The annual testosterone profile in cooperatively breeding superb fairy-wrens, Malurus cyaneus, reflects their extreme infidelity

Superb fairy-wrens are cooperatively breeding birds that combine stable, so cially monogamous pair bonds and high levels of paternal care, with extreme levels of extra-pair mating and high levels of sexual competition. Our aim was to determine which testosterone correlates would prevail in such a lif e history that combines features that are conventionally associated with di vergent hormone profiles. Unlike the situation in other species with monoga mous pair bonds and high levels of paternal care, testosterone was elevated for a very long period of several months. During breeding there was a broa d peak in testosterone followed by a gradual decline: this resembles the pr ofile found in polygynous and promiscuous species. We found that three fact ors correlated with testosterone: development of the sexually selected nupt ial plumage, social status and extra-g-roup mating opportunities. Testoster one started increasing months prior to breeding, when the males that are la ter preferred as extra-group sires develop their nuptial plumage. Although these males did not have higher testosterone levels during breeding, they s ustained high testosterone for much longer, and this might lend reliability to this sexual signal. Dominant males in groups had higher testosterone th an pair-dwelling males and subordinate helpers. This was not due to differe nces in age, reproductive capability or mating opportunities, but was presu mably associated with the assertion of dominance. In contrast to findings i n other species, male testosterone level was not correlated with whether th e resident female was fertile or had dependent nestlings. However, testoste rone was strongly correlated with the total number of fertile females in th e population, and hence with the opportunities for extra-group mating.