SENESCENCE IN NATURAL POPULATIONS OF MAMMALS: A COMPARATIVE STUDY

Here, I use published mortality data from 56 natural populations of mammals to examine evidence for senescence, an increase in the probability of mortality with age. Data on extent of senescence and life history characteristics are compared across taxa in an attempt to test theories for the evolution of senescence in natural populations. In accord with theoretical expectation, senescence is highest in short-lived species with short generation times. In contrast to theoretical expectation, however, senescent increases in mortality rate do not begin until well after age at maturity in most cases. I also present evidence in support of the hypothesis that senescence will be lower in large-brained taxa.     

Effects of age and sex on intra-group spacing behaviour in juvenile savannah baboons,Papio cynocephalus cynocephalus

Twenty juvenile members of known genealogies in two baboon groups were studied over a 16-month period to evaluate a number of predictions about juvenile spacing behaviour based on the natural history of savannah baboons. Young juveniles (1–2·5 years old) approached more frequently and spent more time in proximity to other group members than did old juveniles (3–5·5 years old). In particular, young juveniles associated more closely with their mothers, particular adult males (possible fathers) and age-peers than did old juveniles. Approaches of young juveniles towards unrelated, high-ranking adults were more likely to occur during feeding than were those of old juveniles. Also, following such an approach, young juveniles were more likely than old juveniles to begin feeding immediately. The overall rates of feeding of old juveniles were depressed when they were in proximity to unrelated, high-ranking adults, whereas the feeding rates of young juveniles were not. Juvenile males approached adult males more often than did juvenile females. Juvenile females approached unrelated adult females more often than did juvenile males. Sex differences also existed in juveniles' choices of unrelated adult female neighbours. Juvenile females associated most often with lactating females, whereas juvenile males associated primarily with cycling females. During group resting, juvenile females approached adult females from higher-ranking matrilines more often than they approached adult females from lower-ranking matrilines. Juvenile males did not exhibit this attraction. Also, among old juveniles, females associated closely with their mothers, whereas males did not. Taken together, the results support the hypotheses that juvenile baboons associate with group members in ways that (1) enhance the probability of surviving an early period of high mortality, (2) create opportunities for social learning of sex-typical behaviours/skills, and, for females, (3) facilitate acquisition of familial dominance status.     

Adult-young interactions in island and mainland populations of the deermouse Peromyscus maniculatus

Summary The demographic and ecological characteristics of island populations of small mammals have received increasing attention in recent years, but few studies have compared the behavioral characteristics of island populations with those of mainland populations. Behavior is considered an important variable because it is believed by many to be a crucial factor affecting the population dynamics and demography of natural populations. In particular, among many species of rodents, the social behavior of adults towards juveniles is cited as an important factor influencing dispersal patterns and population regulation. The present study compares social interactions between adults and juveniles of island and mainland populations of the deermouse Peromyscus maniculatus, and attempts to relate differences in behavior to the demographic differences between the two populations. Adult mice were trapped on the mainland of British Columbia and on one of the Gulf Islands off the British Columbia coast, and allowed to breed in the laboratory. Male and female juveniles from both populations were then tested with their own parents and with unrelated male and female adults. The results demonstrate that island adults show almost no aggression towards either own or unrelated young. Mainland adults likewise show little aggression towards their own young, but a proportion of the population, consisting of both male and female adults, shows severe aggression towards unrelated juveniles of both sexes. These results suggest four major conclusions: 1) behavior may be the mechanism responsible for the demographic differences reported for these island and mainland populations; 2) female aggression may be a more important factor in deermouse population dynamics than has been previously recognized; 3) since parents show little aggression towards their own young, adult aggression may be a significant factor in juvenile mortality and emigration only after juveniles have initiated dispersal away from their natal sites; and 4) adult aggression controls the number of both male and female juveniles which are recruited into the population.     

