Helping effort increases with relatedness in bell miners, but ‘unrelated’ helpers of both sexes still provide substantial care

Abstract

Indirect fitness benefits from kin selection can explain why non-breeding individuals help raise the young of relatives. However, the evolution of helping by non-relatives requires direct fitness benefits, for example via group augmentation. Here, we examine nest visit rates, load sizes and prey types delivered by breeding pairs and their helpers in the cooperatively breeding bell miner (Manorina melanophrys). In this system, males remain in their natal colony while young females typically disperse, and helpers of both sexes often assist at multiple nests concurrently. We found extremely clear evidence for the expected effect of genetic relatedness on individual helping effort per nest within colonies. This positive incremental effect of kinship was facultative-i.e. largely the result of within-individual variation in helping effort. Surprisingly, no sex differences were detectable in any aspect of helping, and even non-relatives provided substantial aid. Helpers and breeders of both sexes regulated their provisioning effort by responding visit-by-visit to changes in nestling begging. Helping behaviour in bell miners therefore appears consistent with adaptive cooperative investment in the brood, and kin-selected care by relatives. Similar investment by 'unrelated' helpers of both sexes argues against direct fitness benefits, but is perhaps explained by kin selection at the colony level.

Kinship reduces alloparental care in cooperative cichlids where helpers pay-to-stay

“…Subordinate, non-reproductive members of social groups can gain indirect fitness benefits if they are related to the dominant breeders and distribute their help according to kinship 4 . Among vertebrates, evidence for the importance of indirect fitness benefits exists for some bird species, for example, Seychelles warblers 5 , Long-tailed tits 6,7 and Bell miners 8,9 . However, several studies did not find kinship effects on helping behaviour as predicted by kin-selection theory (in mammals 10 ; fish 11 ; birds 12 ; insects 13 ) and substantial help is sometimes shown by unrelated individuals 9,[14][15][16] .…”

Bell miner provisioning calls are more similar among relatives and are used by helpers at the nest to bias their effort towards kin

“…(c) Mew call similarity best explains individual adjustments in helping effort We show that mew call similarity explained most of the variation in fine-scale individual facultative adjustments in helping effort (see [20]), which was better than the variation explained by simple genetic relatedness of helpers to the focal brood. This strongly suggests that mew call similarity was the behavioural mechanism used by helpers as a proxy for the all-important genetic relatedness to the brood.…”

“…We assessed the relative predictive power of within-subject differences in both genetic relatedness and mew call similarity (relative to breeding females and breeding males) on within-individual variation in nestling provisioning effort of helpers (log-transformed visit rates: these have importantly been demonstrated to have no additional confounding variation from load sizes or prey types delivered per visit [20]). These analyses included individual sex as a factor and individual identity as a random effect.…”

“…While recent analyses have demonstrated a gradual and positive effect of genetic relatedness on individual helper effort in male and female bell miners [20], the mechanism by which this fine-scale kin recognition occurs in such a genetically complex society was unknown. Here, we clearly demonstrate a relationship between the genetic relatedness of individuals and the similarity of their mew call structure ( figure 2).…”

“…Here, we clearly demonstrate a relationship between the genetic relatedness of individuals and the similarity of their mew call structure ( figure 2). This could therefore provide the information needed for a rule-of-thumb to facilitate fine-scale kin-based adjustments in helping effort documented by Wright et al [20] via mew call similarity of helpers versus breeding males (figure 3; extra-pair fertilizations are rare [35]). …”