Abstract
1. Cooperatively breeding species are typically long lived and hence, according to theory, are expected to maximize their lifetime reproductive success through maximizing survival. Under these circumstances, the presence of helpers could be used to lighten the effort of current reproduction for parents to achieve higher survival.2. In addition, individuals of different sexes and ages may follow different strategies, but whether male and female breeders and individuals of different ages benefit differently from the presence of helpers has often been overlooked. Moreover, only one study that investigated the relationship between parental survival and the presence of helpers used capture–mark–recapture analyses (CMR). These methods are important since they allow us to account for the non-detection of individuals that are alive in the population but not detected, and thus, the effects on survival and recapture probability to be disentangled.3. Here, we used multi-event CMR methods to investigate whether the number of helpers was associated with an increase in survival probability for male and female breeders of different ages in the sociable weaver Philetairus socius. In this species, both sexes reduce their feeding rate in the presence of helpers. We therefore predicted that the presence of helpers should increase the breeders' survival in both sexes, especially early in life when individuals potentially have more future breeding opportunities. In addition, sociable weaver females reduce their investment in eggs in the presence of helpers, so we predicted a stronger effect of helpers on female than male survival.4. As expected we found that females had a higher survival probability when breeding with more helpers. Unexpectedly, however, male survival probability decreased with increasing number of helpers. This antagonistic effect diminished as the breeders grew older.5. These results illustrate the complexity of fitness costs and benefits underlying cooperative behaviours and how these may vary with the individuals' sex and age. They also highlight the need for further studies on the sex-specific effects of helpers on survival.
Highlights
Cooperative breeding describes the situation where supernumerary sexually mature individuals, named helpers, assist in raising the offspring of others by bringing additional food to the young
Only one study that investigated the relationship between parental survival and the presence of helpers used capture–mark– recapture analyses (CMR)
We used multi-event CMR methods to investigate whether the number of helpers was associated with an increase in survival probability for male and female breeders of different ages in the sociable weaver Philetairus socius
Summary
Cooperative breeding describes the situation where supernumerary sexually mature individuals, named helpers, assist in raising the offspring of others by bringing additional food to the young. Breeding species are generally long lived (Arnold & Owens 1998) and often live in relatively unpredictable environments (Rubenstein & Lovette 2007) They are typically predicted to maximize their lifetime reproductive success through maximizing survival, because a small increment in survival probability is likely to result in a considerably higher increase in fitness than a small increase in current reproductive output at the expense of survival (Clutton-Brock 1988; Wilbur & Rudolf 2006). This life-history strategy, coupled with the difficulty of measuring small differences in survival in natural populations, could explain why several studies have failed to find a positive effect of helpers on reproductive success (as found in 12 bird species reviewed in Kingma et al 2010)
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