Unexpected diversity in socially synchronized rhythms of shorebirds

Socially synchronized rhythms in shorebirds were assessed during biparental incubation under natural circumstances and were exceptionally diverse, often not following the 24-h day, whereby risk of predation, not starvation, determined some of the variation in incubation rhythms. All organisms have biorhythms, but in social species these have to be synchronized between individuals within a community. Here Martin Bulla et al. address the issue of how parents synchronize their biorhythms when both are caring for their offspring. Using data from 729 nests of 91 populations of 32 species of shorebirds in which parents synchronize their schedules to achieve continuous incubation of the eggs, they show that even under similar environmental conditions and despite day-long environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of captive birds. The risk of predation, not starvation, might be a key determinant of biorhythmic diversity. The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment1,2,3,4. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions1,5, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators6,7,8,9,10. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring)6,7,8,9,11. The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood5,6,7,9. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization12 where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1–19 h, whereas period length—the time in which a parent’s probability to incubate cycles once between its highest and lowest value—varied from 6–43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light–dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity5,6,7,9. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

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