Factors affecting incidence of infanticide and discrimination of related and unrelated neonates in male Mus musculus

doi:10.1016/S0163-1047(83)91159-7

Behavioral and Neural Biology

Volume 37, Issue 1, January 1983, Pages 149-161

https://doi.org/10.1016/S0163-1047(83)91159-7 Get rights and content

Infanticide by males was examined in two strains (C57B1 and DBA) of the house mouse (Mus musculus). Males that had contact with a female within the previous 2–3 weeks rarely committed infanticide when introduced to the home cage of a female and her 1-day-old neonates, even when the female and neonates were of a different strain and from a different colony. In contrast, 90% of C57B1 males that had no contact with a female for more than 7 weeks killed pups when placed in the female's home cage, and 60% killed when a 1-day-old pup was introduced to the male's home cage. No difference in levels of infanticide occurred when grouped males were compared to isolated males. These results indicate that infanticide is not dependent upon recognition of the pups or the female, but depends on the male's previous exposure to females. Infanticidal behavior is not directly determined by genetic relationship, but the factors that inhibit this behavior reduce the probability that a male will kill his own offspring.

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When presented with pups, virgin male mice rapidly approach, investigate and kill pups. Interestingly, following sexual experience this response is inhibited in a time-dependent manner (Tachikawa et al., 2013; Elwood and Ostermeyer, 1984; Vom Saal, 1985; Brooks and Schwarzkopf, 1983). Once infanticidal tendencies are blocked, proximal pup cues elicit caregiving behaviors.
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Fourteen adult C57BL/6 males obtained from Harlan Winkelmann GmbH (Borchen, DE) were used as male intruders. Since C57BL/6J males are known to be potentially infanticidal [32–34] they were regarded as suitable to elicit maternal aggression. To ensure equal experience, each male had been shortly exposed to a dam with offspring at least once before being used as an intruder in the experiment.
Aggressive behavior in males has been intensively investigated regarding the influence of the brain serotonergic system. Despite some inconsistencies, a general conclusion is that low levels of serotonin (5-HT) are associated with high levels of male aggression. The role of the serotonergic system for female aggression is less well researched. Female mice rarely show intraspecific aggressive behavior, except during lactation, when they may exhibit intense aggression towards intruders to protect their pups. The aim of the present study was to investigate the impact of 5-HT transporter (5-HTT) inactivation on maternal aggression in mice. Therefore, lactating homozygous and heterozygous 5-HTT knockout as well as wildtype mice were confronted with male intruders in their home cages. Homozygous 5-HTT knockout dams, which exhibit highest levels of extracellular 5-HT in the brain, were significantly less prone to initiate offensive aggression than wildtype controls. Moreover, they showed longer latencies to attack the intruder, attacked less often and displayed an overall lower frequency of offensive aggressive behavior patterns than wildtype dams. Heterozygous 5-HTT knockout mothers generally showed intermediate levels of aggressive behavior. Thus, our data indicate that higher extracellular including synaptic levels of 5-HT are associated with lower intensity of aggressive behavior in lactating mice, adding support to the inhibitory role of 5-HT in aggression also in females.
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The functional significance of communal nesting and nursing is poorly understood. Female house mice often communally nest, and within these communal nests females appear to indiscriminately nurse all pups, a rare trait for any mammal. In this study, the hypothesis that communal nesting provides protection from conspecific infanticide was tested and supported in semi-natural populations of house mice. Conspecific infanticide in single-mother nests (69%, N = 412) was twice that in communal nests (33%, N = 508). Because this major benefit of communal nesting does not require communal nursing, direct benefits to communal nursing itself were tested. Most proposed benefits should result in heavier weaning weights, but no differences were found between communal and single nests in the semi-natural populations. If communal nursing is to be avoided in communal nests, dams must recognize their own pups. Retrieval tests conducted in the laboratory produced equivocal results. Dams discriminated between pups that differed in age, but not between their own and other age-matched pups. The major survival advantage of communal nesting, coupled with the failure to find nutritional advantages for communally nursed pups, supports a recent suggestion that communal nursing is an unavoidable consequence of communal nesting. This hypothesis is further strengthened by data indicating that communal nesting partners tend to be kin, thereby providing inclusive fitness benefits to communal nursing. Although costs of communal nursing were proposed and tested, no such costs were found. We also show from 15 observations of infanticide that all classes of adults (territorial and non-territorial males, pregnant and non-pregnant females) are infanticidal. These observations are in conflict with previous laboratory studies.
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Parental Behaviour in rodents has traditionally been modelