Rohr-Bender, V. A. | 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↗

bioRxiv2024

Sex and morph variation in activity from early ontogeny to maturity in ruffs (Calidris pugnax)

Intraspecific variation provides the substrate for the evolution of organisms. Ruffs show exceptional phenotypic variation in physiology, appearance and behaviour linked to variation between sexes and male alternative reproductive tactics (ARTs). The male ARTs are associated with the evolution of separate morphs, which are encoded by an autosomal supergene. However, the effects of the supergene on females and chicks are much less well understood. In particular, it is still unknown, whether females also show morph-specific behavioural variation, when behavioural differences emerge during ontogeny, and whether behavioural differences can be detected outside of the breeding context. To address these knowledge gaps, we repeatedly measured the activity in an unfamiliar environment, also known as exploration behaviour, of 109 hand-raised young ruffs throughout their first two years of life. We used automated tracking in an open field arena, and quantified the distance moved within 10 minutes to examine behavioural differences between sexes, morphs and individuals. The activity of young ruffs rapidly increased during the first month after their crouching reflex, a response to potential threats, subsided. Repeatability of individual activity was initially low but increased throughout juvenile ontogeny and was high (R = 0.5) from day 21 onwards. Variation in activity was clearly sex-linked with females moving more than males, indicating potential energetic trade-offs accompanying the strong sexual size dimorphism. In contrast, morph differences in activity remained inconsistent and elusive, both in females and in males. Our results indicate that in species where much of the known behavioural variation is linked to mating tactics, a non-reproductive behaviour can show between-individual variation and clear sex differences, whereas morph differences appear less pronounced.

animal behavior and cognition↗