Figure 9-1

Comparison between humans and captive rats in sex-specific reproductive senescence.

tion is such that oocytes would be available at least as long as animals could expect to live under natural conditions (that is, under the conditions that this physiology evolved) unless there were some adaptive advantage to ceasing reproduction before death.

Consistent with this logic, physiologically postreproductive individuals are very rarely found in nature except in a few species (described below). This is not to say that long intensive field studies will not occasionally identify postreproductive individuals (e.g., Waser, 1978), only that they should always be rare. A typical example is the study of great tits (Parus major) in Wytham Woods near Oxford, U.K., where analysis of demographic data collected over 18 years suggests that 7-year-old females and 8-year-old males may be physiologically postreproductive. But for every thousand females or males fledged, only four females and two males will survive to postreproductive ages, respectively (McCleery and Perrins, 1988).

By contrast, male gamete production is continuous throughout life. Therefore under captive conditions in which the major sources of natural mortality are removed as life span is much extended, it is not surprising that males continue to be reproductively competent somewhat longer than females.

While it is true that menopause in nonhuman primates occurs only near the end of life under captivity (van Wagenan, 1972; Nozaki et al., 1995), this may reflect more about the current state of primate husbandry than about differences among primates in reproductive cessation.

A type of postreproductive condition that is much more common in nature



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