Human uniqueness? Life history diversity among small-scale societies and chimpanzees.

BackgroundHumans life histories have been described as “slow”, patterned by slow growth, delayed maturity, and long life span. While it is known that human life history diverged from that of a recent common chimpanzee-human ancestor some ~4–8 mya, it is unclear how selection pressures led to these distinct traits. To provide insight, we compare wild chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence.MethodsWe employ Life Table Response Experiments to quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to inform differences between humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test several predictions based on life history theory.ResultsChimpanzees’ earlier maturity and higher adult mortality drive species differences in population growth, whereas infant mortality and fertility variation explain differences between human populations. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fertility due to adult mortality attrition. Infant survival is often lower among humans, but lost fitness is recouped via short birth spacing and high peak fertility, thereby reducing selection on infant survival. Lastly, longevity and delayed maturity reduce selection on child survival, but among humans, recruitment selection is unexpectedly highest in longer-lived populations, which are also faster-growing due to high fertility.ConclusionHumans differ from chimpanzees more because of delayed maturity and lower adult mortality than from differences in juvenile mortality or fertility. In both species, high child mortality reflects bet-hedging costs of quality/quantity tradeoffs borne by offspring, with high and variable child mortality likely regulating human population growth over evolutionary history. Positive correlations between survival and fertility among human subsistence populations leads to selection pressures in human subsistence societies that differ from those in modern populations undergoing demographic transition.

Phylogenetically informed imputation methods have rarely been applied to estimate missing values in demographic data but may be a powerful tool for reconstructing vital rates of survival, maturation, and fecundity for species of conservation concern. Imputed vital rates could be used to parameterize demographic models to explore how populations respond when vital rates are perturbed. We used standardized vital rate estimates for 50 bird species to assess the use of phylogenetic imputation to fill gaps in demographic data. We calculated imputation accuracy for vital rates of focal species excluded from the data set either singly or in combination and with and without phylogeny, body mass, and life-history trait data. We used imputed vital rates to calculate demographic metrics, including generation time, to validate the use of imputation in demographic analyses. Covariance among vital rates and other trait data provided a strong basis to guide imputation of missing vital rates in birds, even in the absence of phylogenetic information. Mean NRMSE for null and phylogenetic models differed by <0.01 except when no vital rates were available or for vital rates with high phylogenetic signal (Pagel's λ > 0.8). In these cases, including body mass and life-history trait data compensated for lack of phylogenetic information: mean normalized root mean square error (NRMSE) for null and phylogenetic models differed by <0.01 for adult survival and <0.04 for maturation rate. Estimates of demographic metrics were sensitive to the accuracy of imputed vital rates. For example, mean error in generation time doubled in response to inaccurate estimates of maturation time. Accurate demographic data and metrics, such as generation time, are needed to inform conservation planning processes, for example through International Union for Conservation of Nature Red List assessments and population viability analysis. Imputed vital rates could be useful in this context but, as for any estimated model parameters, awareness of the sensitivities of demographic model outputs to the imputed vital rates is essential.

A key question in both life history evolution and conservation biology is how much the contributions of different demographic processes to the rate of population growth vary from place to place. Using data from a six-year demographic study of five nearby populations, we asked, for the first time, how well the sensitivities and elasticities of the stochastic population growth rate (λs) to the means and standard deviations of underlying vital rates (survival, growth, reversion, and reproduction) can be generalized among populations. Relying on Tuljapurkar's approximation for λs, we used standard mathematical formulas for the sensitivities and elasticities of λs to the vital rate means, and we derived approximations for the sensitivities and elasticities of λs to the vital rate standard deviations. No single vital rate mean or standard deviation had the highest stochastic sensitivity or elasticity across all five populations. However, the summed sensitivities and elasticities of different types of vital rates were much more consistent: the mean survival rates had the highest summed sensitivity and elasticity, and the standard deviations of growth or reversion rates had the highest summed sensitivity or elasticity in all populations. The rank order of the individual sensitivities and elasticities of the vital rate means were, in general, also highly correlated among populations, with the exception of one population that showed a distinctive sensitivity/elasticity pattern, but the sensitivities and elasticities of the vital rate standard deviations were less correlated among populations. Environmental variables (geographical location, elevation, and geological parent material) did not predict the degree of similarity in sensitivity/elasticity patterns between population pairs. Our results show that overall patterns in the stochastic sensitivities and elasticities of mean vital rates can be extrapolated accurately across closely adjacent populations, but that caution is needed when extrapolating sensitivities/elasticities of vital rate standard deviations or when applying the single most important vital rate from one population to others.