Natural settlement dynamics of a young population of Turbinaria ornata and phenological comparisons with older populations

Our study was done during 16 months on two sites at Moruroa to (i) compare Moruroan populations (young) with Tahitian populations (old) of Turbinaria ornata and (ii) follow the natural dynamic of a young population at Moruroa. Moruroan and Tahitian populations have maximal density and maximal reproductive characteristics during the cool and hot seasons, respectively. Recruits are present throughout the year at all sites. Within the reference field site, individuals were initially distributed regularly and then became clumped through time, recruits appeared around adults. Recruits mortality followed a similar pattern, whereas mortality of adults and juveniles was even throughout the monitoring period. Populations initially had similar proportions of recruits, juveniles and adults (28, 40 and 32%, respectively), but at the end, recruits represent 92% of the population. A linear relationship between the number of individuals present within the station and the colonized substrate area was determined (R² = 0.96), suggesting a limiting effect of the substrate against the settlement of T. ornata with time. Moreover, the growth of recruits in the close vicinity of adults was arrested in the young populations: recruitment may therefore, significantly buffer high rates of adult mortality. It may contribute to the invasion and, more interesting to the persistence of T. ornata in new colonized area.     

Social influences on the development of foraging behavior in free-living common marmosets (Callithrix jacchus)

In this study we investigated the extent and pattern of social influences (i.e., the use of a conspecific as a model) on the foraging behavior of immature, wild common marmosets (Callithrix jacchus) as a function of the age of individuals. We compared the foraging activities and interactions with subadult and adult group members (older than 15 months) of young infants (1-2 months old), older infants (3-4 months old), and juveniles (5-10 months old). In addition to measuring the intensities of model-independent foraging (MIF) and merely paying attention to the model's foraging activities, we examined the frequencies of three types of model-dependent foraging (MDF): "follow the model", "manipulate the same object", and "forage together". We found that older infants were the most attentive and most socially-influenced foragers among the three age categories in absolute terms, but were not more attentive than young infants given their low foraging activity. Juveniles, in contrast, tended to have reduced overall foraging intensity compared to infants, but showed relatively more MDF in cases in which they observed subadult or adult models. Female models appeared to be more attractive than male models. These findings suggest that infants are generally more attentive to the foraging behavior of subadults and adults than juveniles, with the latter being more influenced when they had observed a model before. These subtle age-dependent effects of social foraging not only extend the assumption that young primates seek information from adults, they also suggest a complex interplay among physical and cognitive maturation, independence, and social dynamics.     

Mortality,fertility, and the OY ratio in a model hunter–gatherer system

An agent‐based model (ABM) is used to explore how the ratio of old to young adults (the OY ratio) in a sample of dead individuals is related to aspects of mortality, fertility, and longevity experienced by the living population from which the sample was drawn. The ABM features representations of rules, behaviors, and constraints that affect person‐ and household‐level decisions about marriage, reproduction, and infant mortality in hunter–gatherer systems. The demographic characteristics of the larger model system emerge through human‐level interactions playing out in the context of “global” parameters that can be adjusted to produce a range of mortality and fertility conditions. Model data show a relationship between the OY ratios of living populations (the living OY ratio) and assemblages of dead individuals drawn from those populations (the dead OY ratio) that is consistent with that from empirically known ethnographic hunter–gatherer cases. The dead OY ratio is clearly related to the mean ages, mean adult mortality rates, and mean total fertility rates experienced by living populations in the model. Sample size exerts a strong effect on the accuracy with which the calculated dead OY ratio reflects the actual dead OY ratio of the complete assemblage. These results demonstrate that the dead OY ratio is a potentially useful metric for paleodemographic analysis of changes in mortality and mean age, and suggest that, in general, hunter–gatherer populations with higher mortality, higher fertility, and lower mean ages are characterized by lower dead OY ratios. Am J Phys Anthropol 154:222–231, 2014. © 2014 Wiley Periodicals, Inc.     

