Koivula, K. | bioRxiv Science

bioRxiv2025

Sex differences in development alter the fledgling sex ratio in a lekking bird with strong sexual size dimorphism

Sex ratio variation is of fundamental importance for population ecology, and the evolution of sex roles and life-histories. Yet, the ecological mechanisms underlying such variation remain often unknown. Using a multiyear dataset from captive and wild precocial Ruffs Calidris pugnax, we show that poorer survival and later fledging of males relative to females result in a female biased sex ratio of the fledgling population. Both sex-specific survival and maturation time contributed equally to an increasing sex ratio bias from hatching to fledging in this sexually dimorphic bird. On average, juvenile females fledged three days earlier than males due to expedited wing development, despite the higher absolute rate of wing growth in males. We argue that sex differences in growth, maturation, and offspring survival are likely the result of strong pre-sexual selection on males to achieve dominance and may help to enforce the Darwinian sex roles observed in this species.

ecology↗

bioRxiv2016

Unexpected diversity in socially synchronized rhythms of shorebirds

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment1-4. 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 predators6-10. Individuals can temporally segregate their daily activities (e.g. prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (e.g. group foraging, communal defence, pairs reproducing or caring for offspring)6-9,11. The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood5-7,9. Here, we address this 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 parents incubation bout varied from 1 - 19 hours, while period length-the time in which a parents probability to incubate cycles once between its highest and lowest value - varied from 6 - 43 hours. 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 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 captivity5-7,9. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

animal behavior and cognition↗