ADAPTIVE LIMITATION OF THE REPRODUCTIVE RATE OF BIRDS
ADAPTIVE LIMITATION OF THE REPRODUCTIVE RATE OF BIRDS
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鸟类繁殖率的自适应限制
DOI：
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发表时间：
2008
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影响因子：
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通讯作者：
A. Skutch
中科院分区：
文献类型：
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作者：
A. Skutch
总结
有些生物学家认为，鸟类的繁殖率往往与年平均死亡率相适应;另一些生物学家则认为，繁殖率只受父母养育强壮后代的能力的限制。后一种理论，即最大繁殖理论，只有在以下三个相关命题为真的情况下才可能为真：（1）窝卵数的增加比影响繁殖率的其他突变更可能发生;（2）具有浪费性高繁殖率的基因型可以取代具有更保守但足够繁殖率的基因型;（3）过度繁殖率对物种无害。这些假设都没有得到证实。
在一个没有义务充分利用其生殖潜力来维持自身处于有利水平的种群中，限制这种潜力的突变可能会出现并持续存在。它们可能通过以下方式影响这一限制：（一）减少窝卵数;（二）减少窝卵数;（三）雄性不守巢，通常随后不成对;（四）推迟生殖成熟;（五）限制筑巢鸟数量或它们可以养育的后代数量的领土主义的发展;（六）将筑巢限制在传统的地点;以及（vii）耗时的精心筑巢。繁殖率的这些限制必须被看作是适应性的，因为，像其他适应一样，它们使鸟类更完美地适应它们所生活的条件，并减少它们所受到的压力。
在热带和温带地区，只有雌鸟喂养雏鸟的物种的育雏和双亲都喂养的物种的育雏一样大。由此可见，这对父母并没有养育出他们所能养育的那么多的后代。
洞巢鸟比开放巢鸟能养育更多的窝，因为它们的幼崽发育得更慢，每天人均需要的食物更少，这种观点是站不住脚的。在洞穴中长大的雏鸟和在暴露的巢穴中长大的雏鸟一样快，但为了安全，它们在受保护的住所中呆的时间更长。洞巢鸟的大窝显然弥补了获得巢址的困难，这推迟了一些对的繁殖，并阻止了其他对的繁殖。
窝卵的大小绝不是根据父母所能抚养的幼仔数量来调整的。如果有额外的雏鸟，有些鸟会充分照顾它们。在其他物种中，幼鸟很少从所有的卵中长出羽毛。在许多情况下，不同步孵化并不像人们所声称的那样，是一种允许父母根据不同的食物供应来调整他们所抚养的幼仔数量的安排。在许多猛禽中，兄弟相残和同类相食会减少幼仔的数量，有时甚至只剩下一只雏鸟，而不管食物是否充足。
环境越稳定，生育率就越倾向于与死亡率相适应;一个种群遭受灾难性的减少越多，生育率就越接近最大值。
首先，繁殖率是由遗传性状控制的，这些性状可以调节繁殖率以适应稳定的环境，但很少对外部条件或种群密度的短期波动做出反应。种群对其栖息地的最后一次微调是受密度依赖性过程的影响的：要么是密度依赖性的生殖努力调节，要么是密度依赖性的成年或年轻的死亡率，或者是两者的组合。
后生动物的一般进化趋势是产生更少的后代，并更好地照顾他们。如果多产的基因型总是胜过那些养育较小家庭的基因型，从而能够更好地照顾它们的后代，那么这几乎是不可能的。繁殖率的调节是一个独特的进化问题，因为赋予更高生育力的突变，尽管通常对物种有害，但往往会通过它扩散，而其他有害突变则无法做到。然而，它受到许多因素的抵消，主要是生态因素，这些因素的作用微妙，比数字的力量更难理解。
Summary
Some biologists have held that the rate of reproduction in birds tends to be adjusted to the average annual mortality; others, that it is limited only by the parents' ability to raise sturdy young. The latter theory, that of maximum reproduction, is likely to be true only if three related propositions are true:(1) that an increase of clutch size is more likely to occur than some other mutation affecting the rate of reproduction; (2) that a genotype with a wastefully high rate of reproduction can supplant a genotype with a more conservative but adequate rate; and (3) that an excessive rate of reproduction is not harmful to the species. None of these assumptions has been proved.
In a population not obliged to employ its full reproductive potential to maintain itself at a favourable level, mutations which limit this potential may arise and persist. They may effect this limitation by means of:(i) reduction in clutch size; (ii) reduction in the number of broods; (iii) failure of the male to attend the nest, often followed by failure to form pairs; (iv) deferment of reproductive maturity; (v) developments in territorialism that limit the number of nesting birds or the number of progeny they can rear; (vi) restriction of nesting to traditional sites; and (vii) the time-consuming construction of elaborate nests. These limitations of the rate of reproduction must be regarded as adaptive because, like other adaptations, they adjust the birds more perfectly to the conditions in which they live and reduce the stress to which they are subjected.
In both tropical and temperate regions, species in which only the female feeds the nestlings have broods as large as species in which both parents feed them. It follows that the two parents are not rearing as many young as they could nourish.
The view that hole-nesting birds can rear larger broods than open-nesters because their young develop more slowly, and require less food per capita per day, is untenable. Nestlings raised in holes and burrows gain weight about as rapidly as those in more exposed nests, but for safety they remain longer in their protected abodes. The larger broods of hole-nesters evidently compensate for the difficulty of obtaining nest sites, which delays the breeding of some pairs and prevents that of others.
Clutch size is by no means closely adjusted to the number of young the parents can raise. If given additional nestlings, some birds attend them adequately. In other species, young are rarely fledged from all the eggs. In many cases, asynchronous hatching is not, as has been claimed, an arrangement which permits the parents to adjust to a varying food supply the number of young that they rear. In many raptors, fratricide and cannibalism reduce the size of the brood, sometimes to a single nestling, regardless of the abundance of food.
The more stable the environment, the more closely the reproductive rate tends to be adjusted to the mortality; the more a population is subject to catastrophic reductions, the more the rate will approach the maximum.
Primarily, the reproductive rate is controlled by heritable characters, which can adjust the rate to a stable environment but rarely respond to short-term fluctuations in external conditions or population density. The last fine adjustment of a population to its habitat is effected by processes that are density-dependent:either density-dependent regulation of the reproductive effort, or density-dependent mortality of adults or young, or a combination of the two.
The general evolutionary trend in the Metazoa is toward producing fewer offspring and taking better care of them. This would hardly be possible if the more prolific genotype always prevails over those which raise smaller families and in consequence can attend their young somewhat better. The regulation of the rate of reproduction is a unique evolutionary problem, because a mutation conferring greater fertility, although often detrimental to the species, tends to diffuse through it as no other harmful mutation can. Yet it is counteracted by many factors, chiefly ecological, which operate subtly and are more difficult to appreciate than the force of numbers.