Patterns of mortality and age at first reproduction in natural populations of mammals

Delayed male maturity is a cost of producing large sperm in Drosophila

Among fruit-fly species of the genus Drosophila there is remarkable variation in sperm length, with some species producing gigantic sperm (e.g., > 10 times total male body length). These flies are also unusual in that males of some species exhibit a prolonged adult nonreproductive phase. We document sperm length, body size, and sex-specific ages of reproductive maturity for 42 species of Drosophila and, after controlling for phylogeny, test hypotheses to explain the variation in rates of sexual maturation. Results suggest that delayed male maturity is a cost of producing long sperm. A possible physiological mechanism to explain the observed relationship is discussed.

Adult life span as a function of age at maturity

A regression analysis was made of age at first reproduction in female mammals, as a function of body weight, using the data of Wootton. Data on maximal life span, also expressed as a function of body weight, were used to calculate "adult" life span, wherever possible, by subtracting the cognate value for age at first reproduction. Then a regression analysis of adult life span as a function of age at first reproduction was made. In both cases global regression lines (i.e., for whole data sets) were computed by standard least squares and by a robust method, as well as local regression lines for subgroups classified by taxonomic and ecological criteria. The slopes of the various regression lines were found to vary widely as a function of the method of classification. This result argues against the notion that the ratio of life history variables is a constant, or that one life history variable is likely to be a simple function of another. The results for bats are anomalous, in that age at first reproduction appears to be independent of body weight (over about two orders of magnitude). It is concluded that a full understanding of life history variables, such as maximal life span and age at maturity, is likely to depend on combined physiological, ecological, and evolutionary insights.

Evolutionarily stable age at first reproduction in a density-dependent model

We develop a new model of life history evolution to investigate the evolution of age at first reproduction. Density dependence is taken into account. For a given "species", age of maturity, offspring survival, immature survival, adult survival, fecundity, immature age-classes entering in competition with adults and immature competitive ability are traits adjustable by natural selection, and constitute a particular strategy. On the contrary, the type of intraspecific competition (scramble or contest), strength of competition and inherent net reproductive rate Ro(inh) are fixed (specific) characteristics. As a consequence of fixing Ro(inh), the evolution of any trait will affect trade-offs between others. Evolutionarily stable strategies are determined numerically by using the mathematical concept of Lyapunov exponents. Altogether, we consider 960 different hypothetical "species" (i.e. different combinations of fixed traits). Corresponding ESSs are analyzed with respect to their age at first reproduction, adult survival and immature competitive ability components. They appear to be gathered in three groups. One is intuitive and characterized by a reduction of immature competitive ability and a correlation of age of maturity with adult survival; populations reach mainly equilibria. The two other groups respectively include "species" with low age of maturity but high adult survival, and "species" close to semelparity with delayed maturity; immature competitive ability may not be minimized, and populations possibly exhibit complex dynamics. In conclusion, the hypothesis that the evolution of a demographic parameter modifies trade-offs between others turns out to have important consequences. We argue that life history theory cannot ignore the source and mode-of-operation of density dependence and must regard potential short-term instability as essential.

Evolution of life history variation among female mammals

A unified approach is developed for the evolutionary structure of mammalian life histories; it blends together three basic components (individual growth or production rate as a function of body size, natural selection on age of maturity, and stable demography) to predict both the powers and the intercepts of the scaling allometry of life history variables to adult size. The theory also predicts the signs (+, -) of the correlations between life history variables when body size is held constant. Finally, the approach allows us to eliminate body size to predict the dimensionless relationships between the life history variables themselves.

Is the snark still a boojum? The comparative approach to reproductive behavior

One of Frank Beach's many achievements was his stimulating influence on the comparative study of behavior. This review honors that legacy by categorizing and describing the many kinds of comparative approaches in use today for the study of reproductive behavior. The categorization is based on the motives and goals of the researcher, the kinds of questions that can be answered, the number and phylogenetic relatedness of the species being compared, and the method used for analyzing the results. Each approach is illustrated with specific examples from recent research, using studies from the field of hormones and behavior whenever possible.

