New Guinea has quite a few venomous, really dangerous snakes. Death adders (Acanthophis), taipan (Oxyuranus scutellatus), or Papuan blacksnake (Pseudechis papuanus) are elapid snakes that have caused many human fatalities on the island. But the one thing that New Guinea does not have is coral snakes. The only venomous snake that looks like one is a hatchling of the small-eyed snake (Micropechis ikaheka), whose posterior half has alternating white and dark rings. It is therefore somewhat surprising that young ringed pythons (Bothrochilus boa), non-venomous and essentially harmless reptiles (unless you are a mouse, that is) advertise their presence with vivid, orange and black coloration. Their entire body has a shiny gloss to it, and it is impossible not to notice this animal as it glides through the leaf litter and low branches.
I puzzled about this seemingly counterintuitive behavior – why advertise yourself if you have nothing to back it up with, and local predators have never been exposed to venomous snakes of such a coloration? Is it still Batesian mimicry if there is no harmful model? Or perhaps the snake is colorful simply for the sake of being pretty (rather unlikely.) But then, one day while looking for katydids hiding among the leaves of Pandanus plants, I stumbled upon a giant walking stick, Megacrania nigrosulfurea. I was immediately struck by how similar its coloration was to that of a young ringed python that I had photographed a few days earlier: shiny black body, with bright orange legs, plus bright yellow spots. But why would a snake mimic a walking stick?
As it happens, many walking sticks, including the genus Megalocrania, are capable of synthesizing de novo noxious, repellant chemicals (other insects usually sequester such compounds from their food plants), which they then spray at the attackers with surprising accuracy. These compounds, mostly monoterpenes (e.g., iridodial, nepetalactone, anisomorphal), quinoline, and pyrazines, act as powerful deterrents to a wide range of predators, from ants to lizards to birds and rodents (opossums, however, seem to relish toxic walking sticks.) For the young python a walking stick that is avoided by all the same predators that like to munch on small snakes could be a pretty good model to imitate. And a few days later I also found in the same rainforest habitat a burying beetle (Nicrophorus heurni), which showed a similar combination of shiny black and orange coloration. Like the aposematically colored walking sticks, these insects are avoided by many predators thanks to their ability to spray the attacker with some of the stinkiest substances in nature, including phenol, indole and, most importantly, skatole (you can guess what the last one smells like.)
A single, casual observation obviously does not constitute a proof that ringed pythons mimic insects. But this would not be the first example of a vertebrate mimicking an aposematic insect: in southern Africa a small lizard Heliobolus lugubris mimics black and white Anthia ground beetles, known to have powerful chemical defenses. And if a caterpillar can mimic a snake, I don’t see why a snake could not mimic an insect.
Update: The name of the walking stick was misspelled and now has been corrected. Thanks to David Rentz for the tip.