Comparative studies about the relationships between vital rates and ecological traits at the community level are conspicuously lacking for most taxa because estimating vital rates requires detailed demographic data. Identifying relationships between vital rates and ecological traits could help to better understand ecological and evolutionary demographic mechanisms that lead to interspecific differences in vital rates. We use novel dynamic N-mixture models for counts to achieve this for a whole avian community comprising 53 passerine species, while simultaneously accounting for density dependence and environmental stochasticity in recruitment and survival and, importantly, correcting our inferences for imperfect detection. Demographic stochasticity is taken into account in the form of the binomial and Poisson distributions describing survival events and number of recruits. We then explore relationships between estimated demographic parameters (i.e., vital rates) and ecological traits related to migration patterns, diet, habitat and nesting location of each species. The relative importance of recruitment and adult survival as contributors to population growth varied greatly among species, and interspecific differences in vital rates partly reflected differences in ecological traits. Migratory mode was associated with interspecific differences in population growth and density dependence. Resident species had higher population growth rates than long- and short-distance migrants. We found no relationships between diet and population growth rate. Habitat differences were associated with different growth rates: alpine, wetland and farmland species had lower population growth rates than forest species. Differences in population growth rates among nesting locations showed that breeding habitat is essential for population dynamics. Our study reveals relationships between ecological traits and contributions of vital rates to population growth and suggests ways in which patterns of population growth fluctuations in a community might be determined by life history.

In this review we attempt to reconstruct the evolutionary history of hominin life history from extant and fossil evidence. We utilize demographic life history theory and distinguish life history variables, traits such as weaning, age at sexual maturity, and life span, from life history-related variables such as body mass, brain growth, and dental development. The latter are either linked with, or can be used to make inferences about, life history, thus providing an opportunity for estimating life history parameters in fossil taxa. We compare the life history variables of modern great apes and identify traits that are likely to be shared by the last common ancestor of Pan-Homo and those likely to be derived in hominins. All great apes exhibit slow life histories and we infer this to be true of the last common ancestor of Pan-Homo and the stem hominin. Modern human life histories are even slower, exhibiting distinctively long post-menopausal life spans and later ages at maturity, pointing to a reduction in adult mortality since the Pan-Homo split. We suggest that lower adult mortality, distinctively short interbirth intervals, and early weaning characteristic of modern humans are derived features resulting from cooperative breeding. We evaluate the fidelity of three life history-related variables, body mass, brain growth and dental development, with the life history parameters of living great apes. We found that body mass is the best predictor of great ape life history events. Brain growth trajectories and dental development and eruption are weakly related proxies and inferences from them should be made with caution. We evaluate the evidence of life history-related variables available for extinct species and find that prior to the transitional hominins there is no evidence of any hominin taxon possessing a body size, brain size or aspects of dental development much different from what we assume to be the primitive life history pattern for the Pan-Homo clade. Data for life history-related variables among the transitional hominin grade are consistent and none agrees with a modern human pattern. Aside from mean body mass, adult brain size, crown and root formation times, and the timing and sequence of dental eruption of Homo erectus are inconsistent with that of modern humans. Homo antecessor fossil material suggests a brain size similar to that of Homo erectus s. s., and crown formation times that are not yet modern, though there is some evidence of modern human-like timing of tooth formation and eruption. The body sizes, brain sizes, and dental development of Homo heidelbergensis and Homo neanderthalensis are consistent with a modern human life history but samples are too small to be certain that they have life histories within the modern human range. As more life history-related variable information for hominin species accumulates we are discovering that they can also have distinctive life histories that do not conform to any living model. At least one extinct hominin subclade, Paranthropus, has a pattern of dental life history-related variables that most likely set it apart from the life histories of both modern humans and chimpanzees.

