Infanticide Rates Research Articles

The Insectan Apes

I present evidence that humans have evolved convergently to social insects with regard to a large suite of social, ecological, and reproductive phenotypes. Convergences between humans and social insects include: (1) groups with genetically and environmentally defined structures; (2) extensive divisions of labor; (3) specialization of a relatively restricted set of females for reproduction, with enhanced fertility; (4) extensive extramaternal care; (5) within-group food sharing; (6) generalized diets composed of high-nutrient-density food; (7) solicitous juveniles, but high rates of infanticide; (8) ecological dominance; (9) enhanced colonizing abilities; and (10) collective, cooperative decision-making. Most of these convergent phenotypic adaptations stem from reorganization of key life-history trade-offs due to behavioral, physiological, and life-historical specializations. Despite their extensive socioreproductive overlap with social insects, humans differ with regard to the central aspect of eusociality: reproductive division of labor. This difference may be underpinned by the high energetic costs of producing offspring with large brains.

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.

Why leopards commit infanticide

Infanticide is a widespread but seldom observed behaviour that has been shown to convey strong selection forces on some social mammals. However, infanticide in nonsocial species is less clearly understood, particularly the evolutionary function of infanticide. Here we present direct and indirect evidence of infanticide in a large, solitary felid, the African leopard, Panthera pardus, and determine its likely causes in light of six explanatory hypotheses. Observed and inferred rates of infanticide for leopards were among the highest recorded for mammalian carnivores, accounting for almost half of juvenile mortality and nearly a third of all offspring. Our results demonstrate that infanticide in leopards is mainly an adaptive behaviour which provides reproductive benefits to perpetrators. Infanticidal males were typically unrelated to their victims, the death of unweaned offspring shortened the interbirth intervals of mothers, and perpetrators increased their chances of mating with and siring the mother's subsequent litter. Female leopards also appear to have developed a profusion of strategies to counter the risk of infanticide, providing further support for the sexual selection hypothesis. Cannibalism may provide some incentive for infanticide (perpetrators usually consumed their victims) but this is probably an added, minor benefit rather than the primary driver for the behaviour. Our findings suggest that infanticide is a key factor shaping the sociospatial ecology of leopards. They also highlight the importance of accounting for functional components of population dynamics when managing large carnivores, particularly for invasive activities that artificially elevate adult male turnover.

Food distribution influences social organization and population growth in a small rodent

In polygynous mammals, the spatial clumping and predictability of food should influence spacing behavior of females whose reproductive success depends to a great extent on food availability, which would in turn affect male spacing behavior. Changes in the social and mating systems can then influence individual fitness and population dynamics. To test these hypotheses, we manipulated food distribution and predictability in enclosed populations of bank voles (Myodes glareolus) and monitored spacing behavior, survival, and reproduction of adult females and males over 3 months. Food was either spread out (dispersed treatment), spatially clumped and highly predictable (clumped treatment) or spatially clumped but less predictable (variable treatment). We found that females in the clumped treatment were more aggregated and had more overlapping home ranges compared with females in the dispersed and variable treatments. Male spacing behavior followed the same patterns. Despite different social organizations between treatments, no differences in home range size and mating systems were found in females and males. In addition, we found that females in the clumped food treatment had a higher probability of successfully producing weaned offspring, likely due to lower infanticide rates. This led to higher population growth compared with the other 2 treatments. These results suggest a tight relationship between the spatiotemporal distribution of food, social organization, and population dynamics.