Menarche, menopause and reproduction in the Kipsigis of Kenya

Among the Kipsigis, a population of south-western Kenya who do not use contraception, age at menarche and age at last live birth could be determined for a cohort of post-menopausal women, through reference to clitoridectomy ceremonies that can easily be dated. While a woman's age at last live birth was strongly associated with the length of her reproductive lifespan, completed family size was better predicted by age at menarche. The demographic implications of variation in menarcheal age are discussed.

Pattern of covariation between life-history traits of European birds

A large amount of variation is found in most reproductive traits of birds. Clutch size for instance, can vary from 1 to 15 between species of similar body weight. The adaptive significance of this variation is only poorly understood. According to life-history theory, large clutch size and early onset of reproduction are expected when the chances of survival are low. There is some support for the existence of such a relationship from studies of single species. Here I present evidence that, in European birds, clutch size is increased, and onset of reproduction occurs earlier in life, when the probability of survival is low.

Puberty, gonadal steroids and fertility: potential reproductive markers of aging

Three potential predictors (i.e., biomarkers) of longevity in mammals are proposed: (1) age of pubertal onset, (2) concentrations of gonadal steroids and (3) timing of age-related infertility. Ages of pubertal onset and of declining fertility are hypothesized to be positively correlated with longevity. Concentrations of and androgens and estrogens are proposed to be inversely and positively correlated, respectively, with life span. Evidence upon which these hypotheses are based is reviewed and its limitations are discussed. General strategies for testing these hypotheses are outlined. In addition, several pragmatic as well as theoretical issues facing research on biomarkers of aging are discussed, using specific examples derived from studies of female reproductive aging.

A review of why and how we age: a defense of multifactorial aging

Part 1: Longevity is optimized such that reproduction is maximized. Williams (Evolution, 11 (1957) 398-411) proposed pleiotropic genes with beneficial effects during youth and harmful effects at older ages. Because of environmental death (e.g. predation, disease, accidents), even a small increase in younger reproduction could outweigh a large harmful effect at older ages. Guthrie (Perspect. Biol. Med., 12 (1969) 313-324) and Kirkwood (Nature, 270 (1977) 301-304; New Sci., 81 (1979) 1040-1042; Physiological Ecology: An Evolutionary Approach, Blackwell, Oxford, 1981, pp. 165-189; Hum. Genet., 60 (1982) 101-121; Proc. R. Soc. Lond., B205 (1979) 531-546; Handbook of the Biology of Aging, 2nd Edn., Von Nostrand Reinhold New York, 1985, pp. 27-44) proposed that additional longevity requires a further investment of resources when young, thereby reducing the resources available for reproduction when young. The gene(s) controlling this partitioning of resources between younger and older reproduction are a good example of Williams's pleiotropic genes. Population biology provides a great deal of evidence of a tradeoff between younger and older reproduction. A "marginal longevity theorem" is proposed which states that for a population in equilibrium with its environment a marginal change in any gene affecting longevity should cause equal and opposite marginal changes in younger and older expected reproduction. Senescence is not irrelevant in the wild; rather, the amount of senescence in the wild results from a balance between its marginal costs to older reproduction and its associated marginal benefits in younger reproduction. Part 2: The wide variety of damage prevention processes in the body are subject to the problem of diminishing returns. Consequently, a broad spectrum of damage occurs in the body, varying in frequency, harmfulness, and ease of repair. The types of damage which are prevented or repaired tend to be more frequent, harmful, and easily prevented or repaired. In contrast, aging damage (which accumulates) consists of a large number of different types of damage which (when considered separately) are relatively infrequent, less harmful, and/or more difficult to repair. Only when these types of damage accumulate to become very numerous do they (when considered collectively) become significant. Since the selective advantages associated with senescence apply to all parts of the body, primary aging damage occurs in all tissues, cells, and subcellular organelles. The distribution of metabolic resources among the various damage repair and prevention processes is optimized.(ABSTRACT TRUNCATED AT 400 WORDS)