Comparative Demography of Life Span

The literature contains descriptions of only a small number of life span correlates, including the well known relationship between life span and both body mass and relative brain size (1,5-8), and the observation that animals that possess armor (e.g., beetles, turtles) or capability of flight (e.g., birds, bats) are often long lived (9). But, major inconsistencies exist within even this small set of correlates. For example, there are several exceptions regarding the relationship of extended longevity and large body size (e.g., bats are generally small but most of their species are long lived) and this positive relationship may be either absent or reversed within orders. Likewise, the observation that flight ability and extended longevity are correlated does not provide any insight into why within-group (e.g., birds) differences in life span exist, nor does it account for the variation in longevity in insects where adults of the majority of species can fly.

An alternative approach for identifying broad correlates of longevity emerged from an examination of several large-scale databases containing the maximum recorded life spans of both vertebrate and invertebrate species (10-12). Many long-lived species across a wide taxonomic spectrum appear to cluster within one of two general ecology and/or life history criteria:

1. Species that live either in unpredictable environments (e.g., deserts) or where food resources are scarce (e.g., caves; deep water, etc.)

2. Species that exhibit extended parental care and/or live in groups with complex or advanced social behavior (Chapter 2)

These criteria led to a classification system regarding the life span determinants of species with extended longevity (Table 1) that we believe is general and applies to a wide range of invertebrate and vertebrate species:

1. Environmentally selected. This category includes animals whose life histories evolved under conditions in which food is scarce and where resource availability is uncertain or environmental conditions are predictably adverse part of

TABLE 1 The Two General Categories of Factors That Favor the Evolution of Extended Life Span and Examples of Species Within Each

Category

Examples

Environmentally selected

Tortoises, sea turtles, deep-

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