Infanticide risk and male quality influence optimal group composition for Colobus vellerosus

The threat of infanticide by males is suggested to determine upper group size limits for some folivores because large female aggregations attract immigrating males. When groups get large enough to become multimale, infanticide risk should decline because, all other things being equal, more males should deter outside takeovers and the counterstrategies of mothers and sires should lower the consequences of inside takeovers. To determine if this scenario can be generalized to all folivorous primates, we began by examining the influence of female and male group size on 4 increasing levels of infanticide risk in folivorous ursine colobus monkeys (Colobus vellerosus) at Boabeng-Fiema, Ghana. Data from 2004 to 2010 on 7 groups analyzed with generalized estimating equation models showed that infanticide risk is heightened in groups with more females and absolutely more males because male immigration is more frequent. However, when controlling for the effect of female group size on male group size, groups with relatively more males have significantly more male immigration and higher infanticide rates. Thus, multimale groups of C. vellerosus show higher infanticide risk compared with unimale groups. Resident males in unimale groups may be higher quality than those in multimale groups because they perform more energetically costly displays and win more encounters. However, males in multimale groups may also suffer from collective action problems making them unable to prevent male immigration and infanticide. This study highlights the importance of the quality of male defenders in infanticide risk and demonstrates intersexual and intrasexual conflict in group composition in C. vellerosus.

Remaking the Medico-Legal Scene: A Social History of the Late-Victorian Coroner in Oxford

There have been wide-ranging debates about medicine and the law encapsulated in the figure of the coroner in Victorian England. Recently the historical literature on coroners has been enriched by macro-studies. Despite this important research, the social lives of coroners and their daily interactions remain relatively neglected in standard historical accounts. This article redresses that issue by examining the working life of the coroner for Oxford during the late-Victorian era. Edward Law Hussey kept very detailed records of his time in office as coroner. New research material makes it feasible to trace his professional background, from doctor of the sick poor, to hospital house surgeon and then busy coroner. His career trajectory, personal interactions, and professional disputes, provide an important historical prism illuminating contemporary debates that occupied coroners in their working lives. Hussey tried to improve his medico-legal reach and the public image of his coroner's office by reducing infanticide rates, converting a public mortuary, and acquiring a proper coroner's court. His campaigns had limited success because the social scene in which he worked was complicated by the dominance of health and welfare agencies that resented his role as an expanding arm of the Victorian information state.

Effects of siblings on reproductive maturation and infanticidal behavior in cooperatively breeding Mongolian gerbils.

Mongolian gerbils living with their natal families undergo delayed reproductive maturation while helping to rear their younger siblings, whereas those housed away from their natal families may mature earlier but often respond aggressively to unfamiliar pups. We tested whether cohabitation with pups contributes to reproductive suppression and inhibition of infanticidal behavior, using young males and females housed with (1) their parents and younger siblings (pups), (2) parents without pups, (3) mixed-sex littermate groups, or (4) mixed-sex groups of unrelated peers. Maturation in males was inhibited by cohabitation with the parents, while maturation in females was further suppressed in the presence of pups. Males in all housing conditions showed little aggression towards unfamiliar pups, whereas females were usually infanticidal unless housed with pups. Aggression toward pups was especially pronounced in females that were pregnant or undergoing ovulatory cycles. Thus, cohabitation with younger siblings may intensify reproductive suppression and inhibit infanticidal behavior in female gerbils, whereas male gerbils exhibit parentally induced reproductive suppression and low rates of infanticide even in the absence of younger siblings.

Formal submission, tolerance and socioecological models: a test with female Hanuman langurs

Patterns of agonistic relationships among group-living mammalian females are suggested to reflect differences in feeding competition. Socioecological models predict that within-group contest (WGC) competition leads to despotic relationships that are linear, consistent, and stable, in which subordinate individuals use formal submission to acknowledge rank. Between-group contest (BGC) competition and risk of infanticide are predicted to add tolerance, as evidenced by frequent reconciliation, low-intensity aggression and counteraggression. We examined the predicted effects for formal submission and two criteria of tolerance for a group of Hanuman langurs ( Semnopithecus entellus ) in a population known to experience WGC and BGC as well as high rates of infanticide. Results were based on focal animal sampling of eight of 13 adult females (419 h) and ad libitum sampling of all 13 females (711 contact hours) in the group Kailana I (Jodhpur, India). The behaviour ‘presenting’ ( N = 281) qualified as formal submission, adhering to most proposed criteria (e.g. linear, consistent, predictive of other agonistic relationships and hierarchies, 40.9% spontaneous). Thus, regarding formal submission, the predicted effects of WGC hold, and support the explanatory value of this aspect of socioecological models. In contrast, agonistic relationships were nontolerant, with frequent high-intensity aggression (48.9% of N = 231) and almost no counteraggression (