Hunting Ability and Reproductive Success Among Male Ache Foragers: Preliminary Results

Elasticities of matrix elements from population projection matrices are com- monly used to analyze the relative contributions of different life history transitions (birth, survival, growth) to the finite rate of increase (l). Hitherto, comparative demography based on matrix models has relied upon decomposing elasticity matrices into blocks, each con- taining matrix elements deemed to represent recruitment, stasis, or progression to larger size classes. Elasticities across an entire matrix always sum to unity, and different popu- lations and species can be compared on the basis of the relative proportions of these three variables. This method has been widely used, but it contains a weakness in that the value of matrix elements is a function of more than one vital rate. For example, transitions representing progression to larger size classes involve a survival rate as well as a growth rate. Ideally, then, demographic comparisons between populations should be made using elasticities of vital rates themselves, rather than elasticities of matrix elements that are compounds of those rates. Here, we employ the complete set of general equations for the elasticity of vital rates in an entirely new analysis of matrices for 102 species of perennial plants. The results show a surprising similarity to an earlier analysis based upon matrix element elasticity and provide important confirmation of general patterns of correlation between plant life history and demography. In addition, we show that individual vital rate elasticities cannot, on their own, predict variation in life history. Therefore, all three de- mographic processes (survival, growth, and reproduction) are necessary to account for life history variation. The new analysis provides a firmer foundation for comparative demog-

We studied the breeding biology of the south temperate Cape Penduline Tit (Anthoscopus minutus) in order to compare its life history traits with those of related north temperate members of the family Remizidae, namely the Eurasian Penduline Tit (Remiz pendulinus) and the Verdin (Auriparus flaviceps). We used this comparison to test key predictions of three hypotheses thought to explain latitudinal variation in life histories among bird species—the seasonality and food limitation hypothesis, nest predation hypothesis and adult mortality hypothesis. Contrary to the general pattern of smaller clutch size and lower adult mortality among south-temperate birds living in less seasonal environments, the Cape Penduline Tit has a clutch size larger than that of the Verdin and similar to that of the Eurasian Penduline Tit, and higher adult mortality than both of the other two species. The most notable difference between the Cape Penduline Tit and the two other species is in parental behavioural strategy, with the former exhibiting bi-parental care at all stages of nesting together with facultative cooperative breeding, whereas the Eurasian Penduline Tit has uni-parental care and the Verdin has a combination of female-only incubation but bi-parental nestling care. Consequently, in comparison to the other two species, the Cape Penduline Tit exhibits greater nest attentiveness during incubation, a similar per-nestling feeding rate and greater post-fledging survival. Its relatively large clutch size, high parental investment and associated high adult mortality in a less seasonal environment are consistent with key predictions of the adult mortality hypothesis but not with key predictions of the seasonality and food limitation hypothesis in explaining life history variation among Remizidae species. These results add to a growing body of evidence of the importance of age-specific mortality in shaping life history evolution.

Analyzing intraspecific variation in population dynamics in relation to environmental factors is crucial to understand the current and future distributions of plant species. Across ranges, peripheral populations are often expected to show lower and more temporally variable vital rates than central populations, although it remains unclear how much any differences in vital rates actually contribute to differences in population growth rates. Moreover, few demographic studies accounting for environmental stochasticity have been carried out both at continental and regional scales. In this study we calculated stochastic growth rates in five central and six northern peripheral populations of the widespread shortlived herb Plantago coronopus along the Atlantic Coast in Europe. To evaluate at two spatial scales how mean values and variability of vital rates (i.e., fecundity, recruitment, survival, growth, and shrinkage) contributed to the differences in stochastic growth rates, we performed Stochastic Life Table Response Experiment (SLTRE) analyses between and within central and peripheral regions. Additionally, we searched for correlations between vital rate contributions and local environmental conditions. Lower mean values and greater variability for some vital rates in peripheral than in central populations had an overall negative but nonsignificant effect on the stochastic growth rates in the periphery. Different life cycle components accounted for differences in population growth depending on spatial scale, although recruitment was the vital rate with the highest influence both between and within regions. Interestingly, the same pattern of differentiation among populations was found within central and peripheral areas: in both regions, one group of populations displayed positive contributions of growth and shrinkage and negative contributions of recruitment and survival; the opposite pattern was found in the remaining populations. These differences in vital rate contributions among populations within regions were correlated with precipitation regime, whereas at the continental scale, differences in contribution patterns were related to temperature. Altogether, our results show how populations of P. coronopus exhibit life cycle differences that may enable the species to persist in locations with widely varying environmental conditions. This demographic flexibility may help to explain the success of widespread plants across large and heterogeneous ranges.

