Hard-working helpers contribute to long breeder lifespans in cooperative birds

Abstract

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. Helpers had similar effects on female and male breeder survival. 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?’

Context-dependent social benefits drive cooperative predator defense in a bird

“…5,9 Additionally, helping may directly increase survival of current group members (e.g., offspring provisioning can enhance survival of breeders via load lightening). 34 In this case, benefits depend on social context if these group members vary in their relative value to the helper (e.g., they can represent future mates, provide kin-selected and nepotistic benefits, or form a reciprocal social bond with the helper). [6][7][8]12 Despite this, the evolution of group living and the evolution of cooperation are often considered in separate conceptual frameworks, with tests of hypotheses explaining cooperative breeding focused predominantly on offspring provisioning and benefits of helping with reproduction.…”

“…Group size (mean ± SE = 4.9 ± 0.1, range = 3-7) was included in all models as it not only reflects potential benefits of passive group augmentation (Figure 1), but is also necessary to control for potential load-lightening effects in larger groups. 34,70 Although group size is negatively correlated with the probability of breeding position inheritance (LM: R = À0.55, t = À7.84, p < 0.01), both variables were included in models since for models without inheritance (Table S2, models 2, 5, 6 and 7), this yielded a better fit to the data compared to the same models excluding group size (DAICc = 4.4, 4.8, 4.3 and 4.3, respectively), while excluding group size yielded a non-significantly better fit for models including inheritance (Table S2, models 1, 3, 4, and 8; DAICc = 0.9, 0.9, 0.4 and 0.4, respectively). Additional variables that are expected to affect the probability to engage in nest defense were also included in all models: sex (male, female), age (first-year, older), brood size, nest stage (eggs, nestlings), predator type (goanna, goshawk, cuckoo), trial no.…”

Ageing and sociality: why, when and how does sociality change ageing patterns?

“…For example, Lucas & Keller [50] argue that sociality evolves first and lifespan can increase with the evolution of higher social complexity. In particular in small groups, external mortality of a reproductive may indeed not be lower than in solitary organisms (but see [23]) and the effect of a few helpers in parental care may not be sufficient to fully alleviate the costs of reproduction [50]. That the difference in lifespan between reproductives and workers increases with group size royalsocietypublishing.org/journal/rstb Phil.…”

“…Several contributions explore commonalities and differences in ageing patterns that are associated with different degrees of sociality. Using previously published data, Downing et al [23] show that breeders in cooperatively breeding birds survive better when helpers support them in brood care and that the likelihood of survival in breeders with helpers depends on how much breeders invest into brood care. This demonstrates how the social environment can increase survival probability, e.g.…”

Senescence: why and where selection gradients might not decline with age

“…Cooperative breeding systems, defined by the presence of alloparental care by helpers [89], offer an excellent system for empirical and theoretical study for how social interactions could directly alter patterns of senescence. For example, helpers can provide load-lightening benefits to breeders [80] that may delay breeder senescence and extend lifespan [90][91][92]. Helping tendencies may also change with age [93], due, for example, to changing relatedness to its group as an individual helper ages [94], or to changing benefits of independent reproduction [95].…”