Adult sex ratio affects divorce rate in the monogamous endoparasite Schistosoma mansoni

Adult sex ratios: causes of variation and implications for animal and human societies

Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation.

Adaptive value of same-sex pairing in Laysan albatross

Same-sex pairing is widespread among animals but is difficult to explain in an evolutionary context because it does not result in reproduction, and thus same-sex behaviour often is viewed as maladaptive. Here, we compare survival, fecundity and transition probabilities of female Laysan albatross in different pair types, and we show how female-female pairing could be an adaptive alternative mating strategy, albeit one that resulted in lower fitness than male-female pairing. Females in same-sex pairs produced 80% fewer chicks, had lower survival and skipped breeding more often than those in male-female pairs. Females in same-sex pairs that raised a chick sometimes acquired a male mate in the following year, but females in failed same-sex pairs never did, suggesting that males exert sexual selection by assessing female quality and relegating low-quality females into same-sex pairs. Sexual selection by males in a monomorphic, non-ornamented species is rare and suggests that reconsideration is needed of the circumstances in which alternative reproductive behaviour evolves. Given the lack of males and obligate biparental care in this species, this research demonstrates how same-sex pairing was better than not breeding and highlights how it could be an adaptive strategy under certain demographic conditions.

A critical examination of popular assumptions about the benefits and outcomes of monogamous relationships

In this article, we critically examine the social institution of monogamy. First, we discuss the lack of an adequate and consistent definition of the construct of monogamy and consider how common monogamy is. Next, we address perceived benefits of monogamy and whether those ostensible benefits are supported by empirical evidence. We conclude that evidence for the benefits of monogamy relative to other relationship styles is currently lacking, suggesting that, for those who choose it, consensual non-monogamy may be a viable alternative to monogamy. Implications for theories of close relationships are discussed.

Male-biased sex ratio: why and what consequences for the genus Schistosoma?

Schistosomes are the cause of the most significant helminth disease of humans. Their unusual sexual biology is intriguing. Instead of being hermaphroditic, as is the rule in other trematode species, they are gonochoric. Furthermore, their mating system is considered to be monogamous, a characteristic shared by only 1% of living species, and their sex ratio is male-biased. In this paper we propose an explanation of the origin of the male-biased sex ratio in schistosomes and highlight the ecological and evolutionary consequences of this bias. We argue that schistosome gonochorism, monogamy and the biased sex ratio can be integrated into a single evolutionary scheme.