Assessing the Diversity of the Form of Age-Specific Changes in Adult Mortality from Captive Mammalian Populations

International audienceActuarial senescence (i.e., the age-specific increase in mortality rate) is pervasive across mammalian species, but our current understanding of the diversity of forms that actuarial senescence displays across species remains limited. Although several mathematical models have been proposed to model actuarial senescence, there is still no consensus on which model to use, especially when comparing mortality patterns among species. To fill this knowledge gap, we fitted and compared different forms of increase using models commonly used in senescence studies (i.e., Gompertz, Weibull, and logistic) across 61 species of mammalian captive populations using the Bayesian Survival Trajectory Analysis (BaSTA) approach. For as much as 79% of the species, a Gompertz increase of mortality with age was the most parsimonious model that satisfactorily described the shape of age-specific mortality changes in adults. This highlights that the form of the increase in mortality is mostly consistent across mammalian species and follows the Gompertz rule with some rare exceptions. The implications of that result are twofold. First, the Gompertz rate of mortality increase should be used in cross-species comparative analyses of mammals, as already done in some studies. Second, although the Gompertz model accurately describes actuarial senescence in most mammals, there are notable exceptions, and the factors causing this deviation from an exponential mortality increase during the adult stage warrant further investigation

Variation in the ontogenetic allometry of horn length in bovids along a body mass continuum

International audienceAllometric relationships describe patterns of proportional covariation between morphological, physiological, or life-history traits and the size of the organisms among populations or species (evolutionary allometry), or within population, among individuals measured at similar (static allometry), or different (ontogenetic allometry) age or developmental stages. When expressed on a log-log scale, allometric relationships are often described by a linear regression: log(y) = a + b log(x) where y is the trait size; x the body size; and a and b the allometric intercept and slope, respectively (Huxley, 1932). Because population and species mean trait size and body size used to estimate evolutionary allometry result from the proportional growth of both traits, patterns of evolutionary allometry emerge from variation in ontogenetic allom

Sex differences in adult lifespan and aging rates of mortality across wild mammals

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Comparative analyses of longevity and senescence reveal variable survival benefits of living in zoos across mammals

While it is commonly believed that animals live longer in zoos than in the wild, this assumption has rarely been tested. We compared four survival metrics (longevity, baseline mortality, onset of senescence and rate of senescence) between both sexes of free-ranging and zoo populations of more than 50 mammal species. We found that mammals from zoo populations generally lived longer than their wild counterparts (84% of species). The effect was most notable in species with a faster pace of life (i.e. a short life span, high reproductive rate and high mortality in the wild) because zoos evidently offer protection against a number of relevant conditions like predation, intraspecific competition and diseases. Species with a slower pace of life (i.e. a long life span, low reproduction rate and low mortality in the wild) benefit less from captivity in terms of longevity; in such species, there is probably less potential for a reduction in mortality. These findings provide a first general explanation about the different magnitude of zoo environment benefits among mammalian species, and thereby highlight the effort that is needed to improve captive conditions for slow-living species that are particularly susceptible to extinction in the wild

Postreproductive lifespans are rare in mammals

A species has a post‐reproductive stage if, like humans, a female entering the adult population can expect to live a substantial proportion of their life after their last reproductive event. However, it is conceptually and statistically challenging to distinguish these true post‐reproductive stages from the usual processes of senescence, which can result in females occasionally surviving past their last reproductive event. Hence, despite considerable interest, the taxonomic prevalence of post‐reproductive stages remains unclear and debated. In this study we use life tables constructed from published data on wild populations of mammals, and statistical measures of post‐reproductive lifespans, to distinguish true post‐reproductive stages from artefacts of senescence and demography in 52 species. We find post‐reproductive stages are rare in mammals and are limited to humans and a few species of toothed whales. By resolving this long‐standing debate, we hope to provide clarity for researchers in the field of evolutionary biology and a solid foundation for further studies investigating the evolution and adaptive significance of this unusual life history trait

Evolutionary allometry reveals a shift in selection pressure on male horn size

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Modelling allometry: statistical and biological considerations -a reply to Packard

International audienceAllometry studies describe how phenotypic traits increase relative to the increase of the size of the organism. Given that the increase in size of an organism is attributable to growth, a multiplicative process, allometric relationships are often analysed on a proportional scale (e.g. a log-log scale) to account for the multiplicative nature of the data. Nonetheless, the logarithmic transformation of the data when estimating allometric relationships has been the subject of debate. In a series of replies to various case studies of allometry, G. C. Packard has repeatedly criticized this approach under the premise that the logarithmic transformation of the data alters the estimate of the allometric exponent and obscures the biological meaning of the allometric parameters. Recently, Packard (2018) reanalysed data from our study on horn length allometry in bovids (Tidière et al., 2017) and reached conclusions that contrasted with those reported in our original study. Echoing many authors before us, we argue here that logarithmic transformation of the data in allometric studies is justified by the expected distribution of the residual variation in ontogenetic, static and evolutionary allometry. We also point out that allometric slopes thus obtained have a direct biological interpretation in terms of elasticities. Finally, we show that Packard's criticism is based on qualitative and not quantitative assessment of the models fitted on different scales, and his conclusions disregard statistical and biological evidence supporting models fitted on a log-log scale. ADDITIONAL KEYWORDS: biological error-elasticity-fitness function-measurement error-scale

Sex‐specific actuarial and reproductive senescence in zoo‐housed tiger ( Panthera tigris ): The importance of sub‐species for conservation

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Variation in actuarial senescence does not reflect life span variation across mammals

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