Mate rotation by female choice and coercive divorce in Nazca boobies, Sula granti

The formation and dissolution of mating pair bonds have been fruitful areas of investigation for evolutionary biologists. Adaptive mate choice has been the focus of most research on pair dissolution. However, recent work has shown that an important cause of pair dissolution can be involuntary forced divorce, where intruder(s) oust one or both partners. Previously, we presented evidence that, in a male-biased population, females exchanged a reproductively depleted male for a new ‘refreshed’ mate. This ‘mate rotation’ model of divorce could be driven by female choice, forced divorce by unpaired males, or by males forgoing reproduction to recover condition. Here, we examine these alternatives with behavioural and demographic data from our long-term study of banded Nazca boobies. The mate rotation pattern of divorce appears to be driven by a combination of two circumstances involving a female's abandonment of a previous partnership. In some cases, the female appeared to cooperate with a male intruder, causing her former partner to leave the nest. In other cases, the female abandoned her former mate and joined a new male at another nest site. Recent studies that have disentangled age and breeding experience from pair bond length have shown that an initial increase in reproductive output correlated with pair bond length, but after a period of time together the reproductive success of the pair declined. Given the assumed ubiquity of the cost of reproduction, divorce after a lengthy pair bond would seem to be advantageous for one or both mates when fresh potential partners are available.