A mixed model of the evolution of polygyny and sexual size dimorphism in mammals

Abstract

The theory of sexual selection is the most widely accepted theory explaining the evolution of mating systems and secondary sexual characters. Polygyny is the most common mating system in mammals, and there is a strong correlation between the degree of polygyny and the degree of sexual size dimorphism skewed towards males. Sexual selection theory posits that polygyny in mammals has evolved through direct, precopulatory, intrasexual selection in males, and that sexual size dimorphism is a result of male competition for mates. New results that are being obtained with the use of molecular techniques and with comparative phylogenetic methods do not appear to support predictions from this classical model in full. In this article, an expansion of the classical model is presented that combines the effects of at least four forms of selection: natural, precopulatory intrasexual, postcopulatory intrasexual, and intersexual selection. This mixed model consists of an initial phase in which natural selection operates on body size, followed by a second phase dominated by sexual selection and involving increases in sexual dimorphism and coercive behaviour of males towards females. Sexual harassment induces female aggregation, thus creating social potential for polygyny. Males compete for access to the groups of females, following two possible evolutionary scenarios, directional or equilibrium sexual selection, both producing similar behavioural polygyny, but with differences in the intensity of intra-male precopulatory sexual selection. Predictions of the mixed model are as follows: 1) polygyny can exist without high variance in male reproductive success (a fundamental requirement in the classical model); 2) extra-group fertilisation can be common; 3) sexual size dimorphism evolved prior to polygyny; 4) sexual coercion is widespread; and 5) females reduce levels of sexual coercion by joining groups.

Killing conspecific adults in mammals

“…Male adulticide was significantly and positively associated with male-biased size dimorphism in all analyses (table 1), being more prevalent in dimorphic species than in monomorphic ones (61% versus 44%, respectively, using 10% size difference between sexes as a cut-off point to consider a given species as dimorphic) [24]. Sexual size dimorphism is considered a direct consequence of sexual selection in mammals [24,25], and it is widely used as a surrogate of the intensity of the sexual selection operating on each species [25]. Mammals for which male-biased size dimorphism is more frequent, such as carnivores (Otariidae, Felidae, Canidae), primates (Hominidae, Cercopithecidae) and ungulates (Bovidae, Cervidae), are also those where adulticide was more prevalent in our study.…”

Sexual size dimorphism and sexual selection in primates

“…The results of my analysis of sexual size dimorphism in primates are consistent with reproductive skew theory: in large groups, males may lose control of the sexual behaviour of the other members of the group or may concede reproductive opportunities to others. Another explanation is that sexual size dimorphism may have evolved before the evolution of polygyny, due to the effects of natural selection (Cassini 2019). There are several evolutionary mechanisms that can explain the evolution of sexual size dimorphism by natural selection.…”

Sexual aggression in mammals

“…In this review, I found that sexual disturbance is frequent in four mammalian orders and eight families, which are also the most sexually dimorphic and polygynous among mammals (Ralls 1977, Cassini 2020). Taking into account the conclusions of Smuts and Smuts (1993), this review suggests that sexual coercion behaviour, which benefits males and harms females, is systematically present in colonial pinnipeds (mainly family Otariidae) and in the families Hominidae and Cercopithecidae within Primates.…”

“…In summary, joining female groups appears to reduce sexual disturbance. Cassini (2020) suggested that this benefit to females could have elicited a positive feedback loop on the evolution of polygyny in mammals: females aggregated to avoid the costs of disturbance; this in turn facilitated female monopolisation, leading to increased reproductive success of dominant males, increasing the intensity of intrasexual selection, and promoting the evolution of higher levels of polygyny.…”

“…The objectives of this review were as follows: 1) to analyse how widespread sexual disturbance is among mammals, 2) to analyse the types of female response to sexual disturbance, and 3) to characterise the costs of sexual disturbance to females. I discuss the consequences that the existence of sexual disturbance have had on the evolution of polygyny in mammals, mainly focussing on whether it encourages females to concentrate around dominant males that can protect them from disturbance costs, thereby favouring mate monopolisation (Trillmich & Trillmich 1984, Smuts & Smuts 1993, Cassini 1999, 2000, 2020). To analyse how widespread sexual disturbance is among mammals, I followed two methodologies.…”