Infanticide by male house sparrows: gaining time or manipulating females?

The killing of genetically unrelated young by males has been viewed as a strategy that forces victimized females to advance the onset of their next fertile period, thus infanticidal males gain a time advantage that may be crucial to maximize reproductive success. Among females that may raise several broods in a year, a failure occurring relatively earlier in the time-course of the previous breeding attempt may result in an increased investment in the next breeding attempt. This female strategy may be exploited by males in their own interest, and may strongly select for male infanticidal behaviour. I demonstrate that, in the house sparrow, females mated with infanticidal males re-laid earlier, initiated more breeding attempts and fledged more offspring than females mated with non-infanticidal males. These results suggest that both the time saving and the manipulation of female investment are independent mechanisms conferring advantages that may have selected for male infanticide in the studied population.     

Infanticide as an evolutionarily stable strategy

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.     

The variable social organization of hanuman langurs(Presbytis entellus), infanticide,and the monopolization of females

Data from 24 wild populations of hanuman langurs (Presbytis entellus)in south Asia are used to test hypotheses seeking to explain variation in troop structure and the incidence of infanticide. The occurrence of infanticide is associated with a one-male troop structure and not with a high density. The density, predation, and economic-advantage hypotheses, as explanations for the occurrence of one-male and multimale troops, are not supported by the review. However, the monopolization hypothesis is not contradicted; the number of adult males per troop is significantly correlated with troop size and with the number of adult females per troop. Therefore it is suggested that a one-male troop structure will arise if a male is able to monopolize a group of females, a multimale troop if he cannot. One-male troops may predispose to infanticide because of high variance in male mating success and high intermale competition between groups rather than within troops. If female dispersion determines troop structure, it is speculated that females could manipulate males to form a multimale society if the advantages in terms of infant survival and intertroop conflict exceeded the costs in terms of not producing infanticidal “sexy sons.”     

Sport Hunting,Predator Control and Conservation of Large Carnivores

Sport hunting has provided important economic incentives for conserving large predators since the early 1970''s, but wildlife managers also face substantial pressure to reduce depredation. Sport hunting is an inherently risky strategy for controlling predators as carnivore populations are difficult to monitor and some species show a propensity for infanticide that is exacerbated by removing adult males. Simulation models predict population declines from even moderate levels of hunting in infanticidal species, and harvest data suggest that African countries and U.S. states with the highest intensity of sport hunting have shown the steepest population declines in African lions and cougars over the past 25 yrs. Similar effects in African leopards may have been masked by mesopredator release owing to declines in sympatric lion populations, whereas there is no evidence of overhunting in non-infanticidal populations of American black bears. Effective conservation of these animals will require new harvest strategies and improved monitoring to counter demands for predator control by livestock producers and local communities.     

Socio-sexual factors of troop fission in wild Japanese monkeys (Macaca fuscata yakui) on Yakushima Island,Japan

The troop fissions which occurred in a wild population of Japanese monkeys (Macaca fuscata yakui) were observed from 1977 to 1979 on Yakushima Island. The fissions were initiated in the breeding season by non-troop males who established a consort relation with estrous females. In order to analyze the socio-sexual factors which accelerated the fissions, the male emigrations and immigrations before and after two successive fissions, and the copulation frequencies, competition among males and preferences of mating partners in both sexes in the 1977–78 breeding season after the first fission were examined. The results indicated that three factors (a large number of non-troop males, a shortage of troop males and the females' choice of mating partners) effectively influenced on the establishment of consort relationships between non-troop males and estrous females. It is suggested that these factors may exert different effects on the troop disorganization in relation to troop size. In small-sized troops, a large number of non-troop males and a shortage of troop males may lead to stronger competition between them, and the females' choice affected by prolonged intimate relations with the dominant TMs may reduce their priority of access to estrous females. This situation possibly stimulates fission or male emigration in small-sized troops under the natural conditions on Yakushima Island. In contrast, in large-sized troops under isolated conditions, a surplus rather than a shortage of troop males may contribute to troop disorganization, as most former studies have suggested. A higher socionomic sex ratio may decrease the mating activities of subordinate troop males and increase the competition among them. This situation possibly accelerates the fission of large-sized troops through prolonged interactions between females and subordinate or peripheral troop males. A lower ratio and the females' choice, however, raise the mating chances of subordinate troop males and may not promote the fission of large-sized troops under isolated conditions. This study was financed in part by a Grant-in-Aid for Special Project Research on Biological Aspects of Optimal Strategy and Social Structure from the Japan Ministry of Education, Science and Culture, and by the Cooperative Research Fund of the Primate Research Institute, Kyoto University.     

