Offspring Human Fertility Behavior in Biodemographic Perspective (2003) / Chapter Skim
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6. Intraspection Variablity in Fertility and Offspring Survival in a Nonhuman Primate: Behavioral Control in Ecological and Social Sources
6. Intraspection Variablity in Fertility and Offspring Survival in a Nonhuman Primate: Behavioral Control in Ecological and Social Sources
Pages 140-169
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From page 140... ...
Our goal in the present chapter is to investigate fertility and family behavior from the perspective of life histories the schedules of vital rates in a natural primate population. We do this by evaluating the potential fitness consequences, magnitude, and sources of variability in life histories, particularly of females, and in the behaviors affecting them.
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From page 141... ...
Mammals of the same size in different orders differ fairly consistently in slow or fast life history style, and primates, mainly anthropoid primates, have particularly slow life histories. Finally, another interesting feature of the fast-siow continuum is that many life history traits, such as growth rates and adult mortality rates
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From page 142... ...
Both humans and baboons exhibit slow life histories; those of baboons are basically as expected for a primate of their size, whereas some aspects of human life histories tend to be slower than expected (but see, e.g., Hrdy, 1999, and Hawkes, 2002, regarding human "hyperfertility"~. That is, primates in general and anthropoid primates in particular have life histories characteristic of much larger nonprimate mammals.
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From page 143... ...
First, we use matrix demography models to examine the relative strength of selection on different vital rates. In particular, we examine the sensitivity of fitness to comparable changes in infant survival and adult fertility.
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From page 144... ...
Life History Patterns in Amboseli To analyze baboon life histories and life history variability, we used data collected from 1971 to 1999 for approximately 600 individuals living in completely wild-foraging groups of baboons (Alberta and Altmann, 2003~. We constructed life tables with 1-year age classes and the corresponding survivorship and fertility entries of a population projection matrix, shown for females in Table 6-1 and males in Table 6-2.
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From page 145... ...
The fertility entries take into account both adult survival in the interval and birth rate to individuals in that interval; it is based on the full cohort that enters an age class, whether they survive the age class or not. Because baboons, like humans, do not have a distinct birth season, our calculations are based on a birth flow model.
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From page 146... ...
for males is based on the proportion of mating attributable to males of that age class (see text and Alberts and Altmann, 2003, especially p. 78 and Appendix 4.1 for reproductive rate terminology)
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From page 147... ...
Male offspring production declines much more rapidly with age than do birth rates of females, and the sex difference in this decline is much greater than that for survival (Tables 6-1 and 6-2~. A General Approach to Evaluating the Relative Strength of Selection on Different Vital Rates: Perturbations of Female Life Histories Using Matrix Models If one were to compare two family lineages within a population, one lineage in which investment is successfully directed toward increasing infant survival and another in which it is successfully devoted to increasing birth rates, which would be more effective in enhancing population growth iThe estimate of person-years lived in the first year of life used to construct the projection matrix entries is 0.8551 years for females, and the net reproduction ratio the usual NRR known to demographers, but also referred to as the net reproductive rate (Caswell, 2001)
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From page 148... ...
, what would be the relative impact on biological fitness of two different behavioral changes, each of which appeared in some individuals, one a behavioral change that produced a small proportional change in infant mortality, the other a behavioral change that produced a small proportional change in birth rates? Demographic matrix models are a useful too!
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From page 149... ...
Specifically, the elasticity of survival for age class 1 is 0.0972, whereas the elasticity of fertility is 0.0128 at its highest, in age class 6, and declines to an order of magnitude less, 0.0013, by age 19. The importance of survival versus fertility is evidenced by the fact that, for Amboseli baboons, survival accounts for 91 percent of the total elasticities and fertility for only 9 percent, for both males and females (Figure 6-2; see also Alberts and Altmann, 2003)
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From page 150... ...
One is the aggregate approach to the vital rates; that is, age classes are treated as groups of homogeneous individuals. Another is that vital rates are assumed to be independent of each other.
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From page 151... ...
That is, variability solely in first-year infant survival that is within the observed range for baboons has major consequences for estimates of relative fitness of both sexes and for the future of the population. Clearly, the fitness potential is great for variability in effective family and fertility behaviors.
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From page 152... ...
