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Life histories

About mammals > Reproduction of mammals

All of the basic parameters that can be identified in the life cycle of any given mammal species, such as gestation period, litter size, lactation period, time taken to reach sexual maturity, and life span (longevity), contribute to its overall life history. Numerous lines of evidence suggest that these individual components of the life history of a species together constitute an adaptive complex that has been shaped by natural selection. For instance, one fundamental finding is that species that are subject to relatively heavy mortality under natural conditions tend to breed earlier.

Because early breeding typically translates into a higher reproductive turnover, it can be concluded that heavy mortality promotes rapid breeding. In such comparisons between species, it has become commonplace to refer to “reproductive strategies.” However, this is an anthropomorphic term and it is perhaps better to use a more neutral term like “life-history pattern.”

In examining life-history patterns across species, it is essential to take account of the scaling influence of body size. It is only to be expected that large-bodied species will breed more slowly than small-bodied species, as it is generally likely that the time taken to grow to maturity will increase with increasing body size. However, it is also possible for mammals to show divergent life-history patterns even when body size is the same. David Western has aptly referred to these two kinds of difference as “first-order strategies” and “second-order strategies.” For any given animal population, increase in population size is typically geometric until the population reaches a particular level (carrying capacity) determined by limiting factors (e.g. food supply, predation) in its environment. During the geometric phase of population growth, population increase depends on the intrinsic rate of increase (r) that is permitted by the reproductive parameters of the species.

Because it is very difficult and time-consuming to obtain field data on the intrinsic rate of increase for natural populations, especially for large-bodied species, comparisons are often based on the maximum possible intrinsic rate of increase (rmax) that is permitted by standard reproductive values. A number of basic parameters such as age at sexual maturity, gestation period, litter size, interbirth interval, and longevity are used to calculate the rmax value for each species. As expected, for mammals rmax generally declines with increasing body size. In addition, there are distinctions between groups in the value found at any given body size. Primates, for example, generally have markedly lower values of rmax than other mammals. The two parameters that have the greatest influence in the calculation of rmax values are litter size and age at sexual maturity. Hence, the distinction between altricial and precocial mammals, in which litter size is a crucial feature, is clearly connected to a divergence in life-history patterns.

Various attempts have been made to develop general theories to explain the evolution of life-history patterns in mammals and other organisms. One basic problem that is encountered is that differences found between species do not fit well with the patterns that are observed within species, For instance, within a mammal species a particularly long gestation period is typically associated with a decrease in the duration of postnatal growth. In comparisons between species, however, it is found that species with long gestation periods tend to have extended periods of postnatal growth as well. It is therefore unclear how natural selection can shape life-history patterns. One model that has been suggested is “r- and K-selection.” In this, it is proposed that species living in unpredictable environments with occasional catastrophic mortality will be subject to selection to increase rmax (r-selection). By contrast, species that live in predictable environments with moderate mortality will be subject to selection to increase competitiveness at carrying capacity (K-selection). An alternative model is that of “bet-hedging,” in which it is suggested that high mortality among juveniles will favor slow breeding whereas high mortality among adults will favor rapid breeding. In fact, neither of these models fits all of the facts, so a convincing overall model remains elusive.

One point that deserves special mention is a potential link between life span (longevity) and relative brain size. Various authors have reported that mammals with relatively large brains tend to have particularly long life spans. Although this proposed link is controversial, especially because of claims that it may be based on a secondary correlation, there is certainly enough evidence to indicate that some kind of connection exists. Hence, slow-breeding, long-lived mammals may also have relatively large brains as part of their overall life-history patterns.

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