Three extra hints of venom in monitor lizards

(writing in progress)

Beaded lizards (Superfamily Varanoidea: family Helodermatidae: Heloderma suspectum and H. horridum) are the only unquestionably venomous lizards on Earth.

One species, the gila monster (Heloderma suspectum), has whole-body colouration conspicuous enough to warn potential predators of its hidden weapon. No other species of lizard worldwide is known to possess warning colouration (i.e. aposematism) based on a genuine defensive capability, although several species4 mimic venomous organisms and thus qualify as falsely aposematic. 4 E.g. juveniles of a lacertid lizard (Eremias lugubris) in southern Africa are harmless but possess conspicuous colouration similar to that of a venomous carabid beetle (Anthia) common in the same environments.

Although some studies have claimed that certain species of monitor lizards (Superfamily Varanoidea: family Varanidae, closely related to Helodermatidae) are mildly venomous, warning colouration is unlikely to be discovered in any such lizard.

6 The gila monster (Heloderma suspectum) has a specialised diet of helpless juveniles of birds and rodents, for which no venom is needed.

This is partly because of a difference between beaded lizards and monitor lizards. In the former, the main function of venom is self-defence6 from predators (e.g. the coyote (Canis latrans)) of the beaded lizards, whereas in the latter the main function of venom – if it truly exists – is to incapacitate the prey of the monitor lizards.
 
Among the nearly 80 species of monitor lizards, there are two species with conspicuous markings that deserve investigation as possible cases of warning colouration.

Among mangroves in southeast Asia occurs Dumeril’s monitor 8 Varanus dumerilii: hatchlings up to 4-8 weeks old
, the hatchlings of which are ominously coloured with an orange head and a white-banded black body – a pattern quite different from adults’.

Among eucalypts in eastern Australia occurs the lace monitor (Varanus varius), a colour-polymorphic species in which two colour-morphs seem to conform to warning colouration, as follows.

Firstly, one uncommon and geographically restricted colour-morph (colloquially called ‘Bell’s lace monitor’) has a pied pattern all over its body, which at the hatchling stage is too conspicuous 11 The hatchling of ‘Bell’s lace monitor’ is the only example worldwide of a pied lizard. Pied whole-body colouration is common among birds but extremely rare among lizards.

The only other example known to us is a variant of the leopard gecko (Eublepharis macularius), the world’s only domesticated reptile, produced by selective breeding in captivity.

to function mainly for camouflage (i.e. disruptive colouration).

Secondly, adults of the most common colour-morph retain conspicuous dark and pale banding on the mouth which is incongruously conspicuous and occurs in no other species of monitor lizard. 13 The pattern on the rest of the body of this colour-morph of Varanus varius is unremarkable, retaining its function in camouflage.

The production of venom by the lace monitor remains controversial and Dumeril’s monitor has never been investigated for venom.

However, both species would be worth testing for venomousness because their diets indicate that any venom produced would function mainly for defence, not foraging. As adults, Dumeril’s monitor eats mainly invertebrates while the lace monitor eats mainly nestling birds and carrion.

Since whole-body colouration is most conspicuous in the hatchlings of both species – which rely on invertebrates for food – it’s unlikely that any venom produced is needed to subdue the prey of the hatchlings. For these reasons, there remains a possibility of genuine warning colouration in monitor lizards, and confirmation is needed as to whether venom is present in hatchlings of Dumeril’s monitor, and all ages of the lace monitor.

(writing in progress)

Posted on June 9, 2022 02:37 PM by milewski