Final Activity Report Summary - UNGULATE MATE CHOICE (Mutual mate choice in arena-breeding topi antelopes)
The role of mate choice in the evolution of mammalian mating systems has received far less attention than the role of competition between males. In order to clarify the impact of mate choice in ungulate mating systems we addressed three questions: (1) Do males and females choose between mates in a selected model species, the arena-breeding ('lekking') topi antelope? And if so, are the strategies of the sexes compatible or in conflict? (2) Can female mate choice explain the evolutionary origin of lek breeding in topi? (3) How does the intensity of sexual selection through male competition relative to female mate choice vary among bovid species?
(1) In sexual conflict theory it is generally assumed that males should be more persistent than females when it comes to mating, because females typically have to invest more in each offspring. However, where females mate multiply to assure fertilisation, males may end up paying an opportunity costs by mating due to sperm depletion, leading to sex role reversal. We investigated sex roles in a field study on lekking topi antelopes by analysing patterns in matings and aggression. A conflict of interests was indeed identified between females and the preferred males in the arena centers. While the males strived to even out mating efforts between females by preferentially mating with the least mated females, individual females were able to enhance their probability of subsequent mating by aggressive interference with the matings of others. This finding suggests that females in this case have the higher optimum mating rate. Supporting male resistance, male occasionally counterattacked aggressive females, and in these cases they avoided mating with the female aggressor. Role reversal in sexual conflicts over mating rates might be more common in promiscuous mammals than the current state of the debate over sexual conflict warrants, especially where synchronised breeding and strong unanimous mate preferences in females exacerbate sperm depletion.
(2) In lekking ungulates, the small arena territories are always surrounded by an extensive area of larger territories containing significant resources. Earlier studies of lek systems have typically either compared payoffs among lek males or between lek males and resource defenders; however, little is known about the variance in mating success among resource defenders. Since body size reflects fighting ability in bovids, we used the body size of territory holders to estimate the payoffs associated with individual territories in topi antelopes. We found that also resource territorial networks with only slight centripetal clustering have lek-like properties in a negative correlation between centrality and male body size as well as observed mating success. This suggests that female mate choice acts as a selective force not only in classical lek systems but also in territorial systems with more modest clustering. A modelling approach can show how positive feedback between female benefits of choice and male benefits of clustering generates leks through a runaway process.
(3) Focusing on two classical examples of traits selected through male competition, namely male horn length and sexual body size dimorphism, we used the comparative method to identify predictors of the intensity of sexual selection across bovid species. Both traits depended on factors proposed to affect harem size, correlating positively with breeding group size and negatively with territoriality as the male mating strategy. This suggests that while group size increases the opportunity for sexual selection through male competition, territoriality reduces it because territorial males are unable to follow and monopolise female groups as effectively as males in non-territorial species. In contrast, the other main mechanism of sexual selection, female choice, is likely to play a relatively larger role in territorial systems where females can exercise choice by moving between territories.
(1) In sexual conflict theory it is generally assumed that males should be more persistent than females when it comes to mating, because females typically have to invest more in each offspring. However, where females mate multiply to assure fertilisation, males may end up paying an opportunity costs by mating due to sperm depletion, leading to sex role reversal. We investigated sex roles in a field study on lekking topi antelopes by analysing patterns in matings and aggression. A conflict of interests was indeed identified between females and the preferred males in the arena centers. While the males strived to even out mating efforts between females by preferentially mating with the least mated females, individual females were able to enhance their probability of subsequent mating by aggressive interference with the matings of others. This finding suggests that females in this case have the higher optimum mating rate. Supporting male resistance, male occasionally counterattacked aggressive females, and in these cases they avoided mating with the female aggressor. Role reversal in sexual conflicts over mating rates might be more common in promiscuous mammals than the current state of the debate over sexual conflict warrants, especially where synchronised breeding and strong unanimous mate preferences in females exacerbate sperm depletion.
(2) In lekking ungulates, the small arena territories are always surrounded by an extensive area of larger territories containing significant resources. Earlier studies of lek systems have typically either compared payoffs among lek males or between lek males and resource defenders; however, little is known about the variance in mating success among resource defenders. Since body size reflects fighting ability in bovids, we used the body size of territory holders to estimate the payoffs associated with individual territories in topi antelopes. We found that also resource territorial networks with only slight centripetal clustering have lek-like properties in a negative correlation between centrality and male body size as well as observed mating success. This suggests that female mate choice acts as a selective force not only in classical lek systems but also in territorial systems with more modest clustering. A modelling approach can show how positive feedback between female benefits of choice and male benefits of clustering generates leks through a runaway process.
(3) Focusing on two classical examples of traits selected through male competition, namely male horn length and sexual body size dimorphism, we used the comparative method to identify predictors of the intensity of sexual selection across bovid species. Both traits depended on factors proposed to affect harem size, correlating positively with breeding group size and negatively with territoriality as the male mating strategy. This suggests that while group size increases the opportunity for sexual selection through male competition, territoriality reduces it because territorial males are unable to follow and monopolise female groups as effectively as males in non-territorial species. In contrast, the other main mechanism of sexual selection, female choice, is likely to play a relatively larger role in territorial systems where females can exercise choice by moving between territories.