Syntax manipulation changes perception of mobbing call sequences across passerine species

Many species approach predators to harass them and drive them away. Both the intensity of this antipredator strategy and its success are positively related to the size of the group that carries out this mobbing. To recruit individuals to the mob, members of prey species produce mobbing calls. In some songbirds—the Japanese tit, Parus minor, and the southern pied babbler, Turdoides bicolor—mobbing calls are structurally complex and it has been suggested that they convey information by means of compositional syntax, when meaningful items are combined into larger units. These two species combine alert and recruitment calls into an alert and recruitment sequence when attracting conspecifics to cooperate in mobbing a predator. Whether this rudimentary, two‐call, compositional structure is used by other bird species in mobbing calls and how it can alter the ability of heterospecifics to adequately recognize mobbing calls is not well understood. Heterospecifics’ responses to mobs are critical to the success of the mobbing strategy, so it is of great importance to understand whether and how syntax influences these responses. To address these questions, we conducted two playback experiments. Firstly, we investigated whether the great tit, Parus major, extracts different meanings from different individual motifs (i.e., component calls), and from combined motifs in both natural and artificially reversed order. We found that great tits extract different meanings from the two motifs involved in mobbing calls and that they also discriminate for motif order reversal in the mobbing call sequence. Secondly, we investigated whether heterospecifics (the coal tit, Periparus ater, and the common chaffinch, Fringilla coelebs) are sensitive to syntax alteration of great tit mobbing calls. While chaffinches did not respond to great tit mobbing calls, coal tits were sensitive to mobbing call sequence reversal although they did not react in the same way as conspecific subjects. Overall, whereas our results indicate that tits are sensitive to call reversal, this is not to say that tits actually use compositional syntax to increase the information content.     

Male anti-predation services in primates as costly signalling? A comparative analysis and review

In species in which adults of the two sexes show long-term association, males often engage in acts of assistance (“services”) aimed at females and/or their young in various behavioural contexts that are not reciprocated. We conducted a quantitative review of the primate literature on sex differences in involvement in protection against predation. We found that males were more likely to be at the front of group progressions, especially in contexts of increased risk, were more vigilant, sometimes to the point of becoming sentinels, were more active in mobbing, and were far more likely to counter-attack felids and monkey-eating raptors than females did. Our evaluation showed that these services may be an expression of male parental care or aimed at bonded partners, but we also encountered many cases in which non-sires, unrelated to either females or immatures, provide them. We hence investigate explanations invoking group augmentation or costly signalling. Currently, available data on male anti-predation services in nonhuman primates do not allow us to distinguish among them, although costly signalling better explains data on birds and humans. We develop predictions that will allow more detailed tests of the various hypotheses for this understudied phenomenon.     

Long-term familiarity promotes joining in neighbour nest defence

Familiarity plays an important role in the evolution of sociality and cooperation. Familiar individuals may gain a reputation for participating in, or defecting from, cooperative tasks. Previous research suggests that long-term familiarity with territorial neighbours benefits breeders. We tested the hypothesis that great tits (Parus major) are more likely to join in neighbours' nest defence if those neighbours are familiar from the previous year. We show that neighbours that shared a territory boundary the previous year are more likely to join their neighbours' nest defence than neighbours that did not share a boundary before. Closer neighbours did not differ from distant neighbours in their latency to join. For familiar neighbours that joined, there was no difference in call rate in relation to whether one or both members of the focal pair were familiar. First-time breeders (by definition unfamiliar) did not join each other's nest defence. This is the first evidence of a relationship between familiarity and joining in nest defence. Such direct benefits of familiarity may have important implications in the evolution of sociality.     

Mutualism,reciprocity, or kin selection? Cooperative rescue of a conspecific from a boa in a nocturnal solitary forager the gray mouse lemur

Predator mobbing is a widespread phenomenon in many taxa but the evolution of cooperative mobbing as an adaptive behavior is still subject to debate. Here, we report evidence for cooperative predator defense in a nocturnal solitarily foraging primate, the gray mouse lemur (Microcebus murinus). Several mouse lemurs mobbed a snake that held a non-related male conspecific until he could escape. Evolutionary hypotheses to explain cooperative mobbing include (1) by-product mutualism, when individuals defend others in the process of defending themselves; (2) reciprocity, where animals achieve a higher fitness when helping each other than when they do not cooperate; and (3) kin selection where animals help each other only if they share genes by common descent. Owing to the solitary activity of this species, reciprocity seems to be least likely to explain our observations. By-product mutualism cannot be ruled out entirely but, if costs of snake mobbing are relatively low, the available detailed socio-genetic information indicates that kin selection, rather than any of the other proposed mechanisms, is the primary evolutionary force behind the observed cooperative rescue.     

Mobbing behaviour in a passerine community increases with prevalence in predator diet

Mobbing behaviour against predators is well documented but less is known about the factors influencing variation in behavioural response between prey species. We conducted a series of playback experiments to examine how the mobbing responses of prey species differed according to their relative risk of predation by the Eurasian Pygmy Owl Glaucidium passerinum, a predator of passerines. We found that mobbing among 22 passerine prey species was positively correlated with their prevalence in the Pygmy Owl diet. To compare mobbing behaviour between two seasons, we conducted playback experiments during spring (breeding season) and autumn (non‐breeding season). Contrary to previous studies, we found that mobbing intensity was greater during autumn than in spring. Our study shows a differential mobbing response of 22 species to the calls from one predator species and underscores the importance of considering seasonal variation in mobbing behaviour. Mobbing response differences observed among bird species strongly suggest different cooperation behaviour at the community level.     

Two aberrant serpent-eagles may be visual mimics of bird-eating raptors

Two independent cases of visual mimicry involving snake-eating eagles as mimics and more powerful bird-eating hawks as models are proposed. One pair of species is formed by the sympatric West African Serpent-eagle Dryotriorchis spectabilis and Cassin's Hawk Eagle Spizaetus africanus (= Aquila africana ), inhabiting the tropical forests of the African Gulf of Guinea. The second case involves the Madagascar Serpent-eagle Eutriorchis astur and the Madagascar Goshawk Accipiter henstii , both sympatric and endemics of forests in Madagascar. Similarity in plumage colour and pattern as well as in body size and proportions are remarkable in both cases. The species mimicry pairs are in both cases not closely related phylogenetically and greatly differ in diet, making it unlikely that common ancestry or shared foraging strategies explain the resemblance in plumage coloration and pattern. Mimicry may have evolved because the mimic serpent-eagles obtain (the following hypotheses are not mutually exclusive): (1) a foraging advantage by deceiving their snake prey as they may not flee from bird-eating raptors such as the models, (2) a lowered predation or harassment by the models or other predators, and/or (3) reduced mobbing by small birds, which tend to avoid bird-eating raptors. Alternatively, the similarities in plumage described here may be the result of random convergence due to constraints in the evolution of plumage colours and patterns in diurnal raptors.