Abstract
Of all the properties of individual animals of interest to comparative physiologists, age and stage of development are among the most consequential. In a natural population of any species, the survivorship curve is an important determinant of the relative abundances of ages and stages of development. Demography, thus, has significant implications for the study of comparative physiology. When Edward Deevey published his influential summary of survivorship in animal populations in the wild seven decades ago, he emphasized “serious deficiencies” because survivorship curves for natural populations at the time did not include data on the earliest life stages. Such data have accumulated over intervening years. We survey, for the first time, empirical knowledge of early-age survivorship in populations of most major animal groups in a state of nature. Despite wide variation, it is almost universally true that > 50% of newly born or hatched individuals die before the onset of sexual maturity, even in species commonly assumed to exhibit high early-age survivorship. These demographic facts are important considerations for studies in comparative and environmental physiology whether physiologists (i) aim to elucidate function throughout the life cycle, including both early stages and adults, or (ii) focus on adults (in which case early-age survivorship can potentially affect adult characteristics through selection or epigenesis). We establish that Deevey’s Type I curve (which applies to species with relatively limited early mortality) has few or no actual analogs in the real, natural world.
Highlights
In the study of comparative physiology, an important dimension is comparison of the sequential stages of postnatal development within species
For environmental physiologists and others interested in natural populations of a species, the survivorship curve provides invaluable information because it reveals the relative abundances of ages and stages of development
We have examined early-age survivorship data for populations of a wide diversity of animal species living in the wild in relatively untrammeled environments, always expressing survivorship relative to the numbers of individuals born or hatched
Summary
In the study of comparative physiology, an important dimension is comparison of the sequential stages of postnatal development within species. With these considerations in mind, we have surveyed what is known today about the early-age demography of animals in the wild This information is essential for environmental physiologists (physiological ecologists) and for all other comparative physiologists who focus on the on the relationships of animals to their natural environments. Two empirical studies of the early-age demography of Atlantic cod (Gadus morhua) carried out in the decades since Deevey’s publication reveal that the median age at death of cod hatchlings in natural populations is 5–10 days. To understand cod in natural populations, physiologists might adopt the “adult perspective” and exclusively study the properties of adults In this case, it would be important to remember that each adult (i) passed through a stage in the first weeks of its. We conclude that Deevey’s Type I curve (Fig. 1) is misleading and must be revised: there are probably no populations in the wild that adhere even approximately to the curve he drew
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