Complete reproductive skew within white-browed sparrow weaver groups despite outbreeding opportunities for subordinates of both sexes
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The distribution of reproductive success within societies is a key determinant of the outcomes of social evolution. Attempts to explain social diversity, therefore, require that we quantify reproductive skews and identify the mechanisms that generate them. Here, we address this priority using life history and genotypic data from >600 individuals in 40 wild groups of the cooperatively breeding white-browed sparrow weaver, Plocepasser mahali. We show that groups comprise up to six males and seven females, but within-group reproduction is completely monopolised by a single dominant male and female, while extra-group males sire 12–18 % of offspring. Strong within-group kin structure could frequently explain these monopolies, as subordinates had typically delayed dispersal from their natal groups and so frequently (1) lacked within-group outbreeding partners, and/or (2) stood to gain little from contesting dominant reproduction, being almost as related to the dominant’s young as they would have been to their own. Kin structure alone cannot account entirely for these monopolies, however, as they remained complete following the immigration of unrelated males and females. That subordinate females remain reproductively quiescent despite also showing comparable body condition to dominants, overlapping them substantially in age, and showing no evidence of elevated stress hormone levels raises the possibility that they exercise reproductive restraint due instead to a threat of action by dominants and/or deficits in offspring fitness that might arise if subordinates bred. Our findings highlight the complexity of the mechanisms that generate reproductive disparities in animal societies and the challenge of identifying them when skews are complete.
KeywordsPlocepasser mahali Kin structure Cooperative breeding Reproductive conflict Monogamy Inbreeding avoidance
We are grateful to our many field assistants during this period (Tom Reed, Brenna Boyle, Andy Mason, Francesca Jarero, Lexy Russell, Padraig Cregg, Lynda Casson, Sam Farrell, Lynda Donaldson, Jenny Sturgeon, and Robyn Silcock) for their tireless efforts under the Kalahari sun, to Northern Cape Conservation for permission to conduct the research, and to E.O. and Son and all at Tswalu Kalahari Reserve for their exceptional logistical support. We thank two anonymous reviewers for helpful comments that improved an earlier draft of this manuscript. This study was funded by a BBSRC David Phillips research fellowship to AY and a NERC blue skies research fellowship to AY.
All protocols were approved by the University of Pretoria ethics committee and complied with regulations stipulated in the Guidelines for the use of Animals in Research and complied with the current laws of the Republic of South Africa.
Conflict of interest
The authors declare that they have no conflict of interest.
- Arnold TW (2010) Uninformative parameters and model selection using Akaike’s information criterion. J Wildl Manag 74:1175–1178Google Scholar
- Bates D, Maechler M, Bolker B (2012) lme4: Linear mixed-effects models using S4 classes. R package version 0.999999-0, http://cran.r-project.org/web/packages/lme4/index.html. Accessed 12/02/2013
- Birkhead TR, Moller AP (1992) Sperm competition in birds: evolutionary causes and consequences. Academic Press, LondonGoogle Scholar
- Collias NE, Collias EC (1978) Cooperative breeding-behavior in white-browed sparrow weaver. Auk 95:472–484Google Scholar
- Dawson DA, Horsburgh GJ, Kupper C, Stewart IRK, Ball AD, Durrant KL, Hansson B, Bacon I, Bird S, Klein A, Krupa AP, Lee JW, Martin-Galvez D, Simeoni M, Smith G, Spurgin LG, Burke T (2010) New methods to identify conserved microsatellite loci and develop primer sets of high cross-species utility—as demonstrated for birds. Mol Ecol Res 10:475–494CrossRefGoogle Scholar
- Emlen ST (1991) Evolution of cooperative breeding in birds and mammals. In: Krebs JR, Davies NB (eds) Behavioural ecology: an evolutionary approach. Blackwell Science, Oxford, pp 301–337Google Scholar
- Ferguson JWH (1988) Geographic variation in social behavior of white browed sparrow weavers, Plocepasser mahali. S Afr J Zool 23:1–6Google Scholar
- Kempenaers B, Dhondt AA (1993) Why do females engage in extra-pair copulations? A review of hypotheses and their predictions. Belg J Zool 123:93–103Google Scholar
- Nicholls JA, Double MC, Rowell DM, Magrath RD (2000) The evolution of cooperative pair breeding in thornbills Acanthiza (Pardalotidae). J Avian Biol 31:165–176Google Scholar
- R Development Core Team (2013) R: A language and environment for statistical computing. R Foundation for Statistical Computing, ViennaGoogle Scholar
- Richardson DS, Jury FL, Blaakmeer K, Komdeur J, Burke T (2001) Parentage assignment and extra‐group paternity in a cooperative breeder: The Seychelles warbler (Acrocephalus sechellensis). Mol Ecol 10:2263–2273Google Scholar
- Sinclair I, Ryan P (2010) Birds of Africa: South of the Sahara. Struik Publishers, JohannesburgGoogle Scholar
- York JE (2012) The evolution and regulation of dawn chorus performance in white-browed sparrow weaver society. PhD Thesis, University of BristolGoogle Scholar