Autumnal vs spring hatching in the fairy shrimp Siphonophanes grubii (Dybowski) (Crustacea, Anostraca): diversified bet-hedging strategy ?

Polyphenisms, as opposed to polymorphism, refers to coexistence of several distinct phenotypes having a common genotype. Polyphenism can be selected for in unpredictable environments. Here we document and anlyse a case of siphenism in the north-European fairy shrinp Siphonophanes grubii (Dybowski), in relation to the temporary and unpredictable nature of its habitat. The active part of this species'life cycle usually consists of a single, short-lived, spring cohort. Here we report field observations on autumnal hatching and on a long-lived, overwintering cohort; we show that the winter cohort runs the risk of total failure, due to the pond freezing entirely or drying up during winter. If, however, environmental conditions allow winter survival, animals reach a larger size, reproduce for a longer time, and display higher fecundity, than do animals from the spring cohort. Laboratory experiments support the theory that these differences are purely phenotypic and dependent on temperatur. Using an analytical model adapted from Cohen (1966), we propose that the coexistence of both a winter and a spring cohort in the same ponds can be interpreted as a diversified bet-hedging strategy.

Natural selection on fecundity variance in subdivided populations: kin selection meets bet hedging.

In a series of seminal articles in 1974, 1975, and 1977, J. H. Gillespie challenged the notion that the "fittest" individuals are those that produce on average the highest number of offspring. He showed that in small populations, the variance in fecundity can determine fitness as much as mean fecundity. One likely reason why Gillespie's concept of within-generation bet hedging has been largely ignored is the general consensus that natural populations are of large size. As a consequence, essentially no work has investigated the role of the fecundity variance on the evolutionary stable state of life-history strategies. While typically large, natural populations also tend to be subdivided in local demes connected by migration. Here, we integrate Gillespie's measure of selection for within-generation bet hedging into the inclusive fitness and game theoretic measure of selection for structured populations. The resulting framework demonstrates that selection against high variance in offspring number is a potent force in large, but structured populations. More generally, the results highlight that variance in offspring number will directly affect various life-history strategies, especially those involving kin interaction. The selective pressures on three key traits are directly investigated here, namely within-generation bet hedging, helping behaviors, and the evolutionary stable dispersal rate. The evolutionary dynamics of all three traits are markedly affected by variance in offspring number, although to a different extent and under different demographic conditions.

Inbreeding load, bet hedging, and the evolution of sex-biased dispersal

Inbreeding load affects not only the average fecundity of philopatric individuals but also its variance. From bet-hedging theory, this should add further dispersal pressures to those stemming from the mere avoidance of inbreeding. Pressures on both sexes are identical under monogamy or promiscuity. Under polygyny, by contrast, the variance in reproductive output decreases with dispersal rate in females but increases in males, which should induce a female-biased dispersal. To test this prediction, we performed individual-based simulations. From our results, a female-biased dispersal indeed emerges as both polygyny and inbreeding load increase. We conclude that sex-biased dispersal may be selected for as a bet-hedging strategy.

Immunocompetence and nestling survival in the house martin: the tasty chick hypothesis

In altricial birds post-fledging survival is usually positively related to nestling body mass. A large number of studies have shown that the latest hatched chick is the more likely to die, even if food is abundant. Here we suggest that ectoparasites may be a key factor in the evolution and the maintenance of the establishment of weight hierarchies within broods. We prepose the hypothesis that weight hierarchies within broods may be adaptive if the chick in poor condition is the one with the least efficient immune system within a nest. In this case parasites would preferentially feed on such a "tasty chick", because it would allow high reproductive rates for the parasites, without negatively affecting the survival of the other nestlings. This could prevent entire nest failure of the brood or allow the other chicks to grow more efficiently. This hypothesis was investigated in a colony of house martins Delichon urbica. We predicted that immunocompetence was positively correlated with body condition, and that nestlings dying before hedging should have lower immune responses when challenged with an antigen. T-cell immune response to an experimentally injected antigen was strongly positively related to body condition. Non-surviving chicks had low body condition and a weak immune response. The implications of these results are discussed in the context of the adaptive significance of hatching asynchrony.