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.

Climatic influences on animal populations, mediated by changes in condition‐dependent survival or reproduction, have long intrigued ecologists. We analyzed links between winter North Atlantic Oscillations (NAO), a large scale climatic phenomenon affecting weather conditions over the North Atlantic and the Arctic, and average pre‐laying body mass in common eiders. Body mass is a good proxy for condition‐dependent reproductive output in this species. Time series links were assessed for two eider populations breeding at high latitudes, over a 10‐ and a 21‐year time series. Winter NAO affected body mass in both populations and these effects were easier to detect when changes in the series rhythm were assessed using a novel method based on data discretization and information theory, rather than detection based on changes in amplitude, assessed using traditional linear models. Winter conditions affected body condition of eiders in both populations. Different mechanisms, however, are likely to be involved in the two populations, one being presumably affected by direct effects of climate and the other by effects through the food chain. Therefore, the same species can respond along different pathways to the same large scale climatic pattern, an important consideration when seeking to understand or manage the response of species to present and future climate change.

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From the long-term monitoring of isard females ( Rupicapra pyrenaica pyrenaica Bonaparte, 1845) marked in two areas of the Parc National des Pyrénées (France) with a quite constant population size for 20 years, we have conducted an analysis of age variation in recruitment (estimated as the proportion of offspring surviving through the winter per female). As predicted for a well-established high-density population, recruitment rates were low for young females (0.156 at 3 years of age and 0.221 at 4 years of age), but also for prime-aged females (0.414 per year for 5–16 year old females). No female older than 16 that we monitored recruited in any year. Recruitment rates were highly variable among females, among years (0.183 in 2002 vs. 0.635 in 1996 for prime-age females), and among areas (0.562 kid at Mayouret vs. 0.359 at Péguère). The use of an age-structured demographic model allowed us then to compare population dynamics between the well-established isard population in the Park and the colonizing population of isards at Bazès, in the eastern part of the Pyrénées. As predicted, the natural rate of increase of the isard population was much higher at Bazès (1.262) than in the Park (1.037). Likewise, the age structure of isard in the Park markedly differed from that obtained at Bazès (50% of females were younger than 5 years of age, and 19% were older than 10 years of age in the Park versus 67% and only 3.6%, respectively, at Bazès). Such differences in both population growth and age structure between isard populations with contrasted demographic regimes were well illustrated by marked differences in generation time (4.97 years at Bazès vs. 8.25 years in the Park). Our comparative study therefore demonstrates that generation time can vary markedly in relation to the demographic status within a given species.

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Body size variation is an enigma. We do not understand why species achieve the sizes they do, and this means we also do not understand the circumstances under which gigantism or dwarfism is selected. We develop size-structured integral projection models to explore evolution of body size and life history speed. We make few assumptions and keep models simple: all functions remain constant across models except for the one that describes development of body size with age. We set sexual maturity to occur when size attains 80% of the asymptotic size, which is typical of a large mammal, and allow negative density dependence to only affect either reproduction or juvenile survival. Fitness -the quantity that is maximized by adaptive evolution -is carrying capacity in our models, and we are consequently interested in how it changes with size at sexual maturity, and how this association varies with development rate. The simple models generate complex dynamics while providing insight into the circumstances when extremes of body size evolve. The direction of selection leading to either gigantism or dwarfism crucially depends on the proportion of the population that is sexually mature, which in turn depends on how the development function determines the survivorship schedule. The developmental trajectories consequently interact with size-specific survival or reproductive rates to determine the best life history and the optimal body size emerges from that interaction. These dynamics result in trade-offs between different components of the life history, with the form of the trade-off that emerges depending upon where in the life history density dependence operates most strongly. Empirical application of the approach we develop has potential to help explain the enigma of body size variation across the tree of life.