Australia is a land of reptiles, but not particularly large ones, part 1: turtles, tortoises, lizards, and snakes

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Australia seems to be a land of reptiles.

For example,

Why is it, then, that

  • several niches for reptiles are oddly empty in Australia, and
  • the largest living reptiles here are generally smaller than those elsewhere?

It is body mass - not length - that is most important when assessing the size of animals.

Body mass - although seldom measured - determines the energy consumption of a species and its likely demands from, and effects on, an ecosystem.

Length is a particularly poor indicator of body size in reptiles, which vary from linear snakes to hemispherical tortoises.

Readers may ho-hum to hear that the saltwater crocodile (Crocodylus porosus, https://www.inaturalist.org/taxa/26068-Crocodylus-porosus) weighs more than the scrub python (Simalia kinghorni, https://www.inaturalist.org/observations?place_id=any&taxon_id=540202&view=species) of similar extreme length of 6 metres. However, the news that this difference is 30-fold may raise an eyebrow.

This principle applies also within crocodilians, snakes, and lizards, in which the girth of the body is as important for body mass as is the total length.

Bulk is critical to reptiles, partly because it greatly affects the rate of reproduction. The greater the body mass of females, the more eggs or live-young are produced at a time.

Females are, in this context, the more important sex. It takes only one male individual to inseminate many female individuals. Furthermore, some reptiles are parthogenetic, making males redundant. Although particularly large males tend to command attention, they do not necessarily contribute much to the overall energetics of the species.

It is females that are immediately indispensible for survival.

Reptiles provoke exaggeration (when reporting data), macho, and hyperbole (when interpreting data), even among the educated.

Therefore, I sought out well-documented records of maximum body sizes. These are remarkably few for females.

I have tried to avoid other pitfalls. For example,

  • reptiles may continue to grow long after reaching adulthood, and
  • body masses may be doubled when large prey is swallowed whole.

Bearing these caveats in mind, how does Australia compare with other southern continents at the same latitudes?

TESTUDINES (https://en.wikipedia.org/wiki/Turtle)

The living fauna of testudines in Australia falls short in two respects.

Firstly, this is the only inhabited continent that lacks any land tortoise. By contrast, tortoises weighing more than 30 kg are widespread in southern Africa (Stigmochelys pardalis, https://www.inaturalist.org/taxa/40092-Stigmochelys-pardalis) and South America (Chelonoidis denticulatus, https://www.inaturalist.org/taxa/539451-Chelonoidis-denticulatus), at similar latitudes.

Secondly, the freshwater testudines of Australia are limited in body size. The largest species is Carettochelys insculpta, (https://www.inaturalist.org/taxa/39536-Carettochelys-insculpta) of tropical Australia, which can reach 22.5 kg, and lays no more than 30 eggs at a time.

By contrast,

Thus, species extending south to the same latitudes and living in comparable habitats are 2-3-fold more massive than any testudines living in Australia.

CROCODILIA (https://en.wikipedia.org/wiki/Crocodilia)

It has long been customary to regard northern Australia as having the largest crocodiles on Earth. However, this is questionable.

Crocodylus porosus (https://www.inaturalist.org/taxa/26068-Crocodylus-porosus) is only marginally Australian, and is not necessarily more massive than Crocodylus niloticus (https://www.inaturalist.org/taxa/341972-Crocodylus-niloticus) of Africa.

Crocodylus porosus is the most marine of crocodilians, and no individual has been shown to live its full life within Australia.

The largest crocodilian at similar latitudes in South America is Melanosuchus niger (https://www.inaturalist.org/taxa/116532-Melanosuchus-niger), which not only is restricted to fresh water, but spends much of its time in the Amazon rainforests themselves. This species is shorter than its Australian counterpart, but is so heavily armoured that it may rival it in the maximum body mass of females.

Even if we accept that a few exceptional individual males of C. porosus might once have reached 7 m long in Australia, so did C. niloticus in southern Africa, which is the more massive species with a proportionately short tail.