Face to face with the woolly spider monkey. Review of faces in the forest: The Endangered Muriqui Monkeys of Brazil,by Karen B. Strier. New York,Oxford University Press,Inc., 1992, 138 pp, $49.50

During an intertroop encounter, an infanticide was observed in a population of ringtailed lemurs (Lemur catta) at Berenty Reserve, Madagascar. The killer was a male that had not been previously observed to mate with any females in the infant's troop. This observation, combined with other observations of infanticidal behavior by males at Berenty Reserve and the Duke University Primate Center, is consistent with the hypothesis that infanticide is a reproductive tactic for some male ringtailed lemurs. © 1994 Wiley-Liss, Inc.     

Infanticide in a chacma baboon troop

This report describes an infanticide and two attacks of an infant and a juvenile by a natal adult male in a troop of chacma baboons (Papio ursinus). The infanticidal male had become the dominant male in his troop five months before the infanticide suggesting that a trigger for infanticidal behavior is a rise in rank to dominant status.     

How Do Infanticidal Male Bank Voles (Myodes glareolus) Find the Nest with Pups?

Infanticide, the killing of conspecific young, occurs in most mammal species, like in our study species, the bank vole (Myodes glareolus). Infanticide by adult males is regarded as a strong factor affecting recruitment of young into population. It is considered as an adaptive behaviour, which may increase male fitness via resource gain or an increased access to mates. When an intruder is approaching the nest, the mother should not be present, as her nest guarding is very aggressive and successful. Pups use ultrasonic vocalisation to call their mother when mother leaves nest for foraging but it is not know which cues do infanticidal males use to find the nest with vulnerable pups to commit infanticide? We studied whether the pups' sounds or the olfactory cues of the nest guide the males of known infanticidal behavioural trait towards the nest with vulnerable pups. Four nest boxes in a large indoor arena offered different nesting cues: nest odour, pup vocalisation, both odour and sound or control with no cue. The result showed that infanticidal males were more active in their searching behaviour than non‐infanticidal males and seemed to target the nest providing only acoustic cues. Four of the males, all infanticidal, intruded the nest box. Infanticidal males seem to actively search for nests with vulnerable pups by eavesdropping pup begging calls for absent mother. However, under natural conditions, mother presence and aggressive nest protection may be an effective counter strategy against strange male infanticide. When trapping study voles from the wild, we monitored occurrence of male infanticide across the breeding season from early to late summer. Proportion of infanticidal males was between 25 and 29% of all males tested along the breeding season. Our results suggest that male infanticide seems to cause a stable threat for pup mortality in increasing breeding season density.     

The social organization of forest hanuman langurs(Presbytis entellus)

The social organization of hanuman langurs (Presbytis entellus;Colobinae) was studied in Kanha Tiger Reserve, Central Indian Highlands, for 2300 hr (1980–1985), in a mosaic of moist deciduous forest and anthropogenic meadow. The langur population density was 46.15/km ² and the mean troop and band sizes were 21.7 and 14.0, respectively. Of 14 troops, 13 were one-male and 1 was trimale. The population adult sex ratio was 1:2.5. The majority of female sexual solicitations was directed toward the harem male. The birth season was December to May, with an estimated gestation of 171–224 days. A review of langur reproductive seasonality suggests that breeding throughout the year is confined to those populations able to exploit human food sources. Mortality during the first year of life was 40%, including infanticide. A significant positive correlation was found between the age of an infant at death or disappearance and the mother’s subsequent interbirth interval. Five cases of social change are described, including female transfer, one-male to multimale change, troop formation, and gradual and rapid replacement of troop males. Takeover-associated infant killing by band males, in an undisturbed moderate-density population, supported the sexual-selection/infanticide hypothesis but not the social-pathology hypothesis. However, it could not be directly confirmed that an invading infanticidal male gains a reproductive advantage. The male tenure of harems was estimated to be 45 months.     

