Cloacal protuberances and extreme sperm production in Australian fairy-wrens

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Abstract
In many passerine species males have enlarged cloacal protuberances during the breeding season. One hypothesis for the evolution of cloacal protuberances posits that they are a response to sperm competition and thus predicts that both within and between species the size of the protuberance correlates with the number of sperm stored. Here we provide the first intraspecific test of this hypothesis. In Australian fairywrens (Aves: Maluridae) females regularly mate outside of their social group resulting in intense sperm competition among males. Male fairy-wrens develop enlarged cloacal protuberances, and in a study of three species, splendid fairy-wren, white-winged fairy-wren, and variegated fairy-wren, we found significant intraspecific correlations between the size of a male's protuberance and the stored sperm reserves in two of the three species. Males of these species had extreme numbers of sperm in their cloacal protuberances, up to 8.3 billion for splendid fairy-wrens, which should be available for a single ejaculate and is the most ever reported in an avian species. Studies of both captive and wild males showed that individuals can produce as many as 2 billion sperm per day. These data support the sperm competition hypothesis for the evolution of cloacal protuberances and highlight extreme sperm production as one possible outcome of sperm competition.