Sexual selection and the intromittent organ of birds
Sexual selection and the intromittent organ of birds
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DOI：
10.2307/3677097
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发表时间：
1997-03-01
影响因子：
1.7
通讯作者：
Montgomerie, R
中科院分区：
文献类型：
--
作者：
Briskie, JV;Montgomerie, R
与大多数体内受精的动物不同，现存的鸟类很少有雄性插入器官 (10)。尽管之前发表的两个假设试图解释鸟类中罕见的 IO 存在，但系统发育证据表明，大多数鸟类中不存在 IO，这需要解释。先前的假设表明，当在水中发生交配时，IO 有助于最大限度地减少水对精子的损害（水损害预防假说），或者当雄性在交配过程中难以保持平衡时，IO 有助于两性保持生殖器接触（维持生殖器接触假说）。我们发现对这两个想法的支持有限，但也有很多例外，因此都不是特别引人注目。由于在几种不会飞的鸟类中发现了 IO，因此也有人认为丢失 IO 是为了降低飞行成本（飞行成本最小化假说），但运输 IO 的成本预计是最低的。相反，我们认为，IOs 可能已经丢失，以尽量减少在插入期间感染病原体和寄生虫的风险（避免性传播疾病假说）。与大多数其他恒温动物不同，鸟类有一个泌尿生殖系统和胃肠系统共有的泄殖腔。这可能使鸟类更容易感染性传播疾病。不幸的是，由于缺乏有关鸟类性传播病原体的比较信息，无法对这一想法进行令人信服的分析。我们还根据性选择导致的 IO 的成本和收益提出了两个新假设。一方面;当父亲投资较高时，IO 可能会受到青睐 - IO 可以通过确保精子充分沉积到女性生殖道中来增加男性对父权的信心（精子竞争假说）。这一观点得到了 IO 与孵化期间雄性父母照顾之间关联的支持。或者，我们认为雄性 IO 可能在鸟类中基本上消失，因为雌性偏爱没有 IO 的雄性（雌性选择假说），从而使雌性能够更好地控制受精。我们认为，当雌性有能力放弃由具有 IO 的雄性受精的卵子时，这种机制是合理的。这一假设得到了以下事实的支持：没有 IO 的物种通常比有 IO 的物种拥有相对较小的卵，而且可能成本更低。其他四足脊椎动物的初步数据也与雌性选择假说基本一致。尽管鸟类中 IO 的少量独立进化损失排除了对这些假设进行严格的统计检验，但最近发现的雄性生殖器中具有 IO 功能的二次修饰可能有助于我们了解鸟类 IO 在进化过程中保留或丢失的条件。
Unlike most animals with internal fertilisation, few extant bird species have a male intromittent organ (10). 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.