Australia is a land of reptiles, but not particularly large ones, part 2: 'The Great South Land: Here Be Slight Dragons'

...continued from https://www.inaturalist.org/journal/milewski/85581-australia-is-a-land-of-reptiles-but-not-particularly-large-ones-part-1-turtles-tortoises-lizards-and-snakes#

Although Australia is continent-size, it falls short of its surrounding islands in the maximum size of living reptiles.

• The largest of all agamids, Hydrosaurus amboinensis (https://www.inaturalist.org/taxa/1120224-Hydrosaurus-amboinensis), occurs not in Australia but on the islands of eastern Indonesia (https://en.wikipedia.org/wiki/Indonesia).
• The skincid Corucia zebrata (https://www.inaturalist.org/taxa/37194-Corucia-zebrata) of the Solomon Islands (https://en.wikipedia.org/wiki/Solomon_Islands), although arboreal, is reputed to be more massive than even the largest Tiliqua (a genus of unusually thickset, terrestrial skinks) in Australia.
• Not only does Varanus komodoensis (https://www.inaturalist.org/taxa/39449-Varanus-komodoensis and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2748693/) of Indonesian islands (https://upload.wikimedia.org/wikipedia/commons/4/4c/Komodo_dragon_distribution.gif) far outsize its Australian congeners, but the largest varanid in New Guinea (Varanus salvadorii, https://www.inaturalist.org/taxa/39440-Varanus-salvadorii) is also longer, and possibly more massive, than V. giganteus of Australia.
• The largest gekkonid on Earth is Rhacodactylus leachianus (https://www.inaturalist.org/taxa/33862-Rhacodactylus-leachianus) of New Caledonia (https://en.wikipedia.org/wiki/New_Caledonia).
• Even in the case of Pygopodidae (https://en.wikipedia.org/wiki/Pygopodidae), a family restricted to Australasia, the largest species (Lialis jicari, https://www.inaturalist.org/taxa/36977-Lialis-jicari) appears to occur in New Guinea, not Australia.
• The only rhynchocephalians (https://en.wikipedia.org/wiki/Rhynchocephalia) remaining on Earth have survived in New Zealand (Sphenodon punctatus, https://www.inaturalist.org/taxa/200834-Sphenodon-punctatus), not in Australia.

There is a general tendency for the largest animals to have males larger than females. For examples, elephants, giraffes, and kangaroos all have mature males double the body mass of mature females.

Perhaps one reason is that, at the upper limit of body sizes that a habitat can support, animals economise on the number of mature males by means of extreme polygamy and competition among males.

In the extreme sexual dimorphism of the largest crocodilians and lizards, Australia may be unremarkable. However, in the case of snakes, Australia is opposite to the other southern continents, because both P. australis and S. kinghorni have males larger than females, whereas the converse applies to vipers, rock pythons, and anacondas.

If comparisons were standardised to females, the slightness of the Australian reptiles might be even more pronounced than I have shown. For example, it remains possible that the largest female varanid in Australia is less massive than the largest female iguana in South America, which can lay up to 45 eggs per clutch.

Herbivorous reptiles are thickset, partly because greens can only be digested by means of fermentation in bulky guts. The most striking gap in the reptile fauna of Australia is its lack of herbivores, particularly land tortoises but also lizards.

Agamidae and Iguanidae are closely related, and Intellagama lesueurii (https://www.inaturalist.org/taxa/146186-Intellagama-lesueurii) of Australia may resemble juveniles of Iguana iguana. However, no agamid in Australia is herbivorous.

Thus, remarkably, the largest female individual of any kangaroo alive today (35 kg) is smaller than the largest female individual of any tortoise alive today in southern Africa and probably South America - which are likewise grazers.

The lack of herbivorous reptiles in Australia is not explained by isolation.