Secondary sex ratio and maternal dominance rank among wild yellow baboons (Papio cynocephalus) of Mikumi National Park,Tanzania

The Trivers and Willard model predicts that offspring of dominant mothers will be biased toward males and offspring of subordinate mothers towards females, whereas a local-resource-competition hypothesis predicts the reverse. Available data bearing upon these alternative predictions are inconsistent. It has been suggested that the local-resource-competition hypothesis will predominate when resources are scarce, and the Trivers-Willard hypothesis when resources are abundant. The relationship between maternal dominance and secondary sex ratio for 214 offspring of 61 females was examined using four troops of wild yellow baboons living in Mikumi National Park, where the population was increasing in a resource-rich habitat. For all troops combined, no significant relationship was found between offspring sex ratio and maternal rank. The four troops separately showed inconsistent trends, and in no case did the relationship reach conventional levels of statistical significance. In contrast, the local resource-competition hypothesis was supported by a study of yellow baboons done with one troop and 80 offspring in Amboseli National Park, where a massive population decline had occurred. The contrasting Amboseli and Mikumi results may be due to differences in resource-competition at the two study sites, or to stochastic variation. © 1992 Wiley-Liss, Inc.     

Reactions to fear as a proximate factor in the sociospatial organization of baboon progressions

The objective of this study was to test the hypothesis that fearfulness is a key proximate factor determining the nonrandom order repeatedly reported for baboon troops progressing from one location to another. According to this hypothesis, the most vulnerable troop members, the walking immatures, are expected in most circumstances to be cautious and to keep others between themselves and potential danger. The more confident adults, especially large powerful males, should tend toward the front and rear of progressing troops. The fear hypothesis predicts that in progressions toward a fear source, adult males should be near the front of the troop; on the other hand, when the troop is retreating from a frightening or tense situation, adult males should be found more toward the rear. Progressions of chacma baboons away from the location of a severe fright or in retreat from another troop were compared with other progressions. Adult males tended to be more rearward in these situations. The number of adults of either sex interposed between a fear source and the first walking immature was greater for retreating troops than for others. Some adult males continued to be near the front of the troop when retreating from tense situations.     

On the fission of troops of Japanese monkeys

The troop fission of Japanese monkeys has been observed in 11 troops, and the total number of fissions observed numbered 22. Examined cases of fission relative to season, amount of food, troop size and socionomic sex ratio of the main troop and to individuals that acted as nucleus in forming a branch troop and females, exhibited variance, not being uniform. But inquiry into troop fission as seen from the standpoint of the mechanism maintained by Japanese monkeys' society indicated (1) troop fission checks the growth of troop size and increases the socionomic sex ratio of the main troop, (2) troop fission is an effective mechanism for the maintenance of class structure among adult males, and (3) the possibility that troop fission functions to check inbreeding.Thus, it must be said in general that troop fission, along with single desertions of males, plays a very important role as one mechanism for the maintenance of the society of Japanese monkey. However, the branch troop is often composed of a large number of males and a small number of females; in short, it is under very difficult conditions that a branch troop starts on its way to becoming a fully established Japanese monkey troop.The troop fission is an effective mechanism for the maintenance of the main troop, but not too effective for that of the branch troop.It cannot be said that there are abundant data on troop fission of primates other than Japanese monkey. But, from limited data, we can find various forms of troop fission according to each species, and it may be ultimately related to the maintenance mechanism of the primate society.     

Patterns of dominance and affiliation in wild pig-tailed macaques (Macaca nemestrina nemestrina) in West Sumatra

Patterns of agonistic and nonagonistic behaviors were studied in a troop of wild pig-tailed macaques in West Sumatra, Indonesia. The animals were provisioned and the identities of all adult and adolescent individuals were known. The females could be divided into high-, mid-, and low-ranking subsets of individuals. Most grooming occurred within, instead of between members of these subsets. The members of each subset also tended to feed together at the baiting sites, and they probably represented groups of close kin. Among females, grooming appeared to be a conciliatory behavior and was generally performed by the subordinate partner, while mounting was performed by the dominant partner and appeared to be a reassertion of dominance. High-ranking males tended to form agonistic alliances with females and to exchange grooming with estrous females. Low-ranking males did not have such associations with females and were frequently the targets of agonistic alliances of females and juveniles. Mounting between males appeared to be a conciliatory behavior. It seemed effective since severe aggression between males was not observed. The subordinate partner mounted more frequently than the dominant one, but the direction of mounting was apparently controlled by the latter. This suggests that, among pig-tailed macaques, the dominant male plays an important role in the coexistence of males in the troop.     

