ANATOMICAL ADAPTATIONS TO SPERM COMPETITION IN SMITHS LONGSPURS AND OTHER POLYGYNANDROUS PASSERINES

I compared the reproductive anatomy of the polygynandrous Smith's Longspur (Calcarius pictus) with two other polygynandrous passerines (Dunnock [Prunella modularis] and Alpine Accentor [P. collaris]) and with a wide range of socially monogamous species. All three polygynandrous species were found to have enlarged testes and cloacal protuberances (i.e. the site of sperm storage) compared to species with other mating systems. Testes lengths in polygynandrous species averaged 44% longer and cloacal protuberances 213% greater in volume than expected for the body sizes of these species. Testes mass in Smith's Longspurs comprised 4.2% of adult body mass or more than twice (2.0%) that found in the congeneric and monogamous Lapland Longspur (C. lapponicus). Smith's Longspurs also had larger cloacal protuberances, larger seminal glomera and higher sperm stores, suggesting a greater overall rate of sperm production than in Lapland Longspurs. In contrast, females of polygynandrous species did not show any consistent differences in the pattern of sperm storage due to increased sperm production by males, although this needs to be evaluated more thoroughly. The enlarged male reproductive organs of Smith's Longspurs and other polygynandrous species appear to have evolved as a consequence of sperm competition, whereby large sperm reserves function to insure paternity through diluting or displacing the ejaculates of rival males.