Length Of Interbirth Intervals Research Articles

Estimates of life history parameters in a high latitude, arid-country vervet monkey population.

We present data on life history parameters from a long-term study of vervet monkeys in the Eastern Cape, South Africa. Estimates are presented of age at first conception for females and age at natal dispersal for males, along with the probability of survival to adulthood for infants born during the study, female reproductive life-span, reproductive output (including lifetime reproductive success for a subset of females), and inter-birth interval (IBI) duration. We also assess the effect of maternal age and infant survival on length of IBI. We then go on to compare life history parameters for our population with those from two East African populations in Kenya (Amboseli and Laikipia). We find there is broad consensus across the three populations, although mean infant survival was considerably lower for the two East African sites. Such comparisons must be made cautiously, however, as local ecology across the duration of the studies obviously has an impact on the estimates obtained. With this caveat in place, we consider that the concordance between values is sufficient to enable the values reported here to be used in comparative studies of primate life history, although data from habitats with higher rainfall and lower levels of seasonality are needed, and the results presented here should not be seen as canonical.

A model of the\xa0evolution of equitable offers in n-person dictator games with interbirth intervals

People are often generous even when it is not in their apparent self-interest to do so as demonstrated by numerous experiments using the dictator game (DG). More recent research using DGs has varied the number of dictators and recipients and used these games to investigate the bystander effect and congestible altruism. These studies have found that people are less generous when there are others who could be generous (the bystander effect) and also less generous when there are multiple recipients (congestible altruism) though the sum of their generosity to all recipients increases. A working hypothesis is proposed that the context-sensitive generosity observed in n-person DGs can be explained as equitable behavior. From an evolutionary perspective, explaining the evolution of equitable behavior is challenging at best. To provide an evolutionary explanation, a second working hypothesis is proposed: equitable offers evolve because they reduce resource deficits produced by variability in the accumulation of resources and thereby minimize the length of interbirth intervals (IBIs) and increase fitness. Based on this working hypothesis, an evolutionary model was developed for n-person DGs to investigate the evolution of equitable offers as a resource allocation problem when reproduction is constrained by IBIs. Simulations demonstrated that equitable offers could evolve in group-structured populations when there is a cost (i.e., longer IBIs) to running resource deficits. Mean evolved offers also varied as a function of the number of dictators and recipients in patterns consistent with the bystander effect and congestible altruism. Equitable offers evolved because they reduced resource variability among group members and thereby reduced resource deficits, which insured higher average rates of reproduction for more equitable groups of agents. Implications of these results are discussed.

Parental investment in Tibetan populations does not reflect stated cultural norms.

In this paper, we examined both stated norms of sex preference and actual sex-biases in parental investment in a Tibetan pastoralist society. We collected detailed demographic data on infant mortality, infant feeding, the length of interbirth intervals, and a decision when giving gifts, to examine sex-biased parental investment. Our results indicate a mismatch between self-reported son preference and measures of actual parental investment that favor daughters. We interpret this female-biased parental investment as a possible response to daughters generating more economic resources. However, the stated sex preferences of both sexes reflect cultural norms that appear to have remained unchanged over a long period, which may reflect the importance of male roles in the past. Our behavioral measures of parental investment are those most likely to be in the control of women (such as breastfeeding and interbirth interval), so this mismatch between stated and actual investment may be especially true of women.

The Effect of Paternal and Alloparental Support on the Interbirth Interval Among Contemporary North American Families.

The present study investigated whether the length of interbirth intervals between first and second-born children in a North-American middle-class sample could be explained by paternal and alloparental support and firstborn children's gender. The sample consisted of 225 families in which mothers were expecting their second child. Parents reported on paternal and alloparental support (maternal kin, paternal kin, and non-kin support). The results showed that higher maternal kin support and having a firstborn son was linked with shorter interbirth-intervals. Mothers' longer work hours during the pregnancy with the second born was related to longer interbirth intervals. These results highlight the importance of maternal kin support and children's characteristics in understanding the timing of birth when parents have a second child.

Medical, ethical and personal dimensions of parent-offspring conflicts

Medical, ethical and personal dimensions of parent-offspring conflicts

Impact of male infanticide on the social structure of mountain gorillas.

Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The overall rates of infanticide were 2–3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.

Inter-birth interval in zebras is longer following the birth of male foals than after female foals

Mammalian reproductive rates vary among individuals for physiological and environmental reasons. This study aims to determine reproductive rates from an individually monitored population of wild Plains zebras Equus quagga, and to assess the sources of variability in inter-birth intervals. The animals were monitored, where possible, every six months from 2004 to 2011. Thirty nine intervals corresponding to 65 births in 26 mares were identified, using direct observations and faecal steroid monitoring. Mean foaling rate of the population is 0.74 foal/year, and comparable with the literature. There was no significant effect of mother’s age, nor of the season of previous birth on the length of inter-birth intervals. Inter-birth interval was significantly longer when the first foal was a male. This finding indicates that additional costs of having a son may delay future reproduction and thus reduce the total number of offspring a mare can have during her lifetime. Individually-based data provide critical information on the determinants of reproductive rates, and are therefore a key to understanding the causes of variations in life-history traits.