Declining home range area predicts reduced late‐life survival in two wild ungulate populations

Demographic senescence is increasingly recognised as an important force shaping the dynamics of wild vertebrate populations. However, our understanding of the processes that underpin these declines in survival and fertility in old age remains limited. Evidence for age‐related changes in foraging behaviour and habitat use is emerging from wild vertebrate studies, but the extent to which these are driven by within‐individual changes, and the consequences for fitness, remain unclear. Using longitudinal census observations collected over four decades from two long‐term individual‐based studies of unmanaged ungulates, we demonstrate consistent within‐individual declines in home range area with age in adult females. In both systems, we found that within‐individual decreases in home range area were associated with increased risk of mortality the following year. Our results provide the first evidence from the wild that age‐related changes in space use are predictive of adult mortality.     

Population viability analysis of Lower Missouri River shovelnose sturgeon with initial application to the pallid sturgeon

Demographic models for the shovelnose (Scaphirhynchus platorynchus) and pallid (S. albus) sturgeons in the Lower Missouri River were developed to conduct sensitivity analyses for both populations. Potential effects of increased fishing mortality on the shovelnose sturgeon were also evaluated. Populations of shovelnose and pallid sturgeon were most sensitive to age‐0 mortality rates as well as mortality rates of juveniles and young adults. Overall, fecundity was a less sensitive parameter. However, increased fecundity effectively balanced higher mortality among sensitive age classes in both populations. Management that increases population‐level fecundity and improves survival of age‐0, juveniles, and young adults should most effectively benefit both populations. Evaluation of reproductive values indicated that populations of pallid sturgeon dominated by ages ≥35 could rapidly lose their potential for growth, particularly if recruitment remains low. Under the initial parameter values portraying current conditions the population of shovelnose sturgeon was predicted to decline by 1.65% annually, causing the commercial yield to also decline. Modeling indicated that the commercial yield could increase substantially if exploitation of females in ages ≤12 was highly restricted.     

Optimal life‐history schedule in a metapopulation with juvenile dispersal

Previous models have predicted that when mortality increases with age, older individuals should invest more of their resources in reproduction and produce less dispersive offspring, as both their future reproductive value and their prospect of competing with their own sib decline. Those models assumed stable population sizes. We here study for the first time the evolution of age‐specific reproductive effort and of age‐specific offspring dispersal rate in a metapopulation with extinction‐recolonization dynamics and juvenile dispersal. Our model explores the evolutionary consequences of disequilibrium in the age structure of individuals in local populations, generated by disturbances. Life‐history decisions are then shaped both by changes with age in individual performances, and by changes in ecological conditions, as young and old individuals do not live on average in the same environments. Lower juvenile dispersal favours the evolution of higher reproductive effort in young adults in a metapopulation with extinction‐recolonization compared with a well‐mixed population. Contrary to previous predictions for stable structured populations, we find that offspring dispersal should generally increase with maternal age. This is because young individuals, who are overrepresented in recently colonized populations, should allocate more to reproduction and less to dispersal as a strategy to exploit abundant recruitment opportunities in such populations.     

Parental age and lifespan influence offspring recruitment: a long-term study in a seabird

Recent studies of wild populations provide compelling evidence that survival and reproduction decrease with age because of senescence, a decline in functional capacities at old ages. However, in the wild, little is known about effects of parental senescence on offspring quality. We used data from a 21-year study to examine the role of parental age on offspring probability of recruitment in a long-lived bird, the blue-footed booby (Sula nebouxii). Offspring probability of recruiting into the breeding population varied over the life of parents and effects age were similar in mothers and fathers. Offspring recruitment was high when parents were roughly 6-12 years old and low before and after then. Effects of parental age on offspring recruitment varied with lifespan (parental age at last reproduction) and previous breeding experience. Offspring recruitment from young and old parents with long reproductive lifespans was greater than that of offspring from parents with short lifespans at young and old ages. For parents with little previous breeding experience recruitment of offspring decreased with their hatch date, but experienced parents were no similarly affected. We found evidence of terminal effects on offspring recruitment in young parents but not in older parents, suggesting that senescence is more likely a gradual process of deterioration than a process of terminal illness. Failure to recruit probably reflects mortality during the first years after independence but also during the fledgling transition to full independence. Our results show effects of parental age and quality on offspring viability in a long-lived wild vertebrate and support the idea that wild populations are composed of individuals of different quality, and that this individual heterogeneity can influence the dynamics of age-structured populations.     

