapace and plastron of a cheloniid turtle (Chelonia mydas) showing the position and name of scutes from a dorsal and ventral perspective.

Key to the turtles of the Northern Territory

1. Both forelimbs and hindlimbs are paddle-shaped, without webbed, clawed feet (A) 2

Forelimbs and hindlimbs not paddle-shaped, with webbed, clawed feet (B) . . . . . . . . . . Chelidae (side-necked turtles)

2. Nostrils level with the surface of the snout, no fleshy proboscis present; marine-dwelling . . . . . . . . . . 3

Nostrils at the end of a tubular, fleshy snout or proboscis; mostly freshwater-dwelling . . . . . . . . . . Carettochelydidae (pig-nosed turtle)

3. Limbs with claws (A) . . . . . . . . . . Cheloniidae (sea turtles)

Limbs without claws (C) . . . . . . . . . . Dermochelyidae (leatherback turtle)

Key to marine turtle tracks of the Northern Territory

1. Tracks with alternating pattern, flipper marks alternate (A) . . . . . . . . . . 2

Tracks with breaststroke pattern, flipper marks side by side (B) . . . . . . . . . . 3

2. Plastron skid narrower than width of hind flipper tracks . . . . . . . . . . Caretta caretta

Plastron skid wider than or equal to width of hind flipper tracks* . . . . . . . . . . Eretmochelys imbricata or Lepidochelys olivacea

3. Track width <2 m . . . . . . . . . . 4

Track width >2 m . . . . . . . . . . Dermochelys coriacea

4. Front flipper tracks approximately as wide as hind flipper tracks . . . . . . . . . . Chelonia mydas

Front flipper