Calling by Concluding Sentinels: Coordinating Cooperation or Revealing Risk?

Efficient cooperation requires effective coordination of individual contributions to the cooperative behaviour. Most social birds and mammals involved in cooperation produce a range of vocalisations, which may be important in regulating both individual contributions and the combined group effort. Here we investigate the role of a specific call in regulating cooperative sentinel behaviour in pied babblers (Turdoides bicolor). ‘Fast-rate chuck’ calls are often given by sentinels as they finish guard bouts and may potentially coordinate the rotation of individuals as sentinels, minimising time without a sentinel, or may signal the presence or absence of predators, regulating the onset of the subsequent sentinel bout. We ask (i) when fast-rate chuck calls are given and (ii) what effect they have on the interval between sentinel bouts. Contrary to expectation, we find little evidence that these calls are involved in regulating the pied babbler sentinel system: observations revealed that their utterance is influenced only marginally by wind conditions and not at all by habitat, while observations and experimental playback showed that the giving of these calls has no effect on inter-bout interval. We conclude that pied babblers do not seem to call at the end of a sentinel bout to maximise the efficiency of this cooperative act, but may use vocalisations at this stage to influence more individually driven behaviours

The higher the better: sentinel height influences foraging success in a social bird

In all social species, information relevant to survival and reproduction can be obtained in two main ways: through personal interaction with the environment (i.e. ‘personal’ information) and from the performance of others (i.e. ‘public’ information). While public information is less costly to obtain than personal information, it may be inappropriate or inaccurate. When deciding how much to rely on public information, individuals should therefore assess its potential quality, but this possibility requires empirical testing in animals. Here, we use the sentinel system of cooperatively breeding pied babblers (Turdoides bicolor) to investigate how behavioural decisions of foragers are influenced by potential variation in the quality of anti-predator information from a vigilant groupmate. When sentinels moved to a higher position, from where their probability of detecting predators is likely to be greater, foragers reduced their vigilance, spread out more widely and were more likely to venture into the open. Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a profound increase in biomass intake rate. The opposite behavioural changes, and consequent foraging outcomes, were found when sentinels moved lower. A playback experiment demonstrated that foragers can use vocal cues alone to assess sentinel height. This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information, and our results emphasize that a full understanding of the evolution of communication in complex societies requires consideration of the reliability of information

The frequency with which fast-rate chuck calls were given by different classes of sentinels.

<p>Dominant females = DF, dominant males = DM, subordinate females = SF, and subordinate males = SM. Within individual status, there was no difference between sexes (DF-DM, p = 0.12; SF-SM, p = 0.20). Within individual sex, there was a significant effect of dominance for females (DF-SF, p = 0.027, effect size = 0.207) but not males (DM-SM, p = 0.94) – dominant females produced fewer fast-rate chuck calls than subordinate females. Analysed using the package “languageR” and the function ‘pvals.fnc’ (uses Markov Chain Monte Carlo simulations; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025010#pone.0025010-Baayen1" target="_blank">[37]</a>).</p

The frequency with which fast-rate chuck calls were given in conditions with high and low wind.

<p>The production of fast-rate chuck calls was statistically more frequent in high wind than in low wind.</p

The effect of fast-rate chuck calling on bout latency.

<p>(A) Observational data showing the effect of fast-rate chuck calling vs. no calling on the latency between consecutive sentinel bouts. Shown are means (±SE) for each of the eight groups. (B) Playback results showing the effect of fast-rate chuck calls vs. the sound of wingbeats on bout latency. The three letter codes in the legend represent individual group names.</p

Illustrative spectrograms of the two different playback treatments.

<p>In one treatment (A), sentinel calls (the watchman's song) were followed by the fast-rate chuck call; in the other treatment (B), sentinel calls were followed by the sound of wingbeats from a descending sentinel. Spectrograms only show the last section of each playback treatment and not the preceding 2 min of sentinel calling (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025010#s2" target="_blank">Methods</a>).</p

Ontogenetic changes in alarm-call production and usage in meerkats (Suricata suricatta): adaptations or constraints?

In many species, individuals suffer major mortality in their first year because of predation. Behaviours that facilitate successful escape are therefore under strong selection, but anti-predator skills often emerge gradually during an individual’s early development. Using long-term data and acoustic recordings of alarm calls collected during natural predator encounters, we aimed to elucidate two largely unsolved issues in anti-predator ontogeny: (i) whether incorrect predator assignment is adaptively age-appropriate, given that vulnerability often change during development, or whether age-related differences reflect true mistakes made by immature individuals; and (ii) the extent to which the development of adult-like competence in alarm-call production and usage is simply a function of maturational processes or dependent upon experience. We found that young meerkats (Suricata suricatta) were less likely to give alarm calls than were adults, but alarmed more in response t o non-threatening species compared to adults. However, stimuli that pose a greater threat to young than adults did not elicit more calling from young; this argues against age-related changes in vulnerability as the sole explanation for developmental changes in calling. Young in small groups, who were more likely to watch out for predators, alarmed more than less vigilant young in larger groups. Moreover, despite similarities in acoustic structure between alarm call types, calls appeared in the repertoire at different rates and those that were associated with frequently encountered predators were produced relatively early on. These results indicate that experience is a more plausible explanation for such developmental trajectories than is maturation