Pen I, Weissing F J
Department of Genetics, University of Groningen, Haren, The Netherlands.
Proc Biol Sci. 2000 Mar 22;267(1443):539-43. doi: 10.1098/rspb.2000.1034.
In many cooperatively breeding animals, offspring produced earlier in life assist their parents in raising subsequent broods. Such helping behaviour is often confined to offspring of one sex. Sex-allocation theory predicts that parents overproduce offspring of the helping sex, but the expected degree of sex-ratio bias was thought to depend on specific details of female and male life histories, hampering empirical tests of the theory. Here we demonstrate the following two theories. (i) If all parents produce the same sex ratio, the evolutionarily stable sex ratio obeys a very simple rule that is valid for a general class of life histories. The rule predicts that the expected sex-ratio bias depends on the product of only two parameters which are relatively easily measured: the average number of helping offspring per nest and the relative contribution to offspring production per helper. (ii) If the benefit of helping varies between parents, and parents facultatively adjust the sex ratio accordingly, then the population sex ratio is not necessarily biased towards the helping sex. For example, in line with empirical evidence, if helpers are produced under favourable conditions and parents do not adjust their clutch size to the number of helpers, then a surplus of the non-helping sex is expected.
在许多进行合作繁殖的动物中，早年出生的后代会协助父母养育随后出生的幼雏。这种帮助行为通常仅限于某一性别的后代。性分配理论预测，父母会过度繁殖具有帮助行为的性别的后代，但人们认为预期的性别比例偏差程度取决于雌性和雄性生活史的具体细节，这妨碍了对该理论的实证检验。在此，我们阐述以下两种理论。（i）如果所有父母产生相同的性别比例，那么进化稳定的性别比例遵循一条非常简单的规则，该规则对一般类型的生活史均有效。该规则预测，预期的性别比例偏差仅取决于两个相对容易测量的参数的乘积：每个巢穴中具有帮助行为的后代的平均数量以及每个帮助者对后代生产的相对贡献。（ii）如果帮助行为的益处因父母而异，且父母会相应地灵活调整性别比例，那么种群的性别比例不一定会偏向具有帮助行为的性别。例如，根据实证证据，如果在有利条件下产生帮助者，且父母不根据帮助者的数量调整其窝卵数，那么预计会出现非帮助性别的过剩。
