Tolliver, J. D. M. | 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↗

bioRxiv2022

Fitness benefits from co-display favour subdominant male-male partnerships between phenotypes

Male-male competition over matings is a key driving force in the evolution of courtship. Typically, competition is an individual affair selecting for dominance and aggression. Yet, some males forgo direct confrontation and improve their reproductive success through cooperation. Occasionally, this leads to specialized alternative reproductive tactics that operate at the intersection of cooperation and conflict. We used a community game model, informed with empirical data derived from previous studies, to examine cooperation dynamics between lekking male ruffs (Calidris pugnax) using two different tactics: resident and satellite. Residents defend display courts against other residents on leks. In contrast, satellites forgo court defence and engage in cooperative co-display with selected residents. Co-displaying appears to alter female mate choice, yet the exact mechanism and consequences remain unclear. We modelled individual male mating success as a function of lek size, resident rank, and satellite competitiveness. Our most realistic model assumed that co-display draws copulations from residents proportional to the existing mating skew among them. Under this assumption, all residents benefit from co-display over single display when a satellite is on the lek, except for -residents co-displaying with the most competitive satellites on large leks. Thus, satellites could nearly always choose their preferred co-display partner, but achieved the highest copulation rates with lower ranking (subdominant) residents on intermediate sized leks. Co-display between the satellite and lower ranking residents reduced the mating skew among residents. However, since copulations for satellites were similar over a range of potential co-display partners, a variety of co-displaying dyads is to be expected, which is consistent with observations in nature. We conclude that, given our model assumptions, co-displaying reduces the impact of male dominance on reproductive success and ultimately alters the course of sexual selection.

evolutionary biology↗