Estimating abundance, age structure and sex ratio of a recently discovered New Zealand tusked weta

Estimates of abundance, age structure and sex ratio are essential for monitoring the status of populations. We report the first attempt to reliably estimate these parameters in a population of the recently discovered Raukumara tusked weta (Motuweta riparia), which is found almost entirely near streams. On two occasions we searched a 211-m section of creek for 4–5 successive nights and individually marked all weta. We estimated abundance of adults and juveniles using closed-population mark-recapture analysis. The choice of mark-recapture model made a substantial difference to the estimated abundance (116–238) and proportion of juveniles (32–72%). However, no single model was clearly better supported than any other. We therefore used model averaging to account for uncertainty in model choice, giving an estimate of 142 (95% CI 105–231) weta including 56 (95% CI 41–234) adults and 77 (95% CI 46–209) juveniles. This corresponds to a density of 0.11 (95% CI 0.08–0.18) weta per m2, assuming 3 m of habitat on either side of the creek. Recapture probability was much lower for adults (n = 2) than juveniles (n = 10), possibly caused by a difference in handling (adults were held overnight whereas juveniles were not). Slightly more juvenile males (n = 22) were caught than juvenile females (n = 21), but more adult females (n = 51) were caught than adult males (n = 31), suggesting a potentially higher mortality in males. An initial assessment of population size is crucial when considering the conservation status and management strategy of rare animals, such as the highly endangered Middle Island tusked weta. The immediate practical benefit of the methods developed here is therefore clear. In the long-term, these methods also have important implications for the continued monitoring of trends in other threatened invertebrate populations.     

Many Vulnerable or a Few Resilient Specimens? Finding the Optimal for Reintroduction/Restocking Programs

Most reintroduction and restocking programs consist of releasing captive-raised juveniles. The usefulness of these programs has been questioned, and therefore, quality control is advisable. However, evaluating restocking effectiveness is challenging because mortality estimation is required. Most methods for estimating mortality are based on tag recovery. In the case of fish, juveniles are tagged before release, and fishermen typically recover tags when fish are captured. The statistical models currently available for analyzing these data assume either constant mortality rates, fixed tag non-reporting rates, or both. Here, instead, we proposed a method that considers the mortality rate variability as a function of age/size of the released juveniles. Furthermore, the proposed method can disentangle natural from fishing mortality, analyzing the temporal distribution of the captures reported by fishermen from multiple release events. This method is demonstrated with a restocking program of a top-predator marine fish, the meagre (Argyrosomus regius), in the Balearic Islands. The estimated natural mortality just after release was very high for young fish (m ₀ = 0.126 day^-1 for fish 180 days old), but it was close to zero for large/old fish. These large/old fish were more resilient to wild conditions, although a long time was needed to achieve a relevant reduction in natural mortality. Conversely, these large/old fish were more vulnerable to fishing, creating a trade-off in survival. The release age that maximizes the number of survivors after, for example, one year at liberty was estimated to be 1,173 days. However, the production cost of relatively old fish is high, and only a few fish can be produced and released within a realistic budget. Therefore, in the case of the meagre, increasing the number of released fish will have no or scarce effects on restocking success. Conversely, it is advisable implement measures to reduce the high natural mortality of young juveniles and/or the length of time needed to improve fish resilience.     

Senescence in relation to latitude and migration in birds

Senescence is the age‐related deterioration of the phenotype, explained by accumulation of mutations, antagonistic pleiotropy, free radicals or other mechanisms. I investigated patterns of actuarial senescence in a sample of 169 species of birds in relation to latitude and migration, by analysing longevity records adjusted for sampling effort, survival rate and body mass. Senescence might decrease at low latitudes because of elevated adult survival rates and generally slow life histories. Alternatively, the rate of senescence might increase at low latitudes because of the greater impact of biological interactions such as parasitism, predation and competition on fitness through differential effects of age‐specific mortality (e.g. because immunologically naïve young individuals and immuno‐senescent old individuals might die more frequently than individuals belonging to intermediate age classes). Bird migration entails extensive exercise twice annually, with migrants spending more time in benign environments with little abiotic mortality than residents, migrants having higher adult survival rate and lower annual fecundity than residents, and migrants suffering more from the consequences of oxidative stress than residents. The rate of senescence increased with latitude, as expected because of slow life histories at low latitudes. Independently, rate of senescence decreased with increasing migration distance. These findings were robust to control for potentially confounding effects of body mass, age of first reproduction and phenotypic similarity among species because of common descent.     