• As part of Gondwana, Australia and South America supported giant land tortoises (Meiolania, probably up to 500 kg, https://en.wikipedia.org/wiki/Meiolania). This group survived on Lord Howe Island (https://en.wikipedia.org/wiki/Lord_Howe_Island), just east of Australia, as recently as the Pleistocene.
• Modern tortoises have proved to be excellent colonisers of islands.
• Iguanids may also have a Gondwanan ancestor, and in any event have reached remote islands such as the Galapagos (https://en.wikipedia.org/wiki/Gal%C3%A1pagos_Islands), Madagascar (https://en.wikipedia.org/wiki/Madagascar), and Fiji (https://en.wikipedia.org/wiki/Fiji).

ECOLOGICAL EXPLANATION

An important factor may be fire.

All vegetation types in Australia burn, other than a few small pockets of 'rainforest' (https://www.dcceew.gov.au/sites/default/files/documents/mvg1-nvis-rainforests-and-vine-thickets.pdf). By contrast, part of southern Africa and much of South America are free of wildfires.

It is obvious that crocodiles, freshwater turtles, and anacondas are safe from flames. However, scorching is one of the greatest threats to tortoises massive enough to thwart large predators.

Stigmochelys pardalis (https://en.wikipedia.org/wiki/Leopard_tortoise) is commonest and largest-bodied in vegetation such as that dominated by a giant purslane (Portulacaria afra, https://www.inaturalist.org/taxa/326086-Portulacaria-afra) in the Eastern Cape of South Africa. This succulent thicket occurs under a semi-arid climate at the latitude of southern New South Wales (https://en.wikipedia.org/wiki/New_South_Wales), but differs radically from mallee (https://www.anbg.gov.au/photo/vegetation/mallee-woodlands-shrublands.html and https://en.wikipedia.org/wiki/Mallee_(habit)) in being fire-proof.

The largest non-venomous snakes on all three southern continents are restricted to fire-free vegetation. However, a difference remains in the availability of prey such as the fast-growing hares (https://www.inaturalist.org/observations?place_id=113055&taxon_id=43094&view=species), pigs (https://www.inaturalist.org/observations?place_id=113055&taxon_id=42118&view=species), and antelopes (https://www.inaturalist.org/observations?place_id=113055&taxon_id=42241&view=species) of Africa.

Eunectes murinus (https://en.wikipedia.org/wiki/Green_anaconda) also eats prey with no counterparts in Australia, such as giant rodents (https://en.wikipedia.org/wiki/Capybara), and depends on floating meadows that are far more extensive than any similar vegetation on Australian floodplains.

Where does this leave the extinct giant varanid, Megalania prisca (https://prehistoric-fauna.com/Varanus-priscus)?

In a book devoted to this species, 'Dragons in the dust' (https://catalogue.nla.gov.au/catalog/3095349), R E Molnar explains that only a few bones of this species have ever been found, so that its full skeleton can be reconstructed only by guesswork. Past attempts to do so have - perhaps falsely - used the extant Varanus komodoensis as a model.

I have no reason to doubt that mature males may have reached 6 m. However, estimated body masses are another matter. If the extinct giant varanid was slender, in line with other Australian reptiles, its body mass might not have exceeded 250 kg - perhaps inferior to the largest extinct species of Varanus from the fossil beds of the Siwalik Hills (https://en.wikipedia.org/wiki/Sivalik_Hills) in mainland Asia.

Please bear in mind the surviving members of the same family:
.Varanus komodoensis and the largest congener at similar latitudes in New Guinea, to the east, both reach 3 m long, but differ perhaps 6-fold in maximum body mass.

Bulk counts, which is why Varanus salvadorii - although it may turn out to be the longest lizard alive on Earth today (https://www.researchgate.net/figure/Varanus-salvadorii-is-the-largest-monitor-lizard-of-New-Guinea-and-the-only_fig13_236013907) - has yet to gain a reputation as a dragon.

Publicado el 08 de octubre de 2023 a las 10:57 PM por milewski