Infanticide: Let's not throw out the baby with the bath water

Ethology and Sociobiology, 1979

Infanticide among animals is a widespread phenomenon with no unitary explanation. Although the detrimental outcome for the infant is fairly constant, individuals responsible for infanticide may or may not benefit, and when they gain in fitness there may be considerable variation ...

Biological Reviews of the Cambridge …, 1998

I analyse and summarize the empirical evidence in mammals supporting alternative benefits that individuals may accrue when committing nonparental infanticide. Nonparental infanticide may provide the perpetrator with nutritional benefits, increased access to limited resources, increased reproductive opportunities, or it may prevent misdirecting parental care to unrelated offspring. The possibility that infanticide is either a neutral or maladaptive behaviour also is considered. I devote the second half of this article to reviewing potential mechanisms that individuals may use to prevent infanticide. These counterstrategies include the early termination of pregnancy, direct aggression by the mother against intruders, the formation of coalitions for group defence, the avoidance of infanticidal conspecifics, female promiscuity, and territoriality. I evaluate the support for each benefit and counterstrategy across different groups of mammals and make suggestions for future research.

Proceedings of the National Academy of Sciences, 2010

Alpha male chacma baboons experience uncontested access to individual estrus females. Consequently, alpha male paternity certainty is high and underpins significant levels of infanticide by immigrant males that, in turn, has selected for male defense of infants. There is also, however, a high probability that alpha males will be absent during the period when their own offspring are vulnerable, suggesting selection for additional countermeasures. We use data from a long-term study to test the prediction that alpha male chacma baboons cede reproductive opportunities to subordinate males and that this leads to the presence of other fathers that can serve as a buffer against infanticidal attack. We found that subordinate males obtained significantly more conceptive opportunities than predicted by priority of access alone, and that this occurred because alpha males did not consort all receptive periods. There was no evidence that this was due to energetic constraint, large male cohorts, ...

Evolutionary Anthropology, 2005

The students nodded. They had all studied animal behavior, and they knew, for example, that when a new male took over a lion pride, the first thing he did was kill all the cubs. The reason was apparently genetic: the male had evolved to disseminate his genes as widely as possible, and by killing the cubs he brought all the females into heat, so that he could impregnate them.

Behavioral Ecology and Sociobiology, 1997

Territoriality among female rodents may have evolved as an adaptation to intraspeci®c competition for resources or, alternatively, to defend pups against infanticide. In order to evaluate the latter, we analyse the conditions that allow an infanticidal strategy to invade a population of non-infanticidal females, and the circumstances under which infanticide may become an evolutionarily stable strategy (ESS). Our game theoretical analyses indicate that infanticide has to be associated with some direct (cannibalism) or indirect (reduced competition) resource bene®ts in order to invade a non-infanticidal population. We also expect that females will primarily kill litters of nearby neighbors, thereby removing the closest competitors while keeping costs at a low level. However, once established in a population, infanticide may be an ESS, even if females do not gain any resource bene®ts. This is theoretically possible if a female through infanticide can reduce the possibility that other, potentially infanticidal, females establish and/or stay close to her nest. While behavioral data indicate that these special circumstances sometimes occur, they may be too speci®c to apply generally to small rodents. Therefore, we expect that the evolutionary stability of infanticide often requires resource bene®ts, and that female infanticide in small rodents may, in fact, be a consequence rather than a cause of territoriality.

Animal Behaviour, 1985

An evolutionarily stable strategies analysis of infanticide, expressed as two pure strategies, shows that the equilibrial frequency of infanticide depends on only two parameters: the relative advantage of infanticide when others are also infanticidal and the relative advantage of a non-infanticidal strategy if others are non-infanticidal. The possible equilibrial conditions of strictly infanticidal, strictly non-infanticidal, stably polymorphic or history-dependent populations correspond to various earlier models of infanticide. Thus, these earlier models may be subsumed as special cases of the ESS analysis. This permits the comparison of alternative theories of infanticide in specific cases of infanticide in natural populations. In particular, it is argued that if infanticide confers a relative advantage on the perpetrator when other population members are non-infanticidal, this is sufficient to reject a 'maladaptive' explanation for infanticide. The potential existence of history-dependent equilibria suggests there will be extreme difficulty with non-experimental methods that attempt to test the adaptiveness of infanticidal behaviour.