Intrinsic aging-related mortality in birds

Actuarial senescence in captive populations of 28 species of bird was quantified by estimating the parameters of Weibull models fitted to survival curves constructed from data obtained from zoos. Samples of natural and captive populations were compared using phylogenetically independent contrasts, which revealed that extrinsic mortality rates in captive populations are, on average, less than 30% of those of natural populations but that the component of mortality related to aging does not differ significantly between natural and captive birds. This result supports the hypothesis that aging-related mortality is associated with intrinsic causes of death that kill independently of the external environment. A logical implication of this result is that birds in natural populations maintain a high level of physical fitness into old age and do not become more vulnerable to extrinsic mortality factors with increasing age. Additional comparisons showed that the rate of aging in this sample of birds is correlated with body mass, but not with embryonic or postnatal growth rate. These analyses suggest that studies of aging in captive populations can provide powerful tools to help us understand senescence in natural populations.     

猫下丘中央核GABA能神经元年龄相关变化

目的比较研究猫下丘中央核(CIC)GABA能神经元年龄相关变化,探索老年个体听力下降的神经机制。方法Nissl染色显示下丘神经元,免疫组织化学ABC法显示γ-氨基丁酸(GABA)免疫阳性神经元。光镜下观察、拍照,对神经元和GABA能神经元分别计数并换算成密度,测量GABA能神经元直径取平均值。结果GABA阳性反应神经元、阳性纤维及其终末在青年猫及老年猫下丘中央核均有分布。与青年猫相比,老年猫下丘中央核神经元及GABA能神经元密度均显著下降(P<0.01),GABA能神经元下降幅度较大;GABA能神经元胞体直径明显减小(P<0.01),阳性反应明显减弱。结论在衰老过程中猫下丘神经元尤其是GABA能神经元有显著丢失现象,提示GABA能神经元显著减少导致下丘兴奋性和抑制性神经递质之间的平衡失调,可能是引起老年个体听觉功能衰退的重要原因。     

Population variables and causes of mortality in a hedgehog (Erinaceous europaeus) population in southern Sweden

A hedgehog population in southern Sweden was studied by capture-mark-recapture during 1972–1979. Each animal was weighed, sexed and marked with one numbered clip in each ear.
The number of subadult and adult hedgehogs was about 20 in the first two years, 1972–1973. Numbers then increased and approximately doubled and the population peaked in 1978. A decrease came in 1979.
The estimated number of juveniles produced varied between years with the lowest figure (15 juv.) in 1974 when the population was low and the highest in 1977 (39 juv.) when the population was high. As no captured subadult female had suckled young, estimates of recruitment rate assumed that females did not reproduce until they were adult. The average number of juveniles per adult female was 2.8. Recruitment rate did not show any correlation with non-juvenile population densities.
For both juveniles and non-juveniles the annual mortality varied greatly between years. Average yearly mortality was 47% for non-juveniles and 34% for juveniles.
For non-juveniles, most of the total mortality occurred during winter, ranging between 26% and 43% (average 33%), and increased with increasing densities of non juveniles. Juvenile winter mortality varied between 6% and 94%, averaging 33%.
Summer mortality of non-juveniles averaged 15%, ranging between 3% and 22%. Traffic kills were the predominant cause of death. Only a few juveniles were found dead during summer and autumn.
There were no statistical differences in age-specific survival rate between males and females and, generally, no differences in survival rate between the age categories within each sex. It is argued that the population size appears to be more influenced by environmental factors, such as food availability, winter nest sites and winter climate, than by density-dependent factors.     