Evolutionary endocrinology: Integrating proximate mechanisms, ontogeny, and evolved function

Natural selection leads to the evolution of strategies that tend to optimize the allocation of resources to the competing somatic and reproductive demands faced by any organism. This theoretical approach, known as life history theory, is useful for understanding unavoidable trade-offs in resource allocation and the resulting variation in phenotypes (e.g., physical attributes, expressed behaviors, the number and size of offspring, etc.) (Charnov 1993; Stearns 1992). In recent years, life history theory has begun to influence research on the proximate mechanisms mediating ontogenetic strategies. The endocrine system plays a central role in coordinating ontogenetic processes and modulating cross-talk among different functional domains (e.g., hypothalamic–pituitary–gonadal and hypothalamic–pituitary–adrenal axes). These processes are integral to the evolution of phenotypic plasticity (a key feature of adaptive responses) in developmental schedules and other life-history traits. From conception to death, hormones orchestrate growth, differentiation, maintenance, reproduction, and senescence. Pheromones may help parents-to-be to attract each other (Kohl et al., 2001; Hays, 2003). Androgen steroids increase libido encouraging them to mate (Hutchinson, 1995; Stern and McClintock, 1998). Gonadotrophins and gonadal steroids mediate gamete maturation and release. Endometrial steroids intervene in the capacitation of spermatozoa and chemotaxis, luring them towards the oocyte. Recognition, binding, and fusion between sperm and oocyte, and the consequent activation of the egg are mediated by a multitude of hormones (Sun et al., 2005). After fertilization, but before development of the endocrine system, chemical signals help guide cleavage, cell differentiation, migration, and growth. Once developed, the endocrine system transmits information between cells regarding internal and external conditions, coordinating a intricate dance between the epigenome and the genome, steering the organism through its ontogenetic road map. All organisms of a given species follow a similar ontogenetic road map. Yet, as environmental opportunities, restrictions, and challenges are heterogeneously distributed between and within populations, each individual passes life-history mileposts at its own speed and according to its own strategy. The endocrine system is an integral part of the mechanisms that provide organisms with the plasticity and dynamism that are crucial for adapting to their individual contexts. The challenges to be met are broad in type and temporal scales. Some challenges involve almost all biologic systems and take place over long periods of time, such as coordinating proportionate growth or undergoing sexual maturation. Others involve fewer tissues but need to be achieved within minutes or hours, such as adjusting oxygen or blood glucose levels. Some challenges are predictable, such as the sequences of night and day, or the succession of changes that follow conception. Other challenges, such as a tornado or an aneurysm, may be less predictable. Organisms rely on the endocrine system to regulate responses to these challenges as well as to inform ontogenetic strategies based on them. When and how much to grow at each life stage, when to begin sexual maturation, when to stop growing, the timing of the first reproductive venture, the length of inter-birth intervals, and when to stop reproducing are just a few examples of critical life history ‘‘decisions’’ mediated by hormonal pathways. Ontogenetic strategies are modified through neuroendocrine cues that trigger changes in gene expression. Phenotypic plasticity is ultimately achieved by changes in DNA methylation patterns and chromatin remodeling. These epigenetic changes provide great dynamism to genotypic expression. Recent research developments suggest that while some epigenetic DNA methylation patterns are preserved through multiple generations, others are not as stable as originally believed and can change in response to postnatal environmental stimuli at different times within an individual’s life span (McGowan et al., 2008; Szyf et al., 2008). The epigenome, informed by neuro-endocrine cues, is critical for fine tuning the expression of the more static genotype in response to the demands that the environment places on the phenotype. Given the dynamic and complex nature of the endocrine system, a holistic approach to its study should consider not only the longitudinal variation of hormones across days, seasons, and life stages, but also its interactions with the nervous system and the epigenome. Anthropologists are in the privileged position necessary for conducting the multidisciplinary research that will help to realize this admittedly daunting research agenda. The collection of high quality longitudinal data achieved through an intimate knowledge of study participants allows for the description of normal ranges of endocrine function and its variability in response to environmental challenges in nonclinical populations exposed to a broad range of social, economic, and physical contexts. The 2008 Plenary Session of the Human Biology Association explored recent advances in the study of endocrine pathways approached from this holistic perspective. The

Relationship between subsistence and age at weaning in "preindustrial" societies.