Is Mating Alone Enough to Inhibit Infanticide in Male Bank Voles?

Infanticide, the killing of conspecific young, is commonly recognized as an adaptive behavioural strategy enhancing the fitness of the perpetrator. Infanticide is supposed to be inhibited in several male rodent species after mating with a time lag to the time when perpetrators own offspring would be born. This is because males with no parental care do not recognize their own offspring. It has been suggested that copulation alone is enough to inhibit infanticidal behaviour in male rodents. Infanticidal behaviour occurs in more than 50% of male bank voles (Myodes glareolus), and offspring loss because of infanticide may have a great effect on breeding success and population recruitment. In a laboratory experiment, we studied whether infanticidal male bank voles after successful mating stop the killing of pups. Infanticidal males were paired with a female until successful copulation. After the young were born, the males’ infanticidal behaviour was studied from the time of expected birth of own pups until their post‐weaning age. We predicted that mated infanticidal males are inhibited from committing infanticide especially during the time period when pups are less than 10 d old. Against our prediction, 67% of the infanticidal males continued the killing of pups in the age of 3 d. Infanticidal behaviour remained stable, and half of the males were infanticidal still at the age of weaning of pups. Our results are contradictory to previous studies, as we observed no inhibition of infanticide during early life of pups nor increase in infanticide again when their own offspring would have reached the ‘safe’ age and size after weaning. We suggest that mating alone is not sufficient to inhibit infanticide. Thus, we suggest that other cues of the female with whom the male mated with or on her territory are needed for inhibition to occur.     

Intertroop Transfer and Dominance Rank Structure of Nonnatal Male Japanese Macaques in Yakushima, Japan

We examined the interaction between intertroop transfer and male dominance ranks in a wild population of Japanese macaques (Macaca fuscata yakui) in Yakushima using data collected over 15 years. Intertroop transfer tended to maintain a linear, stable, and age-graded dominance rank order among nonnatal males irrespective of variation in troop size or composition. All males that joined a troop at the top of the rank order were prime adults. Among males joining at lower ranks, entry at the most subordinate position in the hierarchy was common. Males joining at lower ranks tended to join troops in which all other resident males were the same age or older. Adult males tended to join troops with few or no males. Young males tended to join troops with many resident males, and in which a relatively large proportion of males was other young ones. Intertroop transfer was responsible for most rank changes of resident males. The most common cause of males rising in rank was the emigration or death of a higher-ranking male. Males fell in rank most frequently as a result of a new male joining the troop at the top of the hierarchy. Rank reversals among resident males were rare. The cumulative effects of male transfers produce sociodemographic variation within a troop over time and sociodemographic diversity among troops in a local population. A key feature of intertroop diversity is that larger troops have a significantly greater proportion of young males than smaller troops. This diversity also creates the potential for intertroop variation in the severity of male competition and provides a range of options for transferring males.     

Even adult sex ratios in lemurs: Potential costs and benefits of subordinate males in Verreaux's sifaka (Propithecus verreauxi) in the Kirindy Forest CFPF,Madagascar

Optimal group size and composition are determined by both the costs and benefits of group living for the group's members. Verreaux's sifakas (Propithecus verreauxi), a diurnal lemur, form multimale multifemale groups with the tendency toward even adult sex ratios despite a small average number of females per group. The unexpected presence of multiple adult males may be explained by tolerance of other group members if subordinate males provide benefits to the group that outweigh the costs associated with their presence. Results based on both demographic data collected over a 13‐year period and behavioral observations suggest that subordinate males provide no benefits in terms of infant survival and defense against group takeover by outside males. Although groups with more males are more likely to win intergroup encounters, subordinate males do not participate in these encounters more often than expected. Subordinate males are not costly to other group members in terms of direct intragroup feeding competition, but aggression rates between dominant and immigrated subordinate males increase in the mating season. Even though subordinate males provide very few benefits to the group, they are not very costly either and thus may be tolerated by resident females and dominant males. This tolerance may help to partially explain the tendency towards their unusual adult sex ratio. Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