C A W Guggisberg, in his book 'Crocodiles: their natural history, folklore and conservation' (https://catalogue.nla.gov.au/catalog/2003481), reports that a specimen shot in 1903 in Malawi, southern Africa, by Hans Besser, "an excellent field naturalist whose reliability cannot be questioned," was 7.6 m long, despite its tail having been shortened further by partial amputation.

The Guinness book of animal records (1995, https://www.amazon.com.au/Guinness-Book-Animal-Records/dp/0851126588) states: "The longest authenticated record of recent years is a male saltwater crocodile (Crocodylus porosus) just over 7 m...long, which lives in the Bhitarkanika Wildlife Sanctuary, Orissa State, India." (Note that the record individual of the Australian species, C. porosus, was not in Australia but on the Asian mainland.)

Crocodylus niloticus is less sexually dimorphic than C. porosus, suggesting that females of the Australian species (which reach only 3 m) may fall short of females of the southern African species. Researcher H B Cott (https://en.wikipedia.org/wiki/Hugh_B._Cott) recorded a female individual of C. niloticus to be 5.6 m long. This is borne out by the fact that the southern African species can lay clutches of up to 95 eggs, compared to only 72 in the case of C. porosus of Asia and Australia.

VARANIDAE (https://en.wikipedia.org/wiki/Varanidae)

Varanus giganteus (https://www.inaturalist.org/taxa/39453-Varanus-giganteus) of Australia, despite its reputation (https://www.google.com.au/search?q=Perenty+australia%27s+giant+lizard&sca_esv=571840155&sxsrf=AM9HkKmC2jnLMvgturnDMJLSkaIyWmwNQw%3A1696838772669&source=hp&ei=dLQjZfHKJZTL2roPxKmj4Ak&iflsig=AO6bgOgAAAAAZSPChPlONWx-0RIfi-8vxQ5Zvx498N47&ved=0ahUKEwjx1KGCweiBAxWUpVYBHcTUCJwQ4dUDCAw&uact=5&oq=Perenty+australia%27s+giant+lizard&gs_lp=Egdnd3Mtd2l6IiBQZXJlbnR5IGF1c3RyYWxpYSdzIGdpYW50IGxpemFyZDIFECEYoAFItWdQiwhY11ZwAXgAkAEAmAG7A6ABmz-qAQowLjMuMjMuNS4xuAEDyAEA-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&sclient=gws-wiz#fpstate=ive&vld=cid:a3482070,vid:bsC1dzwPs2k,st:0), is less massive (up to 15 kg) than its largest congener in southern Africa, namely Varanus niloticus, which is as long but more thickset.

S Spawls and his co-authors, in their authoritative 'Field guide to the reptiles of East Africa' (2002, https://www.abebooks.com/9780126564709/Field-Guide-Reptiles-East-Africa-0126564701/plp), state that V. niloticus has been recorded to reach at least 2.5 m long in southern Africa - implying a body mass over 20 kg. This is borne out by the fact that the Australian species, V. giganteus, lays up to 11 eggs, whereas V. niloticus lays up to 60 eggs, per clutch.

The Australian species does, however, beat any South American lizard in maximum size.

The family Varanidae is absent from the Americas. Although the tegus (largest members of Teiidae) are mainly carnivorous lizards remarkably similar to varanids, they fall short in size.

The largest is Salvator rufescens (https://www.inaturalist.org/taxa/318759-Salvator-rufescens) of semi-arid thorn scrub (chaco, https://en.wikipedia.org/wiki/Gran_Chaco) in South America. With a snout-vent length exceeding 50 cm, and a total length of up to 1.3 m, S. rufescens is larger than any agamid (https://www.inaturalist.org/observations?place_id=6744&taxon_id=31096&view=species) or scincid (https://www.inaturalist.org/observations?place_id=6744&taxon_id=36982&view=species), but smaller than several species of varanid, in Australia.

The largest lizard in South America is herbivorous, but still falls short of the Australian V. giganteus: Iguana iguana (https://www.inaturalist.org/taxa/35342-Iguana-iguana) of the Amazon, which reaches up to 10 kg.

