Male spotted bowerbirds (Ptilonorhynchus maculatus) build and defend a structure of sticks and straw – the bower. They decorate these nests with colourful objects to attract mates during the breeding season. Certain non-resident subordinate males are tolerated by resident males in their bowers over multiple breeding seasons. Previous research has shown that these male coalitions bring indirect benefits to subordinate males. So far, however, it has been unclear whether lower-ranking males also have direct advantages. Our present study shows that in rare cases the lower-ranking birds benefit directly from copulation opportunities.
The cases were observed in the bowers of spotted bowerbirds during the 2018 breeding season. Several non-resident males disrupted ongoing copulations between the bower-owner and a receptive female, and these events were followed by vigorous aggressive interactions. These observations support the hypothesis that subordinate males are sexually mature individuals who occasionally gain access to females while visiting established bowers.
First observation of extremely rare events
The rarity of such events is remarkable. Extensive observations have been made on spotted bowerbirds for several decades – but so far, no attempted copulation from subordinate males had been documented. The recording of four independent observations in different individuals strongly indicates that sneaky copulations are not an isolated and abnormal behaviour but rather a behavioural pattern or alternative reproductive strategy used by subordinate males.
Beta profits from Alpha – male coalitions are profitable
Male-male coalitions have so far been observed particularly in birds such as manakins, grouse, peacocks, wild turkeys and bowerbirds. A common feature of most courtship coalitions is that a dominant “alpha” male accounts for all or most copulations, while subordinate “beta” males abstain from breeding and have no—or very limited—access to mates. Sacrificing reproductive potential for a male association may seem paradoxical, but it has direct and indirect benefits for the subordinate males. The animals benefit indirectly, for example, from taking over the position of the alpha male after his death or from learning behaviour that is important for successful mating from him. As it turns out, they also derive direct benefits from clandestine mating with females.
The article “Sneaky copulations by subordinate males suggest direct fitness benefits from male-male associations in spotted bowerbirds (Ptilonorhynchus maculatus)” by Giovanni Spezie and Leonida Fusani was published in Ethology. The full text can be downloaded here.
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