Behavioral Ecology Advance Access originally published online on July 13, 2007
Behavioral Ecology 2007 18(5):895-904; doi:10.1093/beheco/arm049
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Is extrapair mating random? On the probability distribution of extrapair young in avian broods
a Bird Ecology Unit, Department of Biological and Environmental Sciences, University of Helsinki, PO Box 65, (Viikinkaari 1), FIN–00014, Finland b Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, 9750AA Haren, The Netherlands c School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK
Address correspondence to J.E. Brommer. E-mail: jon.brommer{at}helsinki.fi.
| Abstract |
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A dichotomy in female extrapair copulation (EPC) behavior, with some females seeking EPC and others not, is inferred if the observed distribution of extrapair young (EPY) over broods differs from a random process on the level of individual offspring (binomial, hypergeometrical, or Poisson). A review of the literature shows such null models are virtually always rejected, with often large effect sizes. We formulate an alternative null model, which assumes that 1) the number of EPC has a random (Poisson) distribution across females (broods) and that 2) the probability for an offspring to be of extrapair origin is zero without any EPC and increases with the number of EPC. Our brood-level model can accommodate the bimodality of both zero and medium rates of EPY typically found in empirical data, and fitting our model to EPY production of 7 passerine bird species shows evidence of a nonrandom distribution of EPY in only 2 species. We therefore argue that 1) dichotomy in extrapair mate choice cannot be inferred only from a significant deviation in the observed distribution of EPY from a random process on the level of offspring and that 2) additional empirical work on testing the contrasting critical predictions from the classic and our alternative null models is required.
Key words: extrapair copulation (EPC), likelihood, mate choice, null model, sexual selection.
Received 4 October 2006; revised 25 April 2007; accepted 16 May 2007.
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