According to Michener's Paradox, most altruistic groups in nature should be small, and large groups should not exist. This is because per capita productivity is thought to decrease as groups get larger, meaning that the share of indirect fitness available to each group member declines, which favours dispersal. The empirical evidence for a decrease in per capita productivity is contradictory, however, and limited to the social Hymenoptera. I report that per capita reproductive success decreased with increasing group size across 26 cooperatively breeding bird species. Small groups comprising two or three individuals were the most common (79% of 16,101 groups) and these had the highest per capita reproductive success. This close fit between per capita reproductive success and the distribution of group sizes in nature suggests that it may indeed be difficult for large groups to evolve through indirect fitness benefits alone.

In many species that raise young in cooperative groups, breeders live an exceptionally long time despite high investment in offspring production. How is this possible given the expected trade-off between survival and reproduction? One possibility is that breeders extend their lifespans by outsourcing parental care to non-reproductive group members. Having help lightens breeder workloads and the energy that is saved can be allocated to survival instead. We tested this hypothesis using phylogenetic meta-analysis across 23 cooperatively breeding bird species. We found that breeders with helpers had higher rates of annual survival than those without helpers (8% on average). Increased breeder survival was correlated with reduced investment in feeding offspring, which in turn depended on the proportion of feeding provided by helpers. Female and male breeders showed similar patterns. Our results indicate that one of the secrets to a long life is reduced investment in parental care. This appears to be a unique feature of cooperative societies with hard-working helpers.This article is part of the theme issue ‘Ageing and sociality: why, when and how does sociality change ageing patterns?’.

R script with details of all data manipulations and statistical models (.R file)

Details of the data from which effect sizes were calculated (.txt file)

Details of the data from which effect sizes were calculated (.txt file)

The species search, raw data, publication bias tests and model output associated with the study (.xlsx file)

The species search, raw data, publication bias tests and model output associated with the study (.xlsx file)

In many species that raise young in cooperative groups, breeders live an exceptionally long time despite high investment in offspring production. How is this possible given the expected trade-off between survival and reproduction? One possibility is that breeders extend their lifespans by outsourcing parental care to non-reproductive group members. Having help lightens breeder workloads and the energy that is saved can be allocated to survival instead. We tested this hypothesis using phylogenetic meta-analysis across 23 cooperatively breeding bird species. We found that breeders with helpers had higher rates of annual survival than those without helpers (8% on average). Increased breeder survival was correlated with reduced investment in feeding offspring, which in turn depended on the proportion of feeding provided by helpers. Female and male breeders showed similar patterns. Our results indicate that one of the secrets to a long life is reduced investment in parental care. This appears to be a unique feature of cooperative societies with hard-working helpers.This article is part of the theme issue ‘Ageing and sociality: why, when and how does sociality change ageing patterns?’.

R script with details of all data manipulations and statistical models (.R file)

Male-only parental care, while rare in most animals, is a widespread strategy within teleost fish. The costs and benefits to males of acting as sole carer are highly variable between fish species making it challenging to determine the selective pressures driving the evolution of male-only care to such a high prevalence. We conducted a phylogenetic meta-analysis to examine the costs and benefits of paternal care across fish species. We found no evidence that providing care negatively affects male condition. In contrast to other taxa, we also found limited evidence that male care has evolved as a strategy to improve offspring survival. Instead, we found that males already caring for a brood are preferred by females and that this preference is strongest in those species in which males work harder to care for larger broods. Thus, in fish, investment in offspring care does not constrain a male’s mating success but rather augments it, suggesting that the relatively high prevalence of male-only care in fish may be in part explained by sexual selection through female preference for caring males.