For density–independent populations, the sensitivity of population growth rate to changes in individual vital rates indicates the strength of selection on different parts of the life history. Here I show how this approach may be extended to any density–dependent and/or stochastic population model, including those that show cyclic, quasi–periodic and chaotic dynamics. One calculates the influence of individual vital rates on the outcome of competition between two almost identical life histories. The outcome of this competition is determined by the invasion exponent ϑ introduced by Rand. This is the Lyapunov exponent of the linearized system describing the invasion of a population with one life history by a variant type with another. Demographic sensitivities are given by the partial derivatives of ϑ with respect to the individual vital rates of the invading type. The density–independent analysis is a special case of this general framework. Sensitivities can often be obtained analytically when the population has a stable equilibrium point, and can be calculated by numerical differentiation in other cases. One can also use the methodology to examine selection pressures on the parameters describing density dependence and, if there are trade–offs between vital rates, it can be used to determine optimal life histories. A two age–class example shows that the occurrence of nonlinear dynamics can markedly alter selection pressures on a life history from those which operate when the population has a stable equilibrium point.

BackgroundThe East/West gradient in health across Europe has been described often, but not using metrics as comprehensive and comparable as those of the Global Burden of Disease 2000 and Comparative Risk Assessment studies.MethodsComparisons are made across 3 epidemiological subregions of the WHO region for Europe – A (very low child and adult mortality), B (low child and low adult mortality) and C (low child and high adult mortality) – with populations in 2000 of 412, 218 and 243 millions respectively, and using the following measures: 1. Probabilities of death by sex and causal group across 7 age intervals; 2. Loss of healthy life (DALYs) to diseases and injuries per thousand population; 3. Loss of healthy life (DALYs) attributable to selected risk factors across 3 age ranges.ResultsAbsolute differences in mortality are most marked in males and in younger adults, and for deaths from vascular diseases and from injuries. Dominant contributions to east-west differences come from the nutritional/physiological group of risk factors (blood pressure, cholesterol concentration, body mass index, low fruit and vegetable consumption and inactivity) contributing to vascular disease and from the legal drugs – tobacco and alcohol.ConclusionThe main requirements for reducing excess health losses in the east of Europe are: 1) favorable shifts in all amenable vascular risk factors (irrespective of their current levels) by population-wide and personal measures; 2) intensified tobacco control; 3) reduced alcohol consumption and injury control strategies (for example, for road traffic injuries). Cost effective strategies are broadly known but local institutional support for them needs strengthening.

Most life-history theory assumes the environment is invariant. For the first time, analytical and numerical techniques were employed to investigate the impact of environmental variability on selection pressures (elasticities = proportional sensitivities) on a range of life histories. We find that the impact of variability is influenced significantly by the amount of variability an organism experiences (more variability affects selection pressures more), the correlations between variations among the vital rates (negative correlations are more likely to relax selection on fecundities and increase it on survival rates), and the life history in question (shorter life histories are more affected). In addition, the impact of a variable environment on the elasticities of life histories is sensitive to the sampling distribution used to generate the variability, and it is particularly sensitive to extreme values, such as those caused by occasional catastrophic events. The elasticities of life histories in highly variable environments may bear little relationship to those in a constant environment. In detailed optimality or evolutionarily stable strategy (ESS) modeling, variability in vital rates as small as a standard deviation being 10%-15% of the mean may appreciably alter the conclusions. Thus, it may be very important to consider the possible impact of environmental stochasticity and not to assume that it has no effect.

