Male “Mixed” Reproductive Strategies in Biparental Species: Trivers Was Probably Right, but Why?

Parental behavior correlates to baseline corticosterone of mates and offspring in nesting eastern bluebirds (Sialia sialis)

Raising nestlings in a biparental species involves a complex and dynamic interaction of physiology and behavior among a group of organisms. Parents may be predicted to vary their behaviors based not only upon their own state, but also in relation to the states of both offspring and the other parent. In this study we explore the relationships between parental feeding behaviors and family member condition in eastern bluebirds, with a special emphasis on baseline corticosterone, a hormone associated with energy mediation and stress. We found that the overall number of feeding trips made by both male and female parents were positively correlated to the corticosterone levels of nestlings. Maternal, but not paternal, baseline corticosterone levels were positively correlated to nestling baseline corticosterone levels. Additionally, adult males' feeding behavior was positively correlated to adult females' baseline corticosterone levels. These findings suggest a complex interplay between parental behavior and physiological state not only within a given organism, but also across organisms operating within a family unit. In addition, these results provide evidence that paternal and maternal efforts are influenced by related but distinct pressures, and that male and female parenting may be governed differentially even in species with relatively equitable biparental care.

Mating and Parental Care in Lake Tanganyika's Cichlids

Cichlid fishes of Lake Tanganyika display a variety of mating and parental care behaviors, including polygamous and monogamous mouthbrooding and substrate breeding, cooperative breeding, as well as various alternative reproductive tactics such as sneaking and piracy. Moreover, reproductive behaviors sometimes vary within species both in space and in time. Here, I survey reports on mating and parenting behaviors of Lake Tanganyika cichlid species and address the evolution of mating and parental care patterns and sexual dimorphism. Notes on measures of sexual selection intensity and the difficulties of defining mating systems and estimating selection intensities at species level conclude the essay.

Sperm competition and ejaculate economics

Sperm competition was identified in 1970 as a pervasive selective force in post-copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade-offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non-sperm ejaculate effects, and theoretical integration of pre- and post-copulatory episodes of sexual selection.