American Journal of Physical Anthropology, 2018

Objectives: Infanticide by males is common in mammals. According to the sexual selection hypothesis, the risk is inversely related to infant age because the older the infant, the less infan- ticide can shorten lactational amenorrhea; risk is also predicted to increase when an infanticidal male's chance of siring the replacement infant is high. Infanticide occurs in chimpanzees (Pan troglodytes), a species in which male dominance rank predicts paternity skew. Infanticidal male chimpanzees (if low-ranking) are unlikely to kill their own offspring, whereas those who are cur- rently rising in rank, particularly when this rise is dramatic, have an increased likelihood of fathering potential future infants relative to any existing ones. Given that mothers should behave in ways that reduce infanticide risk, we predicted that female chimpanzees, and specifi- cally those with younger, more vulnerable infants, would attempt to adjust the exposure of their infants to potentially infanticidal males. Specifically, mothers of young infants should reduce their association with adult males in general, and to a greater extent, with both low-ranking males and those rising in rank from a position where paternity of current infants was unlikely, to a rank where the probability of siring the next infant is significantly higher. We also investigated the alternative possibility that rather than avoiding all adult males, mothers would increase asso- ciation with males of stable high rank on the basis that such males could offer protection against infanticide. Materials and methods: We examined data on female association patterns collected from the Budongo Forest, Uganda, during a period encompassing both relative stability in the male hierar- chy and a period of instability with a mid-ranking male rising rapidly in rank. Results: Using linear mixed models, we found that mothers reduced their association with the rank-rising male, contingent on infant age, during the period of instability. We also found evi- dence that females preferentially associated with a potential protector male during the high-risk period. Discussion: Our results support the sexually selected hypothesis for infanticide and demon- strate that female chimpanzees are sensitive to the relative risks posed by adult males.

Behavioral Ecology and Sociobiology, 2011

The evolution of social monogamy in larger mammals is difficult to explain because males usually do not invest much in direct offspring care and might achieve greater fitness by deserting a pregnant female to reproduce with additional females elsewhere. It has been hypothesized that socially monogamous males remain with the female year-round to protect their offspring from infanticide by new immigrant males. We investigated this idea by analyzing all cases of infant loss in a wild population of white-handed gibbons (Hylobates lar; Primates), in which most groups were socially monogamous and some polyandrous (137.5 group years). We examined the influence of (a) male intruder pressure on male immigration rates and (b) the presence of a new male in the group on infant loss. We found no relation between intruder pressure and male immigration rates. Infant loss was lowest (4.5%) for stable monogamy (probable father stayed from conception through infancy) and intermediate (25.0%; p=0.166) for stable polyandry. If a new male immigrated after conception, however, the infant was lost in all cases (p<0.01) independent of the presumed father's presence. Overall, 83.3% of infant losses were associated with the presence of a presumably unrelated male. Although the sample size is small, our results provide the first true support for the idea that the risk of infanticide is an important factor in the evolution of social monogamy in larger mammals.

Current science

The present study reports infanticide among stumptailed macaques. The incident was observed when the study group was manipulated by introducing the βmale to the mother-infant pair at the breeding centre.

Primates, 2019

Infanticide is well documented in chimpanzees and various hypotheses have been proposed to explain this behavior. However, since infanticide by chimpanzees is relatively rare, it has thus far not been possible to thoroughly test these hypotheses. Here we present an analysis of the largest dataset of infanticides from a single community of chimpanzees, a full record of all intra- community infanticides and failed attempts at infanticide over a 24-year period for the Sonso community of chimpanzees in the Budongo Forest, Uganda. We use these data to test four hypotheses for this behavior: the sexual selection hypothesis, male mating competition, resource competition, and meat acquisition. Our dataset consisted of 33 attacks on 30 victims, 11 of which were ‘definite’ infanticides, four of which ‘almost certain’, and nine were ‘suspected’, while nine were ‘attempted’ infanticides. The majority of attacks where the perpetrators were known (23) had only male attackers and victims were disproportionately young (two-thirds of victims with known ages were under 1 week old). Our data best support the sexual selection hypothesis for infanticide. Cannibalism was infrequent and partial, suggesting meat acquisition was a by-product of infanticide, and there was no evidence to suggest that infanticide was part of a male strategy to eliminate future competi- tors. Infanticide by females was rare, but we suggest sexual selection, operating through intra-sexual competition, may also be responsible for infanticide by females.

