Aposematic colouration in snakes

Various snakes show extreme tones inside the mouth in defensive display, e.g.

This is one of the least ambivalent aspects of aposematic colouration in snakes. However, it

  • is only a minor feature of the body as a whole,
  • occurs in only a few taxa of snakes, and
  • does not provoke deep theoretical discussion, other than to comment on how odd it is that Trachydosaurus and Tiliqua also use this tactic, despite lacking venom.

How many harmless spp. of snakes open the mouth to display striking colours in self-defence? Which hues are used, and which are the predators targeted?

Various snakes rattle the tail as part of defensive display, and some display colouration while doing so.

Whereas an open mouth clearly warns of venom (or just bite), warning colouration on the tail is ambiguous, because it could refer to the mouth (particularly where the posture of threat display brings the tail close to the mouth, as in certain Crotalus, https://www.outsideonline.com/outdoor-adventure/exploration-survival/snake-bite-dog-rattlesnake-avoidance-training/), or it could warn of the danger from the cloacal gland (and faeces).

Tail-rattling and hissing/puffing are analogous 'aposematic' sounds, produced by opposite ends of the body. Therefore, it seems logical enough that both can be accentuated by aposematic colouration.

However, the tail has the complication that the snake could be trying to distract the predator away from the head, not to threaten the predator with its hind end. If so, the term 'aposematic' seems inappropriate. Calabaria reinhardtii (https://www.inaturalist.org/taxa/32123-Calabaria-reinhardtii) hides its head in a ball while displaying the tail, with dark-pale contrast on the tail (https://www.inaturalist.org/observations/136487375 and http://www.kingsnake.com/sandboa/calball2.jpg and http://www.kingsnake.com/sandboa/cal_ball.jpg).

So, unless this species releases cloacal defence, it would seem to be a case of non-aposematic colouration, for distraction, or perhaps startling.

Some snakes seem to have an aposematically-coloured tongue, e.g. pink and black in Thelotornis (https://www.inaturalist.org/observations?place_id=any&taxon_id=28332&view=species and https://www.inaturalist.org/observations/87692641 and https://www.inaturalist.org/observations/142618345).

I see this as only a minor aspect of aposematism in snakes. However, it is significant because it announces the animal to be a snake. Apart from snakes, the only herps with forked tongues are varanid lizards.

It is possible that the relatively few types of snake that are whole-body aposematic (mainly dark and pale banded, as epitomised by Bungarus spp., https://www.inaturalist.org/observations?place_id=any&taxon_id=30436&view=species) are uncomplicated, announcing their venomousness much as a skunk announces its noxiousness.

However, what the literature on snakes does not seem to have considered are the possibilities that

  • whole-body aposematism is more about cloacal secretion than venom, or
  • banded snakes may be noxious/poisonous in the sense of their flesh being poisonous for predators to eat, regardless of the question of venom in the head.

This might perhaps help to explain why most of the 'coral snakes' are both reluctant to display their heads and unlikely to bite effectively, having small mouths.

Micrurus (https://www.inaturalist.org/observations?place_id=any&taxon_id=30493&view=species) displays its tail (https://www.canstockphoto.com/eastern-ribbon-coral-snake-micrurus-2377871.html and https://www.reptilesofecuador.com/micrurus_melanotus.html and https://animalia.bio/micrurus-mipartitus and https://www.reptilesofecuador.com/micrurus_scutiventris.html and https://www.alamy.com/andean-redtail-coral-snake-micrurus-mipartitus-esmeraldas-province-western-ecuador-image432368979.html?imageid=B0195DA4-6EFE-4F94-B9F1-1E07317E38A4&p=54509&pn=1&searchId=b5bd9b6231731e6c7664badf55235e49&searchtype=0), and this may be to warn of cloacal secretions, or possibly to allow a predator to taste it without injuring the head ().

Is so, then snakes may be partly analogous to poison frogs (https://www.agefotostock.com/age/en/details-photo/orange-and-black-poison-dart-frog-golfodulcean-poison-frog-golfodulcean-poison-frog-phyllobates-vittatus-morph-with-red-stripe-cutout-costa-rica/BWI-BS445984) rather than Black widow spiders (https://www.smithsonianmag.com/science-nature/eight-fun-facts-about-black-widows-180978098/), in the function of aposematic colouration.

To complicate the matter, of course the cloacal sacs would add to the unpalatability/indigestibility of the whole snake.

It is too easy to jump to the conclusion that if a snake has warning colours or a display of its tail, it is the venom (or pretension to venom in the case of bluffing/mimicking snakes) that is being advertised.

Publicado el sábado, 27 de mayo de 2023 a las 11:00 AM por milewski milewski


Agkistrodon contortrix (https://www.inaturalist.org/taxa/912622-Agkistrodon-contortrix) is extremely well-camouflaged (https://www.inaturalist.org/observations/133349580), and, although venomous, has short fangs by viperid standards.

Its anti-predator warning display is hardy visual, apart from the slight paleness of the mouth (https://www.inaturalist.org/observations/153945051 and https://www.inaturalist.org/observations/135461253 and https://www.inaturalist.org/observations/119053760). Instead, it vibrates its tail (which lacks a rattle) without flaunting the tail.

If handled, it defends itself by means of cloacal discharge, which has a cucumber-like odour.

In juveniles, the tail is yellow-tipped (https://www.inaturalist.org/observations/141425644 and https://www.inaturalist.org/observations/118799561 and https://www.inaturalist.org/observations/164488722 and https://www.inaturalist.org/observations/154264067 and https://www.inaturalist.org/observations/155189970 and https://www.inaturalist.org/observations/149646493), and used as a lure for prey.

Anotado por milewski hace 4 meses

Lampropeltis triangulum (https://www.inaturalist.org/taxa/515419-Lampropeltis-triangulum) is regarded as a textbook-case of mimicry in snakes (https://en.wikipedia.org/wiki/Emsleyan_mimicry).

However, this is only part of the picture.

Consider that L. triangulum, in addition to being (in some forms) banded similarly to venomous Micrurus, also:

a) sprays 'musk', at least when handled by humans,
b) is a mimic of other, widely different, snakes, including Pantheropus guttatus (https://www.inaturalist.org/taxa/73887-Pantherophis-guttatus) and Lampropeltis elapsoides (in the case of subspecies L. t. amaura, L. t. annulata, L. t. gentilis, and L. t. syspila),
c) may eat its 'models', because part of its diet is other snakes, including e.g. venomous rattlesnakes, and
d) has a belligerent defensive display, including not only hissing and striking with the mouth, but also vibrating the tail.

Also odd is that L. triangulum bites slowly, not actually striking when it bites, but instead hanging on with the teeth (perhaps so that musk penetrates the lacerations made when the head is pulled off the bite?).

In reality, L. trangulum is extremely difficult to identify, as opposed to being 'identified with' Micrurus. The colloquial names 'red adder' and house mocassin' are worth noting.

The fact that L. triangulum - like skunks - sprays noxious substances suggests that this species may, in part, be aposematic in its own right, as opposed to merely a mimic.

Anotado por milewski hace 4 meses

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