Postcopulatory guarding strategy in a finite mating period

Journal of Theoretical Biology, 2003

We analyse a co-evolutionary sexual conflict game, in which males compete for fertilizations (sperm competition) and females operate sperm selection against unfavourable ejaculates (cryptic female choice). For simplicity, each female mates with two males per reproductive event, and the competing ejaculates are of two types, favourable (having high viability or success) or unfavourable (where progeny are less successful). Over evolutionary time, females can increase their level of sperm selection (measured as the proportion of unfavourable sperm eliminated) by paying a fecundity cost. Males can regulate sperm allocations depending on whether they will be favoured or disfavoured, but increasing sperm allocation reduces their mating rate. The resolution of this game depends on whether males are equal, or unequal. Males could be equal: each is favoured with probability, p; reflecting the proportion of females in the population that favour his ejaculate (the 'random-roles' model); different males are favoured by different sets of females. Alternatively, males could be unequal: given males are perceived consistently by all females as two distinct types, favoured and disfavoured, where p is now the frequency of the favoured male type in the population (the 'constant-types' model). In both cases, the evolutionarily stable strategy (ESS) is for females initially to increase sperm selection from zero as the viability of offspring from unfavourable ejaculates falls below that of favourable ejaculates. But in the random-roles model, sperm selection decreases again towards zero as the unfavourable ejaculates become disastrous (i.e. as their progeny viability decreases towards zero). This occurs because males avoid expenditure in unfavourable matings, to conserve sperm for matings in the favoured role where their offspring have high viability, thus allowing females to relax sperm selection. If sperm selection is costly to females, ESS sperm selection is high across a region of intermediate viabilities. If it is uncostly, there is no ESS in this region unless sperm limitation (i.e. some eggs fail to be fertilized because sperm numbers are too low) is included into the model. In the constanttypes model, no relaxation of sperm selection occurs at very low viabilities of disfavoured male progeny. If sperm selection is sufficiently costly, ESS sperm selection increases as progeny viability decreases down towards zero; but if it is uncostly, there is no ESS at the lowest viabilities, and unlike the random-roles model, this cannot be stabilized by including sperm limitation. Sperm allocations in the ESS regions differ between the two models. With random roles, males always allocate more sperm in the favoured role. With constant types, the male type that is favoured allocates less sperm than the disfavoured type. These results suggests that empiricists studying cryptic female choice and sperm allocation patterns need to determine whether sperm selection is applied differently, or consistently, on given males by different females in the same population.

Animal Behaviour

Animal Behaviour, 2014

Sperm competition theory predicts that males should allocate sperm prudently in response to perceived return on investment. It is predicted that males should invest in greater sperm stores and allocate more sperm to females in response to increased risk of sperm competition (absence versus presence of competitors), but allocate fewer sperm when intensity of sperm competition (number of competitors) is high, owing to diminishing returns. We tested these predictions in Gammarus duebeni, an amphipod with precopula mate guarding. Contrary to predictions, we found no difference in sperm allocation to females in response to increased risk or intensity of sperm competition. In addition, we observed a decrease in both sperm numbers and sperm length when males were kept with potential competitors for extended periods of time, particularly in larger males. We suggest that G. duebeni invests in mate guarding over sperm production/allocation in response to sperm competition, and we discuss whether competition for resources may also affect investment in ejaculate production.

Cite this article: Mehlis M, Rick IP, Bakker TCM. 2015 Dynamic resource allocation between pre-and postcopulatory episodes of sexual selection determines competitive fertilization success. Proc. R. Soc. B 282: 20151279. http://dx.

Trends in Ecology & Evolution, 1993

Ecology and Evolution, 2017

Sexual conflict over mating rate is both pervasive and evolutionarily costly. For females, the lifetime reproductive fitness costs that arise through interactions with potential mates will be influenced by the frequency of such interactions, and the fitness cost of each interaction. Both of these factors are likely to be influenced by variation in operational sex ratio (OSR) and population density. Variation in OSR-and density-dependent male alternative reproductive tactics (ARTs) may be particularly important if the fitness costs that females experience vary with the reproductive tactics that males express. Using a simple model, we consider several examples of OSRand/or density-dependent variation in male ARTs and the frequency of male-female interactions, and find that variation in the expression of male ARTs has the potential to augment or diminish the costs of frequent male interactions for females. Accurately documenting variation in the expression of male ARTs and associated female fitness costs will benefit future work in this area. density-dependent, male alternative reproductive tactics, OSR-dependent, sexual conflict Sexual reproduction often entails conflict, as reproductive partners frequently differ in their evolutionary interests . One area where such conflict is widely observed and has been greatly investigated is mating rate; whereas females may require relatively few matings to maximize fitness, in many species a male's fitness increases with his mating frequency (Andersson

Functional Ecology, 2014

Original sperm competition theory assumed that males trade expenditure on searching for mates for expenditure on the ejaculate and predicted that males should increase their expenditure on the ejaculate in response to increased risk of competition. A recent extension of this theory has modelled pre-copulatory expenditure in terms of direct contest competition and predicts that when the gains from marginal investment in weaponry are large, males might be expected to allocate resources to armaments even at the expense of the ejaculate. 2. Here, we examine socially cued plasticity in allocation to pre-(body condition) and postcopulatory (testes mass) traits in a male dimorphic beetle, Onthophagus taurus, where major males fight for access to females and minor males obtain reproductive success via sperm competition. Both male morphs were either reared in social isolation or exposed to rivals during the period of sexual maturation following adult emergence. 3. Testes mass was found to be insensitive to social cues of future mating competition for both major and minor males. Major males allocated more to body condition when exposed to rivals, a response expected for a species in which the outcome of dyadic contests strongly affects male reproductive success. In contrast, minor male allocation to condition was insensitive to social cues. 4. Our data illustrate how socially cued plasticity in pre-and post-copulatory traits can depend on the relative importance of these episodes of selection for individual male fitness. In O. taurus dung beetles, males strategically adjusted the amount of resources they allocated to winning pre-copulatory contests over access to females. Strategic allocation to pre-copulatory contest competition did not come at a cost to male investment in sperm competition, suggesting that males may trade investment into contest competition against some other life-history trait, such as longevity. The lack of plasticity in testes size suggests that selection from sperm competition may be a relatively constant feature of this species mating system.

2000

Journal of Evolutionary Biology, 2007

Animal Behaviour, 2007

When a female can mate with more than one male, males are thought to reduce intrasexual competition by prolonging mating, that is, mating longer than necessary for sperm release. Two principal hypotheses have been proposed to explain the advantage of prolonged mating. The extended mate guarding hypothesis suggests that with longer mating, the female is less likely to be inseminated by another male. The ejaculate transfer hypothesis suggests that with longer mating, more of the male's sperm are stored. To our knowledge, this is the first study to test these alternatives simultaneously by manipulating duration of mating and examining effects on sperm release, storage and female polyandry. We found support for the mate guarding hypothesis in the golden orb-weaving spider Nephila clavipes; the longer the mating with the first male, the lower the likelihood of the female copulating with another male. This was not due to influence on female receptivity but on the ability of subsequent males to couple. Inconsistent with the ejaculate transfer hypothesis was the observation that duration of mating with the first male did not linearly correlate with release or storage of its sperm. We suggest that prolonged mating does not result in sexual conflict in this species.