Baboons

Baboons

Infanticide and Maternal Offspring Defence in European Rabbits under Natural Breeding Conditions

AbstractInfanticide is an important source of mortality of dependent offspring in several mammal species, whereas female conspecifics are often the perpetrators. This has led to maternal counter‐strategies, such as the defence of the nests. However, cases of infanticide are hard to detect in the field, and studies on maternal offspring defence behaviour under natural breeding conditions are scarce. We conducted such a study on the European rabbit (Oryctolagus cuniculus), which is usually considered to show low maternal care. The study was carried out over 5 yr on a field enclosure population. We (1) studied infanticide rates and the impact of potential determinants: the group density and age structure of the females’ rank hierarchy within the groups; we used the latter as an estimator of social group stability. We (2) studied if mothers defend their breeding burrow against approaching, potentially infanticidal females. Overall, we recorded infanticide in 5% of all litters; infanticide was the cause in 12% of cases of litter mortality. The proportion of infanticide was 7% higher in groups where same‐age females occupied successive rank positions than in groups where the females’ rank hierarchy had a more heterogeneous and linear age structure. We hypothesize that social instability in the former groups was the reason for the increased infanticide risk. Infanticide rates were not correlated with group density and did not differ among mothers with different social ranks. Infanticide occurred exclusively during the first 10 d after parturition. During this time, mothers stayed closer to their breeding burrows than shortly before parturition or during later lactation. Moreover, mothers were more aggressive against other females in proximity to their breeding burrow than in more distant areas. We suggest that the pattern of spacing behaviour and intrasexual aggression of rabbit mothers are an adaptive response to the occurrence of female infanticidal behaviour.

Correlates of stress in free-ranging male chacma baboons, Papio hamadryas ursinus

In male baboons, social instability is known to increase concentrations of glucocorticoids (GCs), a class of steroid hormones associated with the vertebrate stress response. The stress response may be particularly marked among male chacma baboons, whose social groups often demonstrate a strong mating skew and high rates of infanticide. Here we test this prediction and explore additional correlates of variation in concentrations of GCs measured in faeces. We collected 482 faecal samples from 13 males over 14 months. Changes in the male hierarchy were the major source of GC variation. Such instability occurred as the result of a new immigrant male's take-over of the alpha position, a resident male's rise to the alpha position, and the rise in rank of a young natal male. All three events were correlated with elevated GCs among all males and higher rates of male–male aggression than were found during periods of stability. The interaction between rank and instability had a significant effect on GC concentrations; low-ranking males had higher GCs during stable periods, whereas high-ranking males had higher GCs during unstable periods. Males that dispersed had elevated GCs in the month following immigration but not at other times, including the month preceding emigration. Finally, entering into a sexual consortship led to a temporary increase in a male's GC measures. By contrast, seasonal changes and a male's age were unrelated to GCs. GC variation in chacma males was primarily explained by temporary elevations following events that relate to reproductive fitness.

Do gorilla females join males to avoid infanticide? A quantitative model

Gorilla, Gorilla gorilla, females associate permanently with males. What benefit do the females obtain from the association? Where males are larger than females, as in the gorilla, protection from predation is the long-standing answer. However, protection from infanticidal nonfather males is increasingly suggested as a better hypothesis. Given that a female's alternative strategy is to range alone and mate with many males, a hitherto ignored problem of the anti-infanticide hypothesis is that a female's joining a single male necessarily maximizes the proportion of males in the population who can use the infanticidal strategy. To ask whether lone gorilla females could mate with enough males to decrease the probability of infanticide below that observed in the wild, we use novel modifications of a gas molecule equation to model encounter rates of a lone female with males. The modifications include separation of encounter rates in fertile periods from those in nursing periods and, importantly, a decline function that converts infanticidal males to noninfanticidal, on mating. Parameters include density of males, speed of travel, and duration of fertile and nursing periods. In two other great apes (chimpanzee Pan, orang-utanPongo ), females are more or less solitary, and infanticide rates are very low (≤5%). With these species' values in the model, equivalently low infanticide rates result. Thus, the model appears valid. For the gorilla, the model indicates that under most realistic conditions, solitary females mate with so few males that infanticide rates are more than three times the observed rate of 14% of births. Association with a male thus decreases the probability of infanticide. The anti-infanticide hypothesis for grouping is thus supported. But the antipredation hypothesis is not thereby negated, and remains a powerful hypothesis, given that predators are a danger to the female, as well as her infant.

