If conditions within an organism’s environment occasionally or regularly become harsh, it may be advantageous for an organism to have a resistant stage built into the life cycle. In such a life history strategy, the organism suspends any growth, reproduction, or other activities for a period of time so that they may occur at a later, more hospitable time. This genetically determined resting stage, characterized by the cessation of development and protein synthesis and suppression of the metabolic rate, is called diapause. Many other kinds of resting stages, with different levels of suppression of physiological activities, are known. Some of these resistant stages can be extremely long-lived. In one case, seeds of the arctic lupine, a member of the pea family recovered from ancient lemming burrows in the Arctic, germinated in three days even though they were carbon-dated at more than 10,000years old!

Unfavorable conditions that are relatively predictable probably pose a simpler problem for organisms than do unpredictable conditions. Adaptations to the regular change of seasons in the temperate and polar regions may be relatively simple. For example, many seeds require a period of stratification, exposure to low temperatures for some minimum period, before they will germinate. This is a simple adaptation to ensure that germination occurs following the winter conditions rather than immediately prior to their onset. In contrast, unfavorable conditions that occur unpredictably pose considerable problems for organisms. In fact, unpredictability is probably a greater problem than is the severity of the unfavorable period. How can organisms cope with the unpredictable onset of good or poor conditions?

Many adaptations to this general problem are based on a resting stage that awaits favorable conditions. We will consider two examples from the vertebrates. The first is the red kangaroo. This marsupial inhabits the deserts of central Australia where the onset of rains and the resulting sudden growth of vegetation are extremely unpredictable. Obviously, it is advantageous for a kangaroo female to produce young at a time when plant productivity is sufficient to support her offspring. For such a relatively large mammal, however, gestation (the period of development during pregnancy) is so long that if a female waited to mate and carry the young until after the rains came, the favorable period might be past. The kangaroo’s life history adaptation to this problem involves the use of embryonic diapause during gestation (development in the uterus).

After a 31-day gestation period, the female gives birth to a tiny helpless young typical of marsupials. The newborn crawls into the mother’s pouch and attaches to a teat where it continues to grow and develop. After 235 days it leaves the pouch but remains with the mother and obtains milk from her. Two days after giving birth, the female mates again. The fertilized egg enters a 204-day period of diapause during which it remains in the uterus but does not attach. It then implants, and 31 days later, birth of the second young occurs. Note that the first young leaves the pouch at just this time. Again, the female mates, fertilization occurs, and another diapause follows. The eventual result is that at any one time, the female has three young at various stages of development one in diapause, one in the pouch, and one outside the pouch. Among other benefits, this allows her to freeze the development of an embryo during times of drought and food shortage until the offspring in the pouch is able to leave.

A similar strategy – accelerated development combined with a resting stage – has also allowed amphibians to inhabit deserts. The spadefoot toads, such as Couch’s spadefoot toad, inhabit some of the most severe deserts in North America. Adults of this species burrow deeply into the substrate where it is cooler and perhaps more moist. Here they enter into a resting state in which they are covered with a protective layer of dead skin. When it rains, the adults emerge and congregate to mate at temporary ponds. Development is greatly accelerated: the eggs hatch within 48 hours, and the tadpoles change into toads at 16- 18 days. Consequently, they can complete the life cycle during the brief window of favorable conditions, then return to the resistant resting stage to await the next rainfall. Resting stages thus comprise a series of adaptations that allow the species to avoid the most difficult conditions for life.

如果生物体生活环境的条件偶尔或频繁地变得恶劣，那么生物体在生命周期内建立一个抵抗期可能是有利的。在这样的生活史对策中，生物体会将任何生长，繁殖或其他活动暂停一段时间，以便在以后更为适宜的时间内进行。从遗传学角度来说，这决定了以停止发育、蛋白质合成以及抑制代谢率为特征的休眠期，称之为滞育。众所周知的是，许多其他类型的休眠期，会存在不同程度的生理活动抑制。其中一部分的抵抗期可能会历时非常长。在一个案例中，北极羽扇豆的种子是一种豆类作物，它可以从北极的古代旅鼠洞穴中恢复过来，虽然其碳素测定年代已超过1万年，但在可以在三天内发芽！ 对生物体来说，与不可预知的情况相比，相对可预测的不利情况可能会更容易解决。适应温带和极地地区季节变化可能会相对容易一些。例如，许多种子需要一段时间来分层，在低温下暴露一段时间，然后才会发芽。这是一个简单的适应过程，以确保在冬季之后发芽，而不是在冬季之前突然发芽。相反，不可预测的不利条件则会给生物体造成一个相当大的问题。事实上，相比于不利时期的严重程度，不可预测性可能是一个更大的问题。生物体应该如何应对不可预知的有利或不利情况的发生呢？ 对这个问题来说，许多适应策略都是依赖于这个休眠期，以等待有利条件的发生。我们可以看一看脊椎动物的两个例子。第一个是红袋鼠。这种有袋动物栖息在澳大利亚中部的沙漠地区，那里的降雨，以及由此引发的植被的突然增长是根本无法预测的。显然，雌性袋鼠在植物生产力足以支持其后代生长的时候，有利于其产下幼仔。然而，对于这样一个体积相对较大的哺乳动物来说，妊娠（怀孕期间的发育期）是非常漫长的，以至于如果一位雌性袋鼠直到降雨降临时，才等待交配并产下幼仔的话，这个有利时期可能已经过去了。袋鼠为适应这个问题，采取的方法包括在妊娠期间使用胚胎滞育（在子宫中发育）。 经过31天的妊娠期后，雌性袋鼠产下了一个典型的有袋类幼崽，弱小且无助。小幼崽爬到其母亲的袋子中，叼着乳头，继续生长发育。 235天后，它离开袋子，但仍然与母亲生活在一起，并从母亲处获得乳汁。分娩两天后，雌性会再次交配。受精卵开始进入一个204天的滞育期，在此期间受精卵仍留在子宫内，但并不附着。然后受精卵植入，31天后，产下第二个幼崽。请注意，第一个幼崽会在这个时候离开袋子。再一次，雌性交配，完成受精，并且进入下一个滞育期。最终的结果是，在任何时候，雌性都有三个处于不同发育阶段的幼崽，一个在滞育期，一个在袋子里，另一个在袋子外面。除了其他好处之外，这种方法可以让她在干旱和食物短缺时冻结胚胎的发育，直到袋中的幼崽能够离开。 一个类似的策略是 - 与休眠期相结合来加速发育速度- 这种方法也可以使两栖动物栖息在沙漠中。锄足蟾，如Couch中的锄足蟾，栖息在北美洲一些最干旱的沙漠中。这个物种中的成年动物栖息于基质深处，这里更为凉爽，湿润。在这里，他们进入了休眠期，在这个休眠的状态下，他们给自己的身体覆盖上一层死皮。下雨时，成年动物会出现，并聚集在临时的池塘里，进行交配。其生长速度大大加快：卵在48小时内孵化，蝌蚪在16-18天变成蟾蜍。因此，他们可以在有利条件发生的短暂时间内完成生命周期，然后返回到持久的休眠期中，等待下一次降雨。因此，休眠期包括一系列适应方法，以帮助物种躲避生命中最困难的条件。