Effects of Female Age Structure

Age of females is an important contributor to different levels of fertility and fecundity. Generally, juveniles have much lower fecundity than do adults, and prime-aged adults have much higher fecundity than do older or senesced adults (Figure 3). The larger and longer-lived a species, the greater these age-related differences. For example, Columbian black-tailed deer (Odocoileushemionus columbianus), fecundity of which is illustrated in Figure 3, can live to be 20 years old. Thus, there are many more age classes in the older, senesced category (>7 years old = 14) than in the juvenile categories (fawns and yearlings). Consequently, a population with an older adult age structure (i.e., one that has a greater mean age, because of more individuals in older age classes) tends to be less productive than a population of the same species with a younger age structure because more individuals are present in reproductively senescent age classes. Actions aimed at maximizing productivity thus often try to increase mortality rates on the population above those attributable to natural mortality alone. The goal of these strategies is to decrease the age structure of the adult population in order to have a greater proportion of the population in prime reproductive categories (Figures 1, 3, and 4), thus maximizing per capita productivity.

Figure 2 Age structure of a North American elk (Cervus elaphus) population currently experiencing very high levels of predation on newborn calves. Note that successive recent years of poor recruitment into the population result in a population age structure that is shifted to the right or dominated by older age-class individuals. Such an age structure would further decrease potential recruitment because older individuals show reproductive senescence and produce fewer juveniles than do individuals in their reproductive prime (ages 2-11).

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Figure 2 Age structure of a North American elk (Cervus elaphus) population currently experiencing very high levels of predation on newborn calves. Note that successive recent years of poor recruitment into the population result in a population age structure that is shifted to the right or dominated by older age-class individuals. Such an age structure would further decrease potential recruitment because older individuals show reproductive senescence and produce fewer juveniles than do individuals in their reproductive prime (ages 2-11).

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1 2-6 >7 1 2-6 >7 Percent pregnancy Embryos/adult female

Figure 3 Age effects on probability of pregnancy and numbers of embryos produced by pregnant black-tailed deer (Odocoileus hemionus columbianus). Because deer can live to be 20 years old, there are potentially many more older, reproductively senesced, age classes in the population. Thus, populations that show older mean ages tend to be less productive than populations with younger age structure.

Figure 4 Effects of increasing adult mortality rate on the proportions of oryx (Oryx gazella gazella) >5 ($) and >10 (a) years old on White Sands Missile Range, south-central New Mexico, USA. Note that as total mortality rate increases, a smaller proportion of individuals are able to survive into older age classes. Observed total mortality rate in this population (~0.26) results in a decrease of approximately 52% and 78%, respectively, from the proportions of >5 and >10-year-old oryx in the population, given an approximate 'natural' annual mortality rate of 0.10.

Figure 4 Effects of increasing adult mortality rate on the proportions of oryx (Oryx gazella gazella) >5 ($) and >10 (a) years old on White Sands Missile Range, south-central New Mexico, USA. Note that as total mortality rate increases, a smaller proportion of individuals are able to survive into older age classes. Observed total mortality rate in this population (~0.26) results in a decrease of approximately 52% and 78%, respectively, from the proportions of >5 and >10-year-old oryx in the population, given an approximate 'natural' annual mortality rate of 0.10.

Maternal age can also influence the likelihood of successfully raising a juvenile to recruitment, or age of reproduction. This is largely due to experience of mothers; prime-aged mothers are more likely to be larger and have a higher social rank in long-lived species, which leads to better territories and greater capture of resources, both of which result in greater production and survival of juveniles and hence greater lifetime reproductive success for these individuals. Moreover, birth attributes of neo-nates such as size or birth mass tend to be lower for older, senescent mothers and younger, inexperienced mothers, and birth attributes are strongly related to juvenile survival in many species.

Maternal behavior also varies according to age of females and number of pregnancy experiences, and can affect survival of juveniles. Prime-aged mothers are more successful in rearing juveniles than are younger females, particularly when threatened by predation. Age of the mother has been associated with losses of neonates in the first week of life, when most newborns die; far more juveniles from primaparous females are lost compared to multiparous females. Experienced mothers are less likely to orphan juveniles due to a breakdown in the imprinting process. For many species, prime-aged females protect juveniles better than younger females. For example, when subjected to human disturbance or simulated predator threats, prime-aged female white-tailed deer (Odocoileus virginianus) move their fawns to more secure bedding sites, whereas young mothers often do not. Moreover, experienced mothers do commonly show complex distraction behaviors that may lead a predator away from juveniles, and may also actively defend juveniles by attacking predators.

Effects of age are often correlated with body mass, as older females tend to be larger and live longer. Juveniles born to larger mothers often show greater survival, likely because juveniles born to heavier mothers are larger, and size at birth is strongly related to survival to recruitment. Similar effects may be seen as litter size increases; for example, roe deer (Capreolus capreolus) fawns born to relatively light mothers or in twin or triplet litters had higher mortality rates than those born to heavy mothers or in smaller litters, likely because single-born fawns usually weigh significantly more than individual twins or triplets. For many species maternal mass in late pregnancy is correlated with juvenile birth mass, and probability of survival is lower for juveniles born to females with lower than average body mass. Consequently, maximum juvenile-rearing success occurs when physically mature, multipar-ous females comprise the bulk of the breeding population.

Age of the mother can also affect the sex ratio of juveniles, although climatic and other environmental conditions can also affect sex ratio of juveniles through effects on condition of mothers during gestation. Prime-aged adult females in good condition often produce more male offspring than females in poor condition, and both senescent and younger females are frequently in poorer shape and thus tend to produce more female young. The age structure of a population can consequently affect both juvenile and adult sex ratios, and thus the potential rate of increase of populations, simply by influencing the proportion of females born into the population. This influences population productivity because having more adult females in prime reproductive classes increases population rates of increase in age-structured populations due to their higher fecundity and greater likelihood of successfully raising a juvenile. Because of this, population rate of increase is extremely sensitive to adult female survival.

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