Local Mate Competition Theory Research Articles

Local Mate Competition Promotes Coexistence of Similar Competitors

Limited dispersal of individuals (or gametes if they are the dispersive units) imposes constraints on mating patterns in most natural populations. This aspect of neighbourhood interaction, however, has not received the attention that it fully deserves in the field of competition and coexistence. Hamilton's local mate competition theory (1967) predicts that as a species in a spatially structured habitat tends to become rare, a female-biased sex ratio would result. Such an effect of spatial structure means that a species at low density may gain an advantage in per capita population growth, facilitating coexistence of similar competitors. Thus, while other mechanisms of competitive coexistence that exist require some degree of difference between species, this “local mate competition” hypothesis of coexistence can permit identical species to stably coexist, invalidating the competitive exclusion principle.

Factors influencing brood sex ratios in polyembryonic Hymenoptera.

Copidosoma sp. is a polyembryonic encyrtid wasp which parasitizes isolated hosts. Most broods of this wasp are unisexual, but some contain both sexes and the secondary sex ratio of these is usually highly female biased. The overall population secondary sex ratio is female biased. Walter and Clarke (1992) argue that because the majority of individuals must mate outside the natal patch, the bias in the population secondary sex ratio contradicts predictions made by Hamilton's (1967) theory of local mate competition (LMC). We suggest that the primary sex ratio is unbiased and that Walter and Clarke's results do not cast doubt on LMC. Instead these results imply that ovipositing females make a combined clutch size and sex ratio decision influencing whether individuals developing from a particular brood will outbreed or largely inbreed; for each case the predictions of LMC theory are not violated. If this interpretation is correct, what is of interest is the basis on which this decision is made rather than the population secondary sex ratio. We show that host encounter rate influences the proportions of mixed and single sex broods laid by Copidosoma floridanum, a related polyembryonic parasitoid. Among single-sex broods the primary sex ratio is female biased, but our results are in agreement with LMC theory since offspring developing from these broods will probably mate with siblings from adjacent hosts. We consider the egg load of females to be of major influence on oviposition behaviour, and that the mating structure of parasitoid offspring, potentially differential costs of male and female broods and the natural distributions of hosts both at oviposition and eclosion, require further study.

Unisexual broods and sex ratios in a polyembryonic encyrtid parasitoid (Copidosoma sp.: Hymenoptera).

We examined brood sex ratios of an undetermined species of Copidosoma. Most broods (65%) were unisexual, with a greater proportion of female broods. Some mixed broods contained extremely small proportions of wasps of the opposite sex. Our results imply that the female-biased sex rations in this species cannot be explained by local mate competition theory.

Sex Allocation in Hermaphroditic Sea Basses

Local mate competition theory has emphasized the importance of sperm competition in determining individual patterns of sex allocation in outcrossing hermaphrodites. In this article I develop a more general model of local mate competition that incorporates fertilization efficiency. I use this model to predict interspecific differences in sex allocation. These predictions are tested by using dry weights of male and female gonadal tissue from six species of hermaphroditic sea basses. In all four paired comparisons, when two of the three factors presumed to determine sex allocation patterns are relatively constant, the shift in sex allocation between species is correctly predicted. Neither levels of sperm competition nor the expected effects of spawning behavior on fertilization efficiency can account for all of the differences in sex allocation, although sperm competition does correctly predict the most differences in sex allocation. Overall, the interspecific differences are predicted by either the level of sperm competition or by factors affecting fertilization efficiency. A broader view of local mate competition that includes aspects of sperm competition and the dynamics of fertilization should ultimately provide more accurate predictions of sex allocation in hermaphroditic organisms.

A Test of Local Mate Competition Theory with a Solitary Species of Parasitoid Wasp, Spalangia Cameroni

The effect of conspecific females on offspring sex ratio was examined in the parasitoid wasp Spalangia cameroni. In the presence of a second female, females increased the proportion of sons they produced relative to when they were alone, as predicted by local mate competition theory. However, females did not seem to differentiate between two and more than two females: offspring sex ratios from groups of two, four, six, and ten females were not significantly different. The trace odor of another female was not a sufficient cue for females to increase the proportion of sons they produced. Results indicate that for species with large interfemale variation in offspring sex ratio, it is preferable to test how the presence of other females affects offspring sex ratio by looking for changes within individual females rather than by comparing different-sized groups of females.

Reproductive Strategy of Trissolcus euschisti (Hymenoptera: Scelionidae) under Conditions of Partially Used Host Resources

Ann. Entomol. Soc. Am. 82(2): 172-176 (l989) ABSTRACT Trissolcus euschisti (Ashmead), a scelionid parasitoid of stink bug eggs, did not alter its sex ratio in response to chemical traces left by previously ovipositing wasps as had been predicted by published local mate competition models. Female T. euschistt gen­ erally oviposited male offspring early in an ovipositional sequence. Consequently, stink bug egg masses that have been parasitized by more than one female should produce a higher male/total wasp sex ratio than those parasitized by single females as suggested by Waage (1982) for certain other scelionid parasitoids. Thus, the apparently adaptive outcome of higher sex ratios when more than one foundress oviposits in a patch may be the same as that predicted by local mate competition theory but does not involve local mate competition because of a relatively fixed pattern of sex allocation. Female T. euschistt rejected heavily parasitized egg masses more frequently than they rejected host egg masses of a similar size that were only lightly parasitized.

Adaptive responses to local mate competition by the parasitoid, Telenomus remus

The parasitic wasp, Telenomus remus, lays her eggs in diserete patches of moth eggs, where her offspring develop and mate before dispersal, satisfying conditions for local mate competition (LMC). In the presence of other ovipositing females, wasps lay a higher sex ratio (proportion males), as predicted by LMC theory, and achieve this by a combination of two mechanisms, (1) avoidance of superparasitism and a sequence of sex allocation initially biased towards males and (2) a direct increase in sex ratio in the presence of other wasps, sex ratio increases with the proportion of previously parasitized hosts, as predicted by LMC theory. In both cases, chemical traces left by foraging wasps are indicated as the stimuli causing wasps to increase the proportion of males allocated to hosts.

Optimality, plasticity and selective regime in fig wasp sex ratios

In thirteen species of fig-pollinating wasps the sex ratios of broods with one or two foundress mothers show qualitative agreement with the optimal sex ratios predicted by local mate competition theory. The deviations from these predictions are not random: for a given number of foundresses, the deviations are least in the wasp species that naturally encounter that number of foundresses most frequently. Furthermore, the difference in sex ratio between broods with one and broods with two foundresses is greatest in those species of wasp in which broods of both type commonly occur. These results show the importance of the selective regime in shaping the evolution of sex-ratio adjustment, and suggest that adaptive behavioural plasticity is more developed in species subject to more variable selective regimes.