Seychelles warblers with silver spoons: Juvenile body mass is a lifelong predictor of annual survival,but not annual reproduction or senescence

The environment experienced during development, and its impact on intrinsic condition, can have lasting outcomes for individual phenotypes and could contribute to variation in adult senescence trajectories. However, the nature of this relationship in wild populations remains uncertain, owing to the difficulties in summarizing natal conditions and in long‐term monitoring of individuals from free‐roaming long‐lived species. Utilizing a closely monitored, closed population of Seychelles warblers (Acrocephalus sechellensis), we determine whether juvenile body mass is associated with natal socioenvironmental factors, specific genetic traits linked to fitness in this system, survival to adulthood, and senescence‐related traits. Juveniles born in seasons with higher food availability and into smaller natal groups (i.e., fewer competitors) were heavier. In contrast, there were no associations between juvenile body mass and genetic traits. Furthermore, size‐corrected mass—but not separate measures of natal food availability, group size, or genetic traits—was positively associated with survival to adulthood, suggesting juvenile body mass is indicative of natal condition. Heavier juveniles had greater body mass and had higher rates of annual survival as adults, independent of age. In contrast, there was no association between juvenile mass and adult telomere length attrition (a measure of somatic stress) nor annual reproduction. These results indicate that juvenile body mass, while not associated with senescence trajectories, can influence the likelihood of surviving to old age, potentially due to silver‐spoon effects. This study shows that measures of intrinsic condition in juveniles can provide important insights into the long‐term fitness of individuals in wild populations.     

青年猫和老年猫视神经超微结构观察比较

目的探讨青年猫和老年猫视神经年龄相关的形态学变化及可能造成的生理影响。方法取4只青年猫(2-3岁,2-2.5kg)和4只老年猫(10-13岁,2.5-3.5kg)颅内相对应部分视神经,制作横向半薄切片和超薄切片,半薄切片用甲苯胺蓝硼砂溶液染色,光镜观察、测量视神经的直径(不含外层神经膜);超薄切片标本用醋酸和柠檬酸铅染色,电镜观察、计数视神经纤维密度、测量视神经纤维外径D(含髓鞘)和内径d(不含髓鞘),按一定分级范围算出各种直径的纤维及各种d/D比值的纤维所占百分比,分别画出直方图,对实验结果进行统计学分析并绘制纤维直径谱。结果与青年猫相比,老年猫视神经直径显著增大(P0.05);纤维数量显著下降(P0.05)。纤维直径谱分析结果显示,青、老年猫纤维直径分布范围相似,但老年猫纤维的峰直径及纤维平均直径比青年猫的显著减小(P0.05),老年猫视神经纤维的d/D比值亦明显降低。另外,老年猫视神经中部分轴突肿胀,髓鞘疏松、结构紊乱,板层脱离、空泡化,有的轴索髓鞘溶解。结论在衰老过程中,老年猫视神经纤维丢失,纤维直径减小,d/D比值下降,以及纤维髓鞘的松散解体,这些变化均可能导致视神经纤维对视觉信息的传导速度减慢,是老年个体视觉分析速度下降的重要原因。     

Interactions between year classes in the benthic amphipod Monoporeia affinis: effects on juvenile survival and growth

Summary The effect of 1-year-old Monoporeia (=Pontoporeia) affinis on the survival and growth of young of the year was studied in laboratory experiments. Amphipods were kept in jars with sediment and a continuous supply of cooled water for 3 months. Adults or juveniles were added to a control density of juveniles. Juvenile mortality increased at higher total densities, and was greater when adults were present. The mean length and dry weight of juveniles decreased at higher densities, but adults did not have a greater effect than juveniles. These density-dependent effects are probably caused by competition for food within and between year classes. Thus, a high abundance of juveniles or adults could reduce the recruitment of juveniles into the adult year-class the following year, and the suppression of juvenile growth could lower fecundity. The results of this experiment support the hypothesis, inferred from field data from the Baltic Sea and Swedish lakes, that intraspecific interactions contribute to fluctuations in the abundance of populations of M. affinis.