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I welcome Emlen, Reeve and Keller's recognition that it is difficult to distinguish between cases where subordinates reproduce because dominants are unable to prevent them from doing so and cases where dominants allow subordinates a share of reproduction to induce them to remain in the group and assist in future breeding attempts [as optimal skew models (OSMs) suggest]. I am also in full agreement with them that the presence of overt conflict does not contradict the predictions of OSMs, since dominants might use the responses of subordinates to assess the magnitude of incentives necessary to retain their services. Indeed, as I make this point on page 289 of my review[1xClutton-Brock, T.H. Trends Ecol. Evol. 1998; 13: 288–292Abstract | Full Text | Full Text PDF | PubMed | Scopus (209)See all References[1], I am uncertain whose misunderstandings it is important to correct.However, I believe that the burden of evidence is on those wishing to interpret subordinate breeding as the outcome of reproductive contracts between dominants and subordinates to show that it is not merely the result of a failure of control. This is partly because there is plenty of evidence that dominant animals do not exert full control over subordinate reproduction in societies where subordinates do not assist (and OSMs do not apply). In addition, OSMs rely on a range of untested assumptions, including the assumptions that subordinates that are allowed to breed are less likely to challenge the dominant and that the benefits of retaining a subordinate (to the dominant) exceed the costs of allowing them to breed[1xClutton-Brock, T.H. Trends Ecol. Evol. 1998; 13: 288–292Abstract | Full Text | Full Text PDF | PubMed | Scopus (209)See all References[1].I am also sceptical that testing predictions about the distribution of reproduction will ever provide unambiguous evidence of the kind of social contract envisaged by concession models. Many of the same social and environmental variables are likely to affect both the costs and benefits of attempting to breed (to subordinates) and the benefits of granting reproductive benefits (to dominants) so that firm, contrasting predictions are uncommon. Moreover, the OSMs and incomplete control models (ICMs) published by Emlen and his co-workers represent a small fraction of the range of credible models that could be built to predict skew[2xCant, M.A. Anim. Behav. 1998; 55: 163–169Crossref | PubMed | Scopus (101)See all References, 3xSee all References], so that it is usually possible to suggest reasonable modifications to OSMs and ICMs that are likely to affect their predictions. The most convincing evidence of reproductive contracts of the kind envisaged by OSMs will come from studies that manipulate the benefits of retaining subordinates to individual dominants and show that this affects their tolerance of breeding subordinates. Only direct evidence of changes in the behaviour of dominants is, I believe, likely to show unequivocally whether reproductive contracts occur. My own guess is that, among vertebrates, reproductive contracts will prove to be rare among females but more common between cooperating males[1xClutton-Brock, T.H. Trends Ecol. Evol. 1998; 13: 288–292Abstract | Full Text | Full Text PDF | PubMed | Scopus (209)See all References[1]. |
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