Social relationships between adult central males and kinship groups of Japanese monkeys at Arashiyama with some aspects of troop organization

Using focal-animal and instantaneous sampling techniques, seven adult central males of the Arashiyama-B troop of Japanese monkeys were observed during nonmating seasons from 1976 to 1978. The partners of these males in social activities (allogrooming and agonistic) and nonspecific body-contact, and those found in 1-m proximity were identified. The results indicated that: (1) a lack of observable associations as well as agonism was noticed among the adult central males; (2) a tendency was observed for each adult central male to associate with more than one kinship group and for most of the kinship groups to associate with more than one adult central male each. Many of the adult central males and kinship groups changed associations with each other from year to year; (3) the observed frequency of associations, which finally became zero, between any such male as had kins inside the study troop and his kins was statistically lower than the expected one; (4) some of the female youngsters had strong associations with particular adult central males; and (5) since one kinship group that had lost associations with all the adult central males left the troop, it is suggested that associations with at least one adult central male must be important for binding each kinship group in the troop.     

Inter-male associations in a wild Japanese macaque troop on Yakushima Island,Japan

Adult male association and its annual change were studied in a wild population of Japanese macaques (Macaca fuscata yakui) on Yakushima Island, Japan. Unlike many other Japanese macaque troops, adult troop males frequently maintained proximity and exchanged grooming with one another in both the mating and non-mating seasons, and the dominance relationship rarely appeared in such inter-male associations. The few cases of agonistic interactions occurred mostly when estrous females or food resources were immediately concerned. Although troop males were very intolerant to newly appeared solitary males (new males) during the mating season, close associations were formed between troop males and new males as soon as the mating season terminated. The consort of new males and lower-ranking troop males with estrous females was frequently disturbed, but these males could copulate no less frequently than higher-ranking males. A comparison among macaque species suggests the existence of two forms of inter-male association: (1) the frequent association based on the symmetrical exchange of social behaviors; and (2) the infrequent and asymmetrical association related to the dominance relationship. The form of inter-male association seems to be influenced by whether or not males can keep close associations with females throughout the year.     

Infanticide and effectiveness of pup protection in bank voles: does the mother recognise a killer?

Infanticide, the killing of conspecific young, has been documented in numerous species of mammals, especially rodents. In that infanticide is costly to the victim mother, natural selection should favour counter-strategies by females to protect their pups. We studied the frequency of infanticide by male and female bank voles (Clethrionomys glareolus) and the effectiveness of dams in recognising and deterring infanticide. In trials in which unprotected pups were exposed to voles of both sexes, one third of male and female intruders killed pups. When mothers were present at the nest site, not a single female and only 2 of 25 males were able to commit infanticide. Females acted aggressively towards all intruders and hence did not discern between infanticidal and non-infanticidal males and females. Aggression of dams against any intruder indicates that all strange individuals near the nest site of a territorial species form an equal threat to pups. However, the presence of the aggressive dam is sufficient to deter most intruders from harming offspring by keeping them at a distance from the nest site. Heat run before mating, in a species with postpartum oestrus and vulnerable pups in the nest, also might lure potential infanticidal males away from the nest site and obscure the nest location. Received in revised form: 17 October 2001 Electronic Publication     

Infant Killing, Wounding and Predation in Eulemur and Lemur

Infant killing by primates is highly controversial. Sexual selection of infanticidal males has been disputed, especially for seasonally breeding species, in which death of an infant does not advance conception of the next infant. We report attacks, infants found wounded, and predation in seasonally breeding Eulemur and Lemur at Berenty, Beza Mahafaly and Duke University Primate Center, and review cases seen elsewhere. Observed attacks leading to wounds or death conservatively total twelve by extratroop males, two by troop males, and seven by troop females. Eulemur are occasional vertebrate predators, whose prey includes infant Lemur catta. Wounds inflicted by lemurs are usually abdominal canine slashes or bites to the head, with rare eating, a pattern distinct from carnivore and raptor kills. Infant killing as inferred from corpses is more frequent than previously thought, but still rare. Adaptive advantages of killing plausibly include eliminating resource competitors of females, and sexual selection on males.