We used life history traits to categorize vulnerability of elasmobranchs to exploitation.However, the utility of this approach required that the links between life histories and population dynamics be explored.We constructed standardized three-stage matrix models for 55 species of sharks and rays.Using these models we (1) conducted elasticity analyses to determine how the vital rates of mortality (M) and fertility (f) influence elasmobranch population growth rate r, (2) determined the response of elasticity to changes in the levels of exploitation, (3) estimated sensitivity of elasticity to perturbation in vital rates, and (4) examined the taxonomic distribution of model inputs and species vital rates, such as size at maturity (L mat ), and total length (L max ).We found positive relationships between the elasticity of λ (population growth rate) to changes in juvenile and adult stages to longevity and age of maturity; however, the age of maturity and the elasticity of λ to changes in the adult stage relationship appeared to be invariant.There was a negative relationship between both longevity and age of maturity and the elasticity of λ to changes in inter-stage transitions of the models.Under varying fishing levels, estimates of elasticity were robust to changes in survival.Elasticity and perturbation analyses suggested that compensatory responses to exploitation in elasmobranchs were less likely to be expressed as changes in fertility than as changes in juvenile and adult mortality and stage durations (i.e.changes in age of maturity).Combining vital rates and elasticities, we found similar suites of life histories and demographics within groups at various taxonomic levels.

Several empirical models have attempted to account for the covariation among life history traits observed in a variety of organisms. One of these models, the fast-slow continuum hypothesis, emphasizes the role played by mortality at different stages of the life cycle in shaping the large array of life history variation. Under this scheme, species can be arranged from those suffering high adult mortality levels to those undergoing relatively low adult mortality. This differential mortality is responsible for the evolution of contrasting life histories on either end of the continuum. Species undergoing high adult mortality are expected to have shorter life cycles, faster development rates and higher fecundity than those experiencing lower adult mortality. The theory has proved accurate in describing the evolution of life histories in several animal groups but has previously not been tested in plants. Here we test this theory using demographic information for 83 species of perennial plants. In accordance with the fast-slow continuum, plants undergoing high adult mortality have shorter lifespans and reach sexual maturity at an earlier age. However, demographic traits related to reproduction (the intrinsic rate of natural increase, the net reproductive rate and the average rate of decrease in the intensity of natural selection on fecundity) do not show the covariation expected with longevity, age at first reproducion and life expectancy at sexual maturity. Contrary to the situation in animals, plants with multiple meristems continuously increase their size and, consequently, their fecundity and reproductive value. This may balance the negative effect of mortality on fitness, thus having no apparent effect in the sign of the covariation between these two goups of life history traits.

Understanding how plant fitness varies along natural gradients is critical for predicting responses to environmental change. However, individual vital rates are often used as fitness proxies without knowing how other vital rates vary along the same gradients. We investigated how canopy cover, plant–plant interactions, water availability and soil properties influenced the emergence, survival, seed production and population growth rates of eight annual plant species in semi‐arid Western Australia. We sowed seeds into sun‐exposed and shaded blocks across a reserve, removed all neighbouring plants from half of the interaction neighbourhoods, and used rainout shelters to reduce and increase precipitation relative to ambient plots. Canopy cover had strong negative effects on emergence, but few direct impacts on other vital rates and population growth rates. Direct competitive effects on survival and seed production were rare, although evident for population growth rate for 3/8 species. Competition was stronger in open than shaded plots for half of the species. Canopy cover also interacted with the watering treatment to influence survival of half of the species, but watering alone had few direct impacts on species' vital and population growth rates. We found only positive significant correlations between pairs of rates, and survival and seed production were far more frequently correlated with population growth rate than emergence. Synthesis. Our study illustrates that vital rates can respond to the same local‐scale environmental variation in different ways that are likely not driven by life history trade‐offs. We caution against using emergence as a proxy for population growth rate and emphasise that no single vital rate was a reliable fitness proxy overall. Interactions among abiotic and biotic factors were important drivers of vital and population growth rates for some species, highlighting the need to account for plant–plant interactions when predicting population responses to environmental change.