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Sexual Selection and the Intromittent Organ of Birds

James V. Briskie and Robert Montgomerie
Journal of Avian Biology
Vol. 28, No. 1 (Mar., 1997), pp. 73-86
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3677097
Stable URL: http://www.jstor.org/stable/3677097
Page Count: 14
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Sexual Selection and the Intromittent Organ of Birds
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Abstract

Unlike most animals with internal fertilisation, few extant bird species have a male intromittent organ (IO). Although two previously published hypotheses have attempted to account for the rare presence of IOs in birds, phylogenetic evidence suggests that it is the absence of an IO in the majority of bird species that requires explanation. Previous hypotheses suggested that IOs help minimise water damage to sperm when copulation occurs on the water (Water Damage Prevention Hypothesis) or that IOs help the sexes stay in genital contact when males have difficulty balancing during copulation (Maintaining Genital Contact Hypothesis). We find some limited support for these two ideas but there are so many exceptions that neither is particularly compelling. Because IOs are found in several flightless birds, it has also been suggested that IOs were lost to decrease the costs of flight (Minimising Flight Costs Hypothesis), however the cost of transporting an IO is expected to be minimal. We suggest, instead, that IOs may have been lost to minimise the risk of contracting pathogens and parasites during intromission (Avoidance of Sexually Transmitted Disease Hypothesis). Unlike most other homeotherms, birds have a cloaca that is common to both the urogenital and gastrointestinal systems, which may make birds more susceptible to sexually transmitted diseases. Unfortunately, the lack of comparative information on sexually transmitted pathogens in birds prevents a convincing analysis of this idea. We also present two new hypotheses based on the costs and benefits of IOs as a result of sexual selection. On the one hand, IOs may be favoured when paternal investment is high - an IO can increase a male's confidence of paternity by ensuring that sperm are deposited well up into the female's reproductive tract (Sperm Competition Hypothesis). This idea is supported by an association between IOs and male parental care during incubation. Alternatively, we suggest that male IOs may have largely disappeared in birds because of female preference for males without IOs (Female Choice Hypothesis), thereby allowing females more control over fertilisation. We argue that such a mechanism is plausible when females can afford to abandon eggs fertilised by males with IOs. This hypothesis is supported by the fact that species without IOs generally have relatively smaller, and presumably less costly, eggs than species with an IO. Preliminary data from other tetrapod vertebrates are also largely consistent with the Female Choice Hypothesis. Although the small number of independent evolutionary losses of IOs in birds precludes rigorous statistical tests of these hypotheses, the recent discovery of secondary modifications in male genitalia that function as IOs may help us understand the conditions under which the avian IO has been retained or lost over evolutionary time.

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