Cross-cultural studies have revealed broad quantitative associations between subsistence practice and demographic parameters for preindustrial populations. One explanation is that variationin the availability of suitable weaning foods influenced the frequency and duration of breastfeeding and thus the length of interbirth intervals and the probability of child survival (the "weaning food availability" hypothesis). We examine the available data on weaning age variation in preindustrial populations and report results of a cross-cultural test of the predictions that weaning occurred earlier in agricultural and pastoral populations because dairy and cereal production increased the availability of easily digestible, nutrientrich foods appropriate for weaning. We found that, contrary to predictions, supplementation with liquid foods other than breast milk was delayed in agricultural populations relative to less agriculturally dependent ones and complementary feeding with solid foods was delayed in pastoral populations relative to those less dependent on herding. Although the duration of breastfeeding was longer in populations dependent on hunting, there was no qualitative evidence that such populations lacked foods appropriate for weaning. The patterns observed suggest that the relationships between demography and subsistence observed among preindustrial societies cannot be explained by the "weaning food availability" hypothesis. We discuss the implications for understanding the mechanisms underlying prehistoric human demography, subsistence shifts, allocation to parenting and mating effort, and the evolutionary implications of tradeoffs between diet and disease.

Reproductive intervals in captive female western lowland gorillas with a comparison to wild mountain gorillas.

A wide disparity between parturition and the resumption of sexual behavior in two captive female western lowland gorillas prompted an investigation of interbirth intervals (IBIs) in captive western lowland gorillas that had reared their infants. To determine reproductive intervals, a questionnaire was sent to 30 institutions (19 in North America and 11 in Europe) that maintained captive females who had previously reared or were currently rearing their offspring. Information compiled relates to the length of reproductive intervals, including lactational anestrus, estrous behavior to conception and from parturition to conception, and IBIs. These data are compared with studies published on wild mountain gorillas. The median IBI for 16 mothers still rearing infants when their next conception occurred was 4 years (range 2.33-6.42 years, N = 16). Twenty-one percent of the 52 mothers in this study are no longer reproducing. The mean time without reproduction since separation from their last offspring was 5.43 years. This study indicates that the length of IBIs, lactational anestrus, and intervals from parturition, separation, and estrous behavior to conception compare closely with findings for wild populations. Incompatibility between partners may be a factor limiting further reproduction in some captive mothers in this study.

Sources of variation in interbirth intervals among captive bonnet macaques (Macaca radiata)

Female fitness is a function of variation in the length of females' reproductive careers, the viability of their offspring, and the frequency with which they give birth. Infant loss shortens interbirth intervals in most primate species, but we know considerably less about other factors that contribute to variation in the length of interbirth intervals within groups. In one large captive group of bonnet macaques, maternal parity, age, experience, family size, and recent reproductive history are all associated with variation in the length of intervals that follow the birth of surviving infants. Primiparous females have the longest interbirth intervals, while multiparous females who have produced surviving infants in the past and have raised their last infant successfully have the shortest interbirth intervals. Infant sex and maternal rank have no direct effect upon the length of interbirth intervals. One of the underlying causes of variation in the length of interbirth intervals after surviving births seems to be variation in the timing of conceptions among females. Females who conceive early in the mating season tend to have shorter interbirth intervals than other females. However, females who are multiparous, experienced, and have recently raised infants have late conceptions and short interbirth intervals.

The influence of maternal rank and infant sex on maternal investment trends in rhesus macaques: birth sex ratios, inter-birth intervals and suckling patterns

In the new colony of rhesus macaques at Madingley, no overall difference was found in the length of the inter-birth intervals following the birth of male and female infants. The lack of overall differences in the reproductive costs of raising sons and daughters, was associated with an absence of differences in suckling patterns between male and female infants. It is argued that similar pre-weaning investment in sons and daughters may be related to the presence of similar growth rates for male and female infants during the first year of life. Most high ranking mothers reproduced in consecutive years, while low ranking mothers were more likely to experience two year long inter-birth intervals. Further analyses revealed that this difference was mainly due to the fact that low ranking mothers always failed to reproduce the year after raising a daughter. Thus, no difference was found in the length of inter-birth intervals between high ranking mothers with infants of either sex and low ranking mothers with sons. The daughters of low ranking females suckled more frequently and making more nipple contacts per bout, and such high nipple stimulation was responsible for the reproductive inhibition experienced by their mothers. It is suggested that the high degrees of nipple stimulation may have been a consequence of the high levels of aggression that low ranking females with daughters have been shown to receive, since their mothers could have allowed frequent ventro-ventral contact in order to protect them. Given that low ranking mothers find daughters reproductively costly to raise, it is possible that such high reproductive costs have played a significant role in the evolution of sex ratio biases at birth. As in other primate populations, in this colony low ranking females produced a greater proportion of sons than daughters, and high ranking mothers showed the opposite trend.

Length of interbirth intervals in female psychiatric patients and controls.

Length of 121 interbirth intervals for 77 multiparous female psychiatric patients was contrasted with the length of 605 interbirth intervals for 385 matched population-control women to test a clinical observation that female psychiatric patients often have long intervals between consecutive reproductions. No significant difference was found between all patients versus all controls or between any patient diagnostic group versus controls in studying the length of only those intervals including or subsequent to the first psychiatric hospitalization. Intervals during which this hospitalization occurred were significantly longer for all patients combined, for schizophrenic-like-psychotics, and for psychopaths, each as compared with its parity-matched control group. The clinical observation was not generally supported by the current data.