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles ( Microtus ochrogaster)

Prairie voles ( Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal ‘sensitive period’ during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15–19 days of the 22-day gestational period or postnatally on days 1–7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal ‘sensitive period’ and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.

Malthusian Models and Chinese Realities: The Chinese Demographic System 1700–2000

This article summarizes major recent findings on Chinese demographic behavior and outlines their relevancy for the Malthusian model of comparative population dynamics and Chinese population in particular. Specifically, it considers four distinctive and persistent features of Chinese behavior during the last 300 years—high rates of female infanticide and abortion, high rates of bachelorhood, low marital fertility, and high rates of male and female adoption–and discusses the origins and implications of such a demographic regime for Chinese economic and social development. Contrasting Chinese demographic behavior with European demographic behavior, the article argues the existence of a demographic system and a demographic transition different from current Malthusian and neo‐Malthusian models, and the existence of a system regulating collective demographic behavior in ways distinctly different from Western experience.

Infant injury death in Washington State, 1981 through 1990.

To measure annual rates of infant injury death in Washington State and identify potential risk factors for infant injury death. A population-based case-control study using a linked file of birth and death certificates for the years 1981 through 1990. The leading causes of infant injury death, per 100,000 live births, were intentional injury (9.1); suffocation, choking, or hanging (6.9); motor vehicle crashes (4.4); and drowning (3.2). Unintentional injury death rates fell steeply during the 1970s but remained steady during the 1980s. Infanticide rates rose during the 1980s. Young maternal age (odds ratio, 9.0; 95% confidence interval, 4.5 to 17.9, for age < 20 years vs age > 30 years) and higher parity (odds ratio, 5.0; 95% confidence interval, 3.1 to 8.3, for parity of two or more vs parity of zero) were associated with infant injury death. There was a twofold increase in the risk of infant injury death if the mother smoked, was black, or lived in a rural area. Male sex of the infant, late prenatal care, and low income were associated only with intentional deaths, while higher parity was more strongly associated with unintentional injury deaths. During 1981 through 1990, infant mortality caused by injuries increased in Washington State. Young maternal age and higher parity were strongly associated with infant injury death.

Environmental Influences on Infanticide in Columbian Ground Squirrels

AbstractInfanticide of preweaned and weanling juvenile Columbian ground squirrels (Spermophilus columbianus) has been previously reported, but its prevalence in most populations is poorly known. I examined evidence of infanticide in a six‐year study of four populations in southwestern Alberta, two of which received an experimental supplementation of food for two years. Evidence of infanticide of weanling ground squirrels was uncommon, except for one population and year (previously supplemented population at 1580 m elevation in 1984) in which 9 dead young were found, suggesting an increased rate of infanticide. The rate of total loss of pre‐weaned litters also increased at 1580 m in 1984. At higher elevation sites (2100 m), however, no evidence of infanticide was found under experimental or unmanipulated conditions. One factor that might have influenced the rate of infanticide of weanlings was their degree of development when they first emerged from natal burrows. A review of studies of 11 populations of Columbian ground squirrels suggested that under natural conditions, infanticide may have differed among environments. Differences in density among populations appeared to have little influence on the rate of infanticide, but elevation was significantly negatively associated with infanticide.