SERPENTES (https://en.wikipedia.org/wiki/Snake)

Both the venomous and the non-venomous snakes of Australia fall short of their southern African or South American counterparts in maximum size.

Even if extreme lengths attributed to Oxyuranus scutellatus (https://www.inaturalist.org/taxa/35170-Oxyuranus-scutellatus), viz. more than 3 m, are true, they do not exceed the values attributed to the longest venomous snakes elsewhere.

More importantly, the lack of Viperidae (https://www.inaturalist.org/observations?place_id=any&taxon_id=30667&view=species), which are thickset compared to Elapidae, limits the maximum body mass of venomous snakes in Australia.

The largest in Australia is probably the tropical form of Pseudechis australis (https://www.inaturalist.org/taxa/35161-Pseudechis-australis), which is thickset compared to O. scutellatus and can also exceed 2 m.

However, P. australis falls short of Bitis gabonica (https://www.inaturalist.org/taxa/30853-Bitis-gabonica) of southern Africa and Lachesis muta (https://www.inaturalist.org/taxa/73840-Lachesis-muta) of South America, both of which occur at similar latitudes. This is mainly because B. gabonica has a far greater body width (up to 17 cm, corresponding to a girth of up to 37.5 cm) than any snake in Australia.

R M Isemonger, in his book 'Snakes of Africa' (https://www.abebooks.com/signed-first-edition/Snakes-Africa-Isemonger-R.M-Books-PTY/22383738528/bd), recorded an individual of B. gabonica with length 1.75 m and body mass 8.4 kg. This species produces up to 60 offspring at a time, compared to only up to 20 for P. australis of Australia.

According to 'The Guinness book of animal facts and feats' (https://find.slv.vic.gov.au/discovery/fulldisplay/alma996223373607636/61SLV_INST:SLV), B. gabonica has probably the longest fangs (up to 5 cm), and the greatest average yield of venom, of any snake on Earth. Lachesis muta is not as thickset as B. gabonica, but is more thickset than any long venomous snake in Australia, and clearly exceeds the maximum length of P. australis (3.4 m vs 2.7 m).

This gives the South American species, L. muta, a maximum body mass of probably more than 10 kg, without peer in Australia.

Simalia kinghorni, largest snake in Australia, is so slender that it has never been recorded to exceed 26 kg, and can lay only up to 20 eggs per clutch. Research by R G Shine (https://www.publish.csiro.au/wr/wr04084 and https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/jzo.12837) has shown that large male individuals of S. kinghorni are about 50% more massive than their mates.

The southern African Python natalensis (https://www.inaturalist.org/taxa/111451-Python-natalensis), although no longer than 5.6 m, may exceed 100 kg, and it is females that are largest, laying up to 100 eggs per clutch. Furthermore, the largest prey item (59 kg) known to have been swallowed by an African python, belonging to Aepyceros melampus (https://www.inaturalist.org/taxa/42278-Aepyceros-melampus), is itself more than double the body mass of the largest snake in Australia.

Eunectes murinus (https://www.inaturalist.org/taxa/32213-Eunectes-murinus) of the Amazon, which exceeds 200 kg, is at least as long (up to 8.5 m) as the Australian S. kinghorni, and incomparably more thickset. Females of E. murinus are much more massive than males.

to be continued in https://www.inaturalist.org/journal/milewski/85595-australia-is-a-land-of-reptiles-but-not-particularly-large-ones-part-2-the-great-south-land-here-be-slight-dragons#...

Publicado el octubre 8, 2023 03:37 MAÑANA por milewski milewski

Comentarios

Well done! Thoughtful, well researched and , for me, very interesting.

Publicado por ptexis hace 10 meses

An excellent read and a real eye opener! Thank you!

Publicado por mattcampbellaus hace 10 meses

Interesting read!

Publicado por johannesvanrooyen hace 10 meses

@johannesvanrooyen @ptexis @mattcampbellaus

Many thanks to you for your appreciative words.

Publicado por milewski hace 9 meses

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