History Compass, 2016

Historians have assumed that early modern Europeans did not practice neo-naticide similar to the great Asian civilizations, but sex-ratio studies are only now entering the demographic literature. This article passes in review both published and unpublished research on sex ratios at baptism in Italy, France, England and colonial Acadia, together with juvenile sex ratios drawn from censuses in Germany, France and Italy. Both endemic and conjunctural imbalances appear everywhere, but they could target females or males depending upon the context. It is still considered newsworthy that in much of the world, parents select the sex of their children before bringing a pregnancy to term. In China, the sex ratio at birth is currently 116 males for every female, while in India, the rate is 111, significantly above the well-established biological norm of 105. 1 This sex preference creates well-publicized difficulties for young men seeking brides (The Economist, April 18 2015). Why kill females preferentially? The literature often lays the blame on misogynistic ideologies, suggesting that it would be sufficient to combat them with propaganda in order to eradicate the practice. There are several better reasons: first of all, in agricultural economies requiring strenuous ploughing with large animals and equally strenuous field and forest work far from home, males were better value. In patrilocal societies where husbands, or their families, received a sizeable dowry for the bride (which served as a security cushion for her and her children in the event of the premature death of either spouse), parents were unequal to the task of providing those for several daughters. Finally, if the aim is to keep the future population stable in order not to overstretch resources, then killing future child-bearers is simply more efficient than killing males and females indiscriminately. Today unwanted pregnancies are usually terminated by abortion, but in the past, the safer solution was to kill the newborn or expose it to the elements. Infanticide, like abortion, may be human universals, that is, part of the behavioural repertoire of every known society, although its frequency would vary according to local environmental conditions. 2 Humans are not alone in this behaviour: mothers in many species of mammals will sometimes cull their offspring at birth. In Darwinian language, infanticide, or abortion that has replaced it, are adaptive mechanisms involving some kind of rational decision-making on the part of the parent, which is usually the mother. 3 In most societies, newborns are not considered full-f ledged persons when leaving the womb. Rather, some sort of ceremony confers a name and social identity on them, sometimes providing an additional set of symbolic kin. Returning to the great Asian civilizations where sex-selective behaviour persists, parents enact strategies to better themselves and assess the likelihood of survival and future of the newborn infant. In traditional China and Japan, neonatal infanticide was a kind of post-natal abortion that allowed parents to choose the number, the spacing and the sex of their offspring, while coping better with short-term difficulties like famine. 4 In his compelling recent study on northeastern Japan, Fabian Drixler suggests that one-third of live births ended with infanticide during the 18th century, despite government disapproval of the practice. 5 Historians sometimes

Amer J Primatol, 2008

We report here one observed and two potential cases of infanticide during a brief period of 1 month after a dominant male replacement in one group of black capuchin monkeys in Iguazú National Park, Argentina. We also compile infant disappearances and demographic data in seven groups followed from 1-14 years. Behavioral and molecular data showed that the probability that an infanticidal male would kill his own progeny is very low in this species. Females that lost infants less than 6 months old had shorter interbirth intervals than females whose infants survived (14.1275.32 months, n 5 17 vs. 20.4275.65 months, n 5 34). Females whose infants die shortly after takeovers mate with the presumed infanticidal male during the most fertile days of their subsequent estrous periods giving this male a high probability of siring the new progeny. We recorded 181 proceptive periods and 52 births from 18 adult females in two groups. Most proceptive periods were concentrated during a conception season, but there was an increase in sexual behavior after male takeovers. Seven females copulated while pregnant after the observed male takeover, an unusual behavior in this species in years of group stability. Of 24 infants born during takeover years, 62.5% did not survive the first year, whereas only 22.5% of 80 infants died in years without male replacements. We found a significant positive association between infant mortality and male takeovers, but not with food provisioning. The main cause of infant mortality in this population is associated with male takeovers. Our results suggest that infanticide can have an important effect on the behavior of this species, selecting for female behaviors that function to reduce infanticide risk. Am.

Royal Anthropological Institute - Occasional Paper - Dunbar's Number, 2019

There has been a long history of attempts to explain the evolution of monogamy in mammals in general (Kleiman 1977), and primates in particular (Alexander 1979), partly because of the implications that it may have for the evolution of monogamy in humans. Monogamy is a complex term though, with some species invariably found in pairs (such as owl monkeys), whereas other species can be monogamous, but flexibly take up other mating systems (such as callitrichid monkeys). Humans fit with the latter flexible group, as well as having the additional distinction between their mating system and their marriage system. However, it is fairly well established that monogamy is common in birds (90% of species) (Lack 1968) due to the short time period between mating and egg laying that allows a male to increase his reproductive success through sharing high levels of offspring care (incubating, hatching and rearing), rather than leaving the female he has just mated with to look for other females with whom to mate. The particular reproductive strategy of mammals, with a long gestation period followed by lactation, reduces the opportunities for a male to provide direct care for his offspring, therefore most males move on after mating to seek other fertile females. As a result, monogamy is rare among mammals (5% of species) (Lukas and Clutton-Brock 2013). It is surprising, then, that monogamy is unusually prevalent in one particular mammal order, namely the primates (where 30% of species are monogamous) (Opie, et al. 2013a), with monogamy occurring in all the major primate families (Opie, et al. 2012).

Primates, 1992

Adult resident males of one-male-multi-female primate groups housed at the Hannover Zoo exhibited aggression, when confronted with nonadult individuals, which were fathered by other males: (1) a new adult resident male in a group of blue monkeys killed a 5.8-month-old female infant; (2) a new adult resident male in a group of white collared mangabeys injured a 24.0-month-old female and an 18.9-month-old male severely; they would have died without veterinary care; and (3) the resident male of a group of drills threatened an 1.8-month-old foreign female infant seriously; efforts to introduce the infant were discontinued. Pathological explanations are unlikely because the adult males showed no aggression towards own nonadnlt offspring under the same captive conditions. By and large, the events support the theory that infanticide is the result of sexual selection among males.

Current Zoology