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 intracommunity 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 competitors. Infanticide by females was rare, but we suggest sexual selection, operating through intra-sexual competition, may also be responsible for infanticide by females.

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 infanticide 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 lowranking) are unlikely to kill their own offspring, while those who are currently rising in rank, particularly when this rise is dramatic, have a 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 specifically 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 lowranking 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 association with males of stable high rank on the basis that such males could offer protection against infanticide. Materials and MethodsWe examined data on female association patterns collected from the Budongo Forest, Uganda, during a period encompassing both relative stability in the male hierarchy and a period of instability with a mid-ranking male rising rapidly in rank. ResultsUsing 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 evidence that females preferentially associated with a potential protector male during the high-risk period. DiscussionOur results support the sexually-selected hypothesis for infanticide, and demonstrate that female chimpanzees are sensitive to the relative risks posed by adult males.

Baboons are well studied in savannah but less so in more closed habitats. We investigated predation on mammals by olive baboons (Papio anubis) at a geographical and climatic outlier, Gashaka Gumti National Park (Nigeria), the wettest and most forested site so far studied. Despite abundant wildlife, meat eating was rare and selective. Over 16 years, baboons killed 7 bushbuck (Tragelaphus scriptus) and 3 red-flanked duiker (Cephalophus rufilatus), mostly still-lying ‘parked' infants. Taking observation time into account, this is 1 predation per group every 3.3 months - far lower than at other sites. Some features of meat eating resemble those elsewhere; predation is opportunistic, adult males monopolize most prey, a targeted killing bite is lacking and begging or active sharing is absent. Carcass owners employ evasive tactics, as meat is often competed over, but satiated owners may tolerate others taking meat. Other features are unusual; this is only the second study site with predation records for bushbuck and the only one for red-flanked duiker. The atypical prey and rarity of eating mammals probably reflects the difficulty of acquiring prey animals when vegetation cover is dense. Our data support the general prediction of the socioecological model that environments shape behavioural patterns, while acknowledging their intraspecific or intrageneric plasticity.

Maternal cannibalism has been reported in several animal taxa, prompting speculations that the behavior may be part of an evolved strategy. In chimpanzees, however, maternal cannibalism has been conspicuously absent, despite high levels of infant mortality and reports of non-maternal cannibalism. The typical response of chimpanzee mothers is to abandon their deceased infant, sometimes after prolonged periods of carrying and grooming the corpse. Here, we report two anomalous observations of maternal cannibalism in communities of wild chimpanzees in Uganda and Ivory Coast and discuss the evolutionary implications. Both infants likely died under different circumstances; one apparently as a result of premature birth, the other possibly as a result of infanticide. In both cases, the mothers consumed parts of the corpse and participated in meat sharing with other group members. Neither female presented any apparent signs of ill health before or after the events. We concluded that, in both cases, cannibalizing the infant was unlikely due to health-related issues by the mothers. We discuss these observations against a background of chimpanzee mothers consistently refraining from maternal cannibalism, despite ample opportunities and nutritional advantages. We conclude that maternal cannibalism is extremely rare in this primate, likely due to early and strong mother-offspring bond formation, which may have been profoundly disrupted in the current cases.

Carrying food to water and either dunking or manipulating it before consumption has been observed in various taxa including birds, racoons and primates. Some animals seem to be simply moistening their food. However, true washing aims to remove unpleasant surface substrates such as grit and sand and requires a distinction between items that do and do not need cleaning as well as deliberate transportation of food to a water source. We provide the first evidence for food washing in suids, based on an incidental observation with follow-up experiments on European wild boar (Sus scrofa) kept at Basel Zoo, Switzerland. Here, all adult pigs and some juveniles of a newly formed group carried apple halves soiled with sand to the edge of a creek running through their enclosure where they put the fruits in the water and pushed them to and fro with their snouts before eating. Clean apple halves were never washed. This indicates that pigs can discriminate between soiled and unsoiled foods and that they are able to delay gratification for long enough to transport and wash the items. However, we were unable to ascertain to which degree individual and/or social learning brought this behaviour about.

The biological basis of homosexuality is the subject of ongoing investigations. Researchers in this field explore potential causes through two complementary approaches. On the one hand, there is a focus on mechanisms that generate the trait (proximate causes), with evidence for a multifacetted influence of genes and intrauterine hormones. On the other hand, researchers also consider the functions that might have favored the evolution of homosexuality (ultimate causes). Several functional hypotheses can be applied to bisexual behavior (substitution or practice for heterosexual activities; alliance formation through homosexual interactions). Exclusive homosexuality, however, requires different explanatory perspectives (e.g., kin selection, i.e., homosexuals reproduce indirectly; parental manipulation, i.e., parents turn younger offspring into helpers for older siblings; balanced polymorphism, i.e., male homosexual behavior is a by‐product of selection for other traits, such as higher fecundity of female relatives). Such multiplicity of explanatory approaches reflects the diverse nature of same‐sex sexual behavior.

The killing of immature conspecifics (fetuses, infants, children, dependent offspring) by adult males or females, both unrelated and related, is not an uncommon occurrence among human and nonhuman animal societies. Although not universally accepted, adaptationist approaches attempt to explain why infanticide may provide a reproductive benefit to the perpetrator. These hypotheses focus on resource competition (improving access to food or care for own infants), parental manipulation (terminating investment in children that compromise lifetime reproductive prospects, often leading to biases against a particular sex), sexual selection (eliminating competitors' offspring to avoid misdirected parental investment), and exploitation (cannibalism). Given that infanticide is a predictable response to certain ecological and psychosocial circumstances, social engineering may be an effective tool in reducing its frequency among human societies.