Abstract
Ecological conditions are expected to have an important influence on individuals' investment in cooperative care. However, the nature of their effects is unclear: both favorable and unfavorable conditions have been found to promote helping behavior. Recent studies provide a possible explanation for these conflicting results by suggesting that increased ecological variability, rather than changes in mean conditions, promote cooperative care. However, no study has tested whether increased ecological variability promotes individual-level helping behavior or the mechanisms involved. We test this hypothesis in a long-term study population of the cooperatively breeding banded mongoose, Mungos mungo, using 14 years of behavioral and meteorological data to explore how the mean and variability of ecological conditions influence individual behavior, body condition, and survival. Female body condition was more sensitive to changes in rainfall leading to poorer female survival and pronounced male-biased group compositions after periods of high rainfall variability. After such periods, older males invested more in helping behavior, potentially because they had fewer mating opportunities. These results provide the first empirical evidence for increased individual helping effort in more variable ecological conditions and suggest this arises because of individual differences in the effect of ecological conditions on body condition and survival, and the knock-on effect on social group composition. Individual differences in sensitivity to environmental variability, and the impacts this has on the internal structure and composition of animal groups, can exert a strong influence on the evolution and maintenance of social behaviors, such as cooperative care.
Highlights
In cooperatively breeding groups, individuals can gain direct fitness by competing with other group members for reproductive opportunities or gain indirect fitness by helping to care for relatives’ offspring
The variability of ecological conditions altered the composition of banded mongoose groups and the patterns of reproductive and cooperative behavior within them
0.0 0.5 1.0 1.5 2.0 2.5 Number of babysitters per day better condition, leading to older males increasing their investment in direct fitness returns by mate-guarding more. These findings demonstrate that individual differences in sensitivity to ecological conditions, and the knock-on effects this has on the composition of animal groups, can influence selection on social behaviors such as cooperative care
Summary
Individuals can gain direct fitness by competing with other group members for reproductive opportunities or gain indirect fitness by helping to care for relatives’ offspring. According to Hamilton’s (1964) rule, selection will favor helping behavior when br > c, that is, when the fitness benefits to others (b), weighted by the actor’s relatedness to them (r), outweigh the fitness costs incurred by the actor (c). Individuals tend to increase their investment in helping behavior when opportunities to occupy independent breeding positions are limited, but this can occur both where conditions are favorable and all positions are occupied, that is, habitats are saturated (Komdeur 1992; Schoepf and Schradin 2012), or where adverse conditions limit the total number of these positions (Russell 2001; MacColl and Hatchwell 2002; Hatchwell and Sharp 2013). Ecological conditions can promote helping by increasing the net fitness benefits to individuals that help, for example, through reduced aggression from group members, but again this has been shown in both favorable (Dickinson and McGowan 2005; Baglione et al 2006; Koenig et al 2011) and adverse conditions (Shen et al 2012)
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