The effects of prenatal stress and postweaning housing conditions on parental and sexual behavior of male Long-Evans rats

Male offspring of stressed and control female rats were housed in pairs with a control male or a female from 22 to 90 days of age, then they were screened for infanticide and tested for parental behavior and sexual behavior. Male rats housed with females were less likely to be infanticidal than males housed with other males, but prenatal stress had no effect on infanticide rate. Control males housed with males showed a longer latency to carry pups than did males in the other three groups, and there was a stress × days interaction in the frequency of parental behavior, with prenatally stressed males showing more parental behavior than control males after 4 days of pup exposure. In the male sexual behavior tests, prenatally stressed males had longer intromission latencies and shorter postejaculatory intervals than did control males, while males housed with females had fewer mounts without intromissions and shorter ejaculation latencies than did males housed with other males. Female sexual behavior was not influenced by stress or housing, but control males showed more aggression toward stud males than did stressed males. Stressed male rats had smaller testes than did control males. Correlations among these measures indicate that prenatal stress results in a general syndrome of behavioral change.

Early dry season subsistence ecology of Cuiva (Hiwi) foragers of Venezuela

The subsistence ecology of Venezuelan Cuiva foragers during the early dry season is described. Data on diet, time allocation, demography, and physical measurements are presented. Analyses show that the Cuiva depend primarily on game and wild roots during the early dry season for their subsistence. Sex differentials in productive efficiency, total contribution to the diet, and time allocation to food acquisition and other activities are also examined. As in most other foraging societies, men specialize in hunting while women specialize in gathering. During the early dry season, men provide more calories than women and are the more efficient food producers. However, men spend slightly less time than women in food acquisition. Demographic data show that child mortality rates, female infertility rates, female infanticide rates,and the sex ratio among juveniles are high in the Cuiva population. Comparisons between the patterns found among the Cuiva and other foraging populations are made.

Peopling of the Arctic: A Computer Simulation

The research described in this paper involved the development of a computer program designed to simulate population growth and migration patterns among hunter-gatherers, especially with respect to the Arctic. The program, which handles up to 200 discreet geographical locations, each with its own particular demographic and environmental characteristics, begins with an initial population and its vital statistics and simulates the events that occur through time. The fertility and mortality rates used in the simulations were those of modern and former Eskimo populations and other anthropological populations. The program was run under five different conditions. Condition 1 included high mortality and fertility rates and no female infanticide and resulted in extinction with little population dispersion. Condition 2, a situation of low mortality and high fertility with no infanticide, resulted in the occupation of nearly the entire Arctic in 1300 years. Condition 3 included the same mortality and fertility rates as condition 2, with the incorporation of a 30% rate of female infanticide. Under this condition, the population declined very slowly, while migration proceeded to some extent. Condition 4 represented a situation of very high fertility and mortality with 30% female infanticide and resulted in relatively rapid growth and migration rates. Condition 5, which incorporated the same high fertility and infanticide rates as condition 4 and lower mortality rates, produced very rapid population growth and migration. Key words: computer simulation, demography, Eskimos, female infanticide, fertility, paleodemography

Social and hormonal factors influencing infanticide and its suppression in adult male Long-Evans rats (Rattus norvegicus).

In a series of six experiments, the experiential and hormonal factors influencing infanticide in Long-Evans hooded rats were examined. Males, but not females, purchased from a breeder as adults exhibited infanticide at high rates, whereas males reared in the laboratory until adulthood were less likely to be infanticidal. Sexual experience reduced the rate of infanticide in laboratory-reared males, and cohabitation with a female through pregnancy and lactation (including exposure to pups) inhibited infanticide in previously infanticidal males. Fourteen days of cohabitation with a pregnant female following copulation inhibited infanticide in males purchased from a breeder, but copulation without cohabitation and copulation followed by cohabitation with a nonpregnant (ovariectomized) female did not inhibit infanticide. Castration of laboratory-reared males in adulthood did not reduce their rate of infanticide, whereas testosterone implants increased the number of males exhibiting infanticide. The importance of hormonal and experiential factors for controlling infanticide in male rats is discussed.