Life History Hypotheses Research Articles

A reconstruction of the invasion of land by Jamaican crabs (Grapsidae: Sesarminae)

Several decapod groups independently colonized freshwater and terrestrial habitats and became independent from the sea. These invasions were accompanied by analogous reproductive and developmental traits such as large eggs and an abbreviated, lecithotrophic development. Here, we present the first empirical study on the evolution of reproductive and developmental traits that accompany the invasion of land by crabs. As crucial steps in the colonization, we identify the transitions of the larval nursery, first, from the marine plankton into landlocked-brackish nurseries and, second, into fresh water. During these invasions, the early life-cycle stages were facing new ecological conditions and selective agents. We test hypotheses on the evolution of egg size and the mode of development in relation to the larval ecology of recent species and draw conclusions on their evolutionary past. As a model we focus on the genus Sesarma, that colonized Jamaica relatively recently and comprises species with a larval development in marine, brackish and freshwater habitats. In addition, we compare representatives of the crab genera Armases, Sesarma and Uca that invaded brackish-nursery habitats independently. The analysis reveals that in each genus the transition from marine to brackish nurseries resulted in fewer and larger eggs, an abbreviated development and higher endotrophic potential of larvae, and a wider tolerance to physicochemical stress (salinity). Size at metamorphosis, however, did not change in brackish species, suggesting that it is constrained. Within the Sesarma-lineage, egg size increases considerably from marine to freshwater species. The duration of embryonic development, the size and endotrophic potential of larvae are positively correlated, but the duration of the larval phase is negatively correlated with egg size. Hypotheses suggesting that large eggs evolved as a response to limited food or intense predation are inadequate to explain the initial egg-size increase in brackish species. We suggest that the specific abiotic environment of the brackish nurseries ultimately selected for increased egg size. These particular larval nurseries of brackish species of Armases, Sesarma and Uca are nutrient rich but ephemeral habitats with unfavourable physicochemical conditions, which strongly favour a swift larval phase and possibly large body size and higher salinity-stress resistance of larvae. The reason for the further and substantial increase in egg size in freshwater species remains unknown. The ‘food-limitation’ hypothesis derived from laboratory experiments, however, is inadequate to explain this increase. Our results support general life-history hypotheses (‘safe harbour’ hypothesis) that predict the evolution of large eggs if post-embryonic stages face high risk of mortality, but not the predicted positive relationship between egg size and instantaneous egg stage mortality. On the contrary, we find a negative relationship, suggesting that larger eggs are a ‘safer harbour’ than smaller eggs. We outline a scenario for the invasion of land by crabs and propose a two-step model: as a first step, an instant shift of the larval development from offshore into landlocked-brackish nurseries, and, as a second step, from there into freshwater nurseries.

Influence of trematode infections on in situ growth rates of Littorina littorea

The influence of various species of digenean trematodes on the in situ growth rate of Littorina littorea (Gastropoda: Prosobranchia) from different habitats was investigated. The main results showed: (1) that infections either reduced or had no significant effect on growth in comparison with uninfected snails; (2) that the same type of infection could have a differential effect on growth depending on the habitat/population from which the snails originated. These findings are consistent with the life history hypothesis expecting no or a stunting effect of trematode infections on the growth of longer-lived snails, but do also emphasize that growth rates following trematode invasion can be significantly modified by environmental conditions, such as, food availability.

Life history characteristics and strategies of the American eel, Anguilla rostrata

Several life history hypotheses for the American eel, Anguilla rostrata, were examined using seaward-migrating silver-phase eels collected in the Annaquatucket River, Rhode Island, U.S.A. Female eels were significantly larger and older than males. Female eels also had a significantly higher mean growth rate. The addition of life history data from Annaquatucket River eels to published silver eel data from locations throughout the eels' range shows that female size at migration is positively correlated with latitude (r = 0.56, p = 0.05) but male size is not (r = 0.54, p = 0.17). Female age was not related to latitude (r = 0.57, p = 0.27) but male age showed a positive relationship (r = 0.87, p = 0.05). Growth rates for females and males were inversely related to latitude (r = -0.98, p = 0.02 and r = -0.95, p = 0.05, respectively). Differences between the latitudinal relationships and life history traits of the sexes may be due to differences in life history strategies.

Cooperative breeding in birds: a comparative test of the life history hypothesis

In approximately 3.2% of bird species individuals regularly forgo the opportunity to breed independently and instead breed cooperatively with other conspecifics, either as non–reproductive ‘helpers’ or as co–breeders. The traditional explanation for cooperative breeding is that the opportunities for breeding independently are limited owing to peculiar features of the specie's breeding ecology. However, it has proved remarkably difficult to find any common ecological correlates of cooperative breeding in birds. This difficulty has led to the ‘life history hypothesis’, which suggests that the common feature of cooperatively breeding birds is their great longevity, rather than any particular feature of their breeding ecology. Here, we use a comparative method to test the life history hypothesis by looking for correlations between life history variation and variation in the frequency of cooperative breeding. First, we find that cooperative breeding in birds is not randomly distributed, but concentrated in certain families, thus supporting the idea that there may be a common basis to cooperative breeding in birds. Second, increases in the level of cooperative breeding are strongly associated with decreases in annual adult mortality and modal clutch size. Third, the proportion of cooperatively breeding species per family is correlated with a low family–typical value of annual mortality, suggesting that low mortality predisposes cooperative breeding rather than vice versa. Finally, the low rate of mortality typically found in cooperatively breeding species is associated with increasing sedentariness, lower latitudes, and decreased environmental fluctuation. We suggest that low annual mortality is the key factor that predisposes avian lineages to cooperative breeding, then ecological changes, such as becoming sedentary, further slow population turnover and reduce opportunities for independent breeding. As the traditional explanation suggests, the breeding habitat of cooperatively breeding species is saturated, but this saturation is not owing to any peculiar feature of the breeding ecology of cooperative breeders. Rather, the saturation arises because the local population turnover in these species is unusually slow, as predicted by the life history hypothesis.