Age-specific variation in reproductive effort can affect population dynamics, and is a key component of the evolution of reproductive tactics. Late-life declines are a typical feature of variation in reproduction. However, the cause of these declines, and thus their implications for the evolution of life-history tactics, may differ. Some prior studies have shown late-life reproductive declines to be tied to chronological age, whereas other studies have found declines associated with terminal reproduction irrespective of chronological age. We investigated the extent to which declines in late life reproduction are related to chronological age, terminal reproductive attempt or a combination of both in the Thorn-tailed Rayadito (Aphrastura spinicauda), a small passerine bird that inhabits the temperate forest of South America. To this end we used long-term data (10 years) obtained on reproductive success (laying date, clutch size and nestling weight) of females in a Chilean population. Neither chronological age nor terminal reproductive attempt explained variation in clutch size or nestling weight, however we observed that during the terminal reproductive attempt older females tended to lay later in the breeding season and younger females laid early in the breeding season, but this was not the case when the reproductive attempt was not the last. These results suggests that both age-dependent and age-independent effects influence reproductive output and therefore that the combined effects of age and physiological condition may be more relevant than previously thought.

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Data used in Hammers et al. Breeders the receive help age more slowly in a cooperatively breeding bird. Nature Communications.

Data used in Hammers et al. Breeders the receive help age more slowly in a cooperatively breeding bird. Nature Communications.

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Species showing color polymorphisms—the presence of two or more genetically determined color morphs within a single population—are excellent systems for studying the selective forces driving the maintenance of genetic diversity. Despite a shortage of empirical evidence, it is often suggested that negative frequency-dependent mate preference by males (or diet choice by predators) results in fitness benefits for the rare female morph (or prey type). Moreover, most studies have focused on the male (or predator) behavior in these systems and largely overlooked the importance of female (or prey) resistance behavior. Here, we provide the first explicit test of the role of frequency-dependent and frequency-independent intersexual interactions in female polymorphic damselflies. We identify the stage of the mating sequence when frequency-dependent selection is likely to act by comparing indexes of male mate preference when the female has little (females presented on sticks), moderate (females in cages), and high (females free to fly in the field) ability to avoid male mating attempts. Frequency-dependent male preferences were found only in those experiments where females had little ability to resist male harassment, indicating that premating interactions most likely drive negative frequency-dependent selection in this system. In addition, by separating frequency-dependent male mating preference from the baseline frequency-independent component, we reconcile the seemingly contradictory results of previous studies and highlight the roles of both forms of selection in maintaining the polymorphism at a given equilibrium. We conclude that considering interactions among all players—here, males and females—is crucial to fully understanding the mechanisms underlying the maintenance of genetic polymorphisms in the wild.