JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Wiley-Blackwell and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. ert, J. 1989. An analysis of demographic tactics in birds and mammals. -Oikos 56: 59-76.Using field data on fecundity, age at first reproduction and adult life expectancy, we reconsider the so-called r-K gradient by analyzing relationships between these three demographic parameters in 80 mammal species and 114 bird species. After the allometric effect of adult body weight is removed, the three variables remain correlated. The existence of demographic tactics which are independent of adult body weight is demonstrated by multivariate analyses of these variables. These analyses confirm the importance of ecological and phylogenetic constraints. The main structure is a time-scale gradient ranking species according to turn-over, both in birds and mammals. A second gradient ranking species according to iteroparity level appears significantly both in birds and mammals. In mammals, this pattern is related to patterns of parental investment. © OIKOS lationships are well supported for many biological characteristics such as gestation (incubation) time, age at first reproduction, fecundity, metabolic rate (Brody 1945, Sacher and Staffeldt 1974, Blueweiss et al. 1978, McNab 1980, Western and Ssemakula 1982, Clutton-Brock and Harvey 1983, Peters 1983, Calder 1984, Al-lain6 et al. 1987). About the second question, several authors have shown that covariations between some life history traits still exist after accounting for adult body weight (e.g. Stearns 1983 and Harvey and Zammuto 1985 for mammals; Harvey and Clutton-Brock 1985 for primates; Saether 1987 for passerines, Saether 1988 for European birds) but none has only considered simultaneously the basic demographic parameters: age at first reproduction, fecundity and adult life expectancy.The third question has been stated explicitly by Wittenberger (1981) without answering it.After investigating these questions with the help of multivariate analysis of bird and mammal data we discuss the effects of phylogeny and ecology in the elaboration of demographic tactics. OIKOS 56:1 (1989) Mammals 1 0.70 -0.55 0.79 1 -0.56 -0.77 -0.80 1 Data and methods DataThe published literature was reviewed for birds and mammals and demographic parameters collated at the species level. Because demographic parameters have been estimated on an annual basis, this data set includes only species in which the young do not breed within their half first year. Indeed, the large intra-annual variability of these parameters for species with reproduction during the first year prevents from defining parameters on an annual basis (see e.g. Eberhardt 1985). T...

Oli and Dobson proposed that the ratio between the magnitude and the onset of reproduction (F/ alpha ratio) allows one to predict the relative importance of vital rates on population growth rate in mammalian populations and provides a reliable measure of the ranking of mammalian species on the slow-fast continuum of life-history tactics. We show that the choice of the ratio F/ alpha is arbitrary and is not grounded in demographic theory. We estimate the position on the slow-fast continuum using the first axis of a principal components analysis of all life-history variables studied by Oli and Dobson and show that most individual vital rates perform as well as the F/ alpha ratio. Finally, we find, in agreement with previous studies, that the age of first reproduction is a reliable predictor of the ranking of mammalian populations along the slow-fast continuum and that both body mass and phylogeny markedly influence the generation time of mammalian species. We conclude that arbitrary ratios such as F/ alpha correlate with life-history types in mammals simply because life-history variables are highly correlated in response to allometric, phylogenetic, and environmental influences. We suggest that generation time is a reliable metric to measure life-history variation among mammalian populations and should be preferred to any arbitrary combination between vital rates.

In human populations, women consistently outlive men, which suggests profound biological foundations for sex differences in survival. Quantifying whether such sex differences are also pervasive in wild mammals is a crucial challenge in both evolutionary biology and biogerontology. Here, we compile demographic data from 134 mammal populations, encompassing 101 species, to show that the female’s median lifespan is on average 18.6% longer than that of conspecific males, whereas in humans the female advantage is on average 7.8%. On the contrary, we do not find any consistent sex differences in aging rates. In addition, sex differences in median adult lifespan and aging rates are both highly variable across species. Our analyses suggest that the magnitude of sex differences in mammalian mortality patterns is likely shaped by local environmental conditions in interaction with the sex-specific costs of sexual selection.