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

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

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

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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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Does tooth wear influence ageing? A comparative study across large herbivores

International audienceWe test whether the intensity of tooth wear influences the strength of actuarial senescence across species of large herbivores. We collected from the literature data on tooth wear in the wild (measured as the slope of the regression of log-transformed M1 crown height on age), longevity (measured as the age at which 90% of individuals are dead) and two metrics of actuarial senescence in captive populations (rate of senescence between 6 and 12 years of age and Gompertz rate of senescence). Between-species differences and variation in tooth height accounted for most of the observed variation in tooth wear among large herbivores: tooth height and tooth wear were positively correlated. In contrast, tooth wear was little influenced by sex, body mass, or taxonomy. No marked between-sex differences in longevity occurred. Males senesced faster than females when tooth wear was low (for both senescence metrics), while between-sex differences in actuarial senescence when tooth wear was high depended on the metric used to measure actuarial senescence. While longevity was mostly independent of the intensity of tooth wear, we found general support for a positive relationship between both measures of actuarial senescence and tooth wear. These patterns were consistent whether hypsodonty was controlled for or not. Although varying according to sex and to the metric used for assessing actuarial senescence, our findings suggest overall that tooth wear could be positively associated with actuarial senescence among large herbivores. Further longitudinal studies focusing on changes within individuals will be required to test whether a mechanistic link between tooth wear and actuarial senescence occurs in large herbivores

Does grandparental care select for a longer lifespan in non-human mammals?

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Lifespan decreases with proportion of sons in males but not females of zoo‐housed tigers and lemurs

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