Polyandry as a mediator of sexual selection before and after mating

The Darwin–Bateman paradigm recognizes competition among males for access to multiple mates as the main driver of sexual selection. Increasingly, however, females are also being found to benefit from multiple mating so that polyandry can generate competition among females for access to multiple males, and impose sexual selection on female traits that influence their mating success. Polyandry can reduce a male's ability to monopolize females, and thus weaken male focused sexual selection. Perhaps the most important effect of polyandry on males arises because of sperm competition and cryptic female choice. Polyandry favours increased male ejaculate expenditure that can affect sexual selection on males by reducing their potential reproductive rate. Moreover, sexual selection after mating can ameliorate or exaggerate sexual selection before mating. Currently, estimates of sexual selection intensity rely heavily on measures of male mating success, but polyandry now raises serious questions over the validity of such approaches. Future work must take into account both pre- and post-copulatory episodes of selection. A change in focus from the products of sexual selection expected in males, to less obvious traits in females, such as sensory perception, is likely to reveal a greater role of sexual selection in female evolution.

Female territoriality in a paternal mouthbrooding cardinalfish to avoid predation against spawned eggs

Generally, paternal mouthbrooding cardinalfishes are characteristic of sex-role-reversed animals: females have a higher potential reproductive rate and are more active in mating competition than males, and the operational sex ratio (OSR) is female-biased. However, one species of cardinalfish, Apogon notatus (Houttuyn, 1782), shows unusual sex roles: females alone defend their breeding territories to form pairs, even though the OSR is male-biased. This is inconsistent with the general rule that breeding territoriality is shown by the more abundant sex. We examined the function of female breeding territory in this fish using field observations. Prior to the breeding season, large females established their territories earlier than small females. Earlier settlers occupied deeper areas with larger boulders where conspecifics were less likely to aggregate. As the level of conspecific aggregation increased, spawning females suffered from frequent intraspecific interference and subsequent egg predation, leading to increased time or energy spent on territorial defense. For the females, territories that have more boulders and fewer conspecifics might be of higher quality because such places are safe from egg predation and less costly to defend. We conclude that females defend their breeding territories to avoid predation of spawned eggs rather than to guard high-quality mates or to increase mating opportunities.

Temporal and spatial opportunities for polygamy in a monogamous seahorse, Hippocampus whitei

The sex with the higher potential reproductive rate is expected to mate polygamously unless there are temporal or spatial constraints on mate availability. We investigated whether such constraints were evident in a population of the monogamous seahorse Hippocampus whitei (family Syngnathidae). Across the whole study site, breeding was more asynchronous than expected by chance. Our findings are thus compatible with the hypothesis that asynchronous breeding may promote and/or maintain monogamy. Asynchrony per se was unlikely to explain monogamy entirely, however, as temporal opportunities for polygamy remained and the males that were nearest one another had the lowest level of asynchrony. Moreover, each animal's home range overlapped with home ranges of potential mates other than their partner, implying a lack of spatial constraints on polygamy. We suggest that H. whitei mated monogamously because the benefits of polygamy were reduced by (1) only small differences in the potential reproductive rates of males and females and/or (2) a mate familiarity effect that increased reproductive success in successive matings. Further research could investigate relationships between mating pattern and varying intersexual differences in potential reproductive rates across syngnathid species.

The population dynamics of a tropical possum, Trichosurus vulpecula arnhemensis Collett

The population ecology of Trichosurus vulpecula has been studied extensively in temperate Australia and in New Zealand. This paper provides the results of a trapping study of a population of the northern brushtail possum (T. vulpecula arnhemensis Collett 1897) in the wet–dry tropics of Australia’s Northern Territory. Possums were readily trapped and the population had a comparatively high density for Australian brushtails of around 3 per hectare. The core home-range size and range length for males was 1.12 ha and 165 m; this was a little larger than for females (0.89 ha and 155 m). The possums were not very agressive when handled and were apparently quite socially tolerant. The ready availability of nutritious food sources throughout the year enables them to breed continuously, producing 1.7 young per year per adult female. If environmental conditions become unfavourable with a series of poor wet seasons or frequent fires, the habitat will resemble the less-productive eucalypt forest not occupied by possums. Mortality of pouch young and immatures will increase under these conditions but with a potential reproductive rate of nearly two per year, populations of the northern brushtail can readily recover from short periods of unfavourable conditions.