Therefore, to retain reasonable sample sizes, we collapse the vital rates from 1-year age intervals into a few meaningful and manageable 60 tO5~0O ~~ fj j 3'Nyayo's Hightail's °14 `' Dotty's 19~80mo's 358Viola's Hook's 0 range shift ~ ~ > ~ Linda's > Weaver's 58 25 N9 8> O~ONzige's Lodge 0 >0~ 0 > ~ ~3 (food-enhanced) ~ Joy's 1971 1981 1991 2001 FIGURE 6-4 Amboseli study groups that contributed data to the present analyses.
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From page 153... ...
For evaluating the extent to which offspring survival, offspring quality, and adult fertility are fooci limited for the Amboseli baboons, we macle comparisons for two instances in which baboon groups chose living situations that changed fooci availability. First, we compared the totally wilclforaging groups in the Amboseli population with a group of baboons that moved into an area near the park's tourist lodges and supplemented their diet with cliscarcleci fooci scraps from the lodge (Altmann et al., 1993; Altmann and Muruthi, 1988; Hahn et al., 2003; Kemnitz et al., 2002;
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From page 154... ...
The greatest proportional increase was in offspring survival. The home range and consequent lifestyle chosen by these females required tolerance of close association with humans, of higher density of baboons in close proximity, and of higher rates of aggressive interactions (Altmann et al., 1993; Kemnitz et al., 2002; Muruthi, 1989~.
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From page 155... ...
was again greatest for offspring survival. In both cases of improved foraging environment, rapid offspring growth rates may be the life history variable mediating the improved survival and fertility that resulted.
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From page 156... ...
In the aggregate, fertility rates do not appear to exhibit density dependence. We postulate a scenario in which in high-density conditions the lower reproductive rates for females with surviving offspring are offset by the higher fertility that occurs when infants die, resulting in no effect on the aggregate fertility measurements used in our matrix models.
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From page 157... ...
Interbirth interval following birth of a surviving offspring is a function of immediate experienced density as measured by group size (total group size and number of adult females in the group) after controlling for change in foraging environment.
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From page 158... ...
Despite widespread, though not ubiquitous (e.g., Watts, 1996) , findings of dominance effects on foraging in a number of primate species, and despite long-postulated fitness consequences of dominance status, adequate data for rigorous tests remain scarce (van Schaik, 1983, and sequelae; Sterck et al., 1997~.
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From page 159... ...
are functions of social status. NOTE: High-ranking females experience higher reproductive rates and their offspring mature at younger ages than do low-ranking females (after controlling for changes in foraging environment and group size)
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From page 160... ...
but also potentially choosing enhancement of offspring survival over greater mating opportunities. In summary, by altering their home range and their social environments through dispersal (males)
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From page 161... ...
. Isslon ·Reproduction is earlier ·Reproductive rate ·Reproduction is declines earlier ·Reproductive rate increases · Low status is · Reproductive rate ·Infant survival improves costly increases FIGURE 6-8 Overview of behaviors affecting fertility and offspring survival by fully wild-foraging baboons in Amboseli over three decades.
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From page 162... ...
Enhanced fitness through the actions of others has been postulated for the evolution of alloparental care among some primate species and more specifically as an explanation for humans having "faster" reproductive rates than expected for their body size and otherwise slow life history components (see particularly Hrdy, 1999; Ross and McLarnon, 2000; Hawkes, 2002, and references therein)
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From page 163... ...
Because differences in survival of immature young will have the greatest impact on population growth and individual fitness for species with life histories characteristic of human and nonhuman primates, understanding fertility, parental behavior, offspring behavior, and the mechanisms producing variability in each must of necessity hold a central place in understanding primate adaptation and human .
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From page 164... ...
Studies of physiological mechanisms, ontogenetic effects, the effect of life history trade-offs on observed behavior (see above) , and heritable differences in fertility and parental behaviors will also be essential to the agenda of elucidating ecological and evolutionary perspectives on fertility and parental care behaviors in nonhuman primates.
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From page 165... ...
Pp. 351-361 in Evolution of Life Histories of Mammals: Theory and Pattern, M.S.
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From page 166... ...
2001 Evolution of mammal life histories. Evolutionary Ecology Research 3:521-535.
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From page 167... ...
Blurton Jones 2003 The evolution of human life histories: Primate tradeoffs, grandmothering socioecology, and the fossil record. In Primate Life Histories and Socioecology.
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From page 168... ...
Wasser 2000 Lifetime reproductive success, longevity, and reproductive life history of female yellow baboons (Papio cynocephalus) of Mikumi National Park, Tanzania.
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From page 169... ...
Wasser, S.K., and A.K. Starling 1988 Proximate and ultimate causes of reproductive suppression among female yellow baboons at Mikumi National Park, Tanzania.
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