Aposematism and crypsis in a rodent: antipredator defence of the Norwegian lemming

Abstract

Aposematism is unusual in herbivorous mammals, and exceptions help clarify its ecology and evolution. The Norwegian lemming differs from other rodents in colouration and behaviour. One hypothesis is that its black, yellow and white colours, loud calls and ferocious defence reduce predation by conspicuous aposematism. Another hypothesis is that the colouration is cryptic. These alternatives are tested in a detectability experiment comparing lemmings and sympatric grey-sided voles. All 18 observers detected a higher proportion of the lemmings, corroborating conspicuousness. Unlike smaller rodents, Norwegian lemmings often call from a distance at predators. The aposematism hypothesis predicts that cryptically coloured Alaskan brown lemmings will not call. In the field, Norwegian lemmings gave antipredator calls at a human observer in 36 of 110 encounters, but brown lemmings did so in only 1 of 39 cases. Most Norwegian lemmings called if surprised within a few metres but froze or fled silently farther away, relying on crypsis against distant predators. Small juveniles called as often as adults, a possible case of automimicry. In an earlier experiment, Norwegian lemmings, in contrast with grey-sided voles, aggressively resisted attacks by a major avian predator of rodents. The tests corroborate the hypotheses that (1) distinctive, contrast-rich colouration, antipredator calls and threat postures of the Norwegian lemming form a multimodal suit of aposematic traits, warning predators that this is a more dangerous prey than the smaller sympatric voles, and (2) discriminability from undefended species is an important adaptive reason for conspicuous distinctness of many aposematic signals.

Venom in Furs: Facial Masks as Aposematic Signals in a Venomous Mammal

“…Although wounds were distributed across age classes, juvenile animals were the most aggressive, fiercely attacking measurers when handled, and often growling or even shrieking. Such behaviour is also exhibited by juvenile Norwegian lemmings Lemmus lemmus , that lack the attack power of an adult, but especially use loud calls as a form of mimicry [ 43 ]. Taitt [ 44 ] suggested that Norwegian lemmings use a strategy of crypsis when resident and aposematism when migrating, being both noisier and more aggressive during migration.…”

Biased generalization of salient traits drives the evolution of warning signals

“…Generalization bias towards orange/ red thus appears as a reasonable driver of this warning signal evolution. The evolution of aposematism in Epipedobates poison frogs [39], as well as in red salamanders [40] and in Norwegian lemmings from an appearance like Siberian brown lemmings [41], also seem to fit this scheme of exaggeration of previously existing characteristics. Thus, aspects of brownish -greenish dorsal colour, disruptive markings or countershading, originally serving as camouflage, could be used by predators to discriminate unprofitable from profitable prey and then evolve to warning signals through salience-driven generalization bias.…”

Aposematic signaling and seasonal variation in dorsal pelage in a venomous mammal

“…As a loris begins to disperse, they mature, and their defensive capabilities increase, making them less vulnerable to predation or attack by a conspecific, but their higher levels of aggression may persist. Automimicry of this kind has been observed in other taxa, including the Norwegian lemming ( Lemmus lemmus ), the juveniles of which emit loud warning calls at the same frequency as an adult despite not possessing the same physical power to attack (Andersson, 2015 ). Similarly, dendrobatid poison frog ( Oophaga pumilio ) juveniles display the same aposematic colouration as the adults despite lacking their chemical potency (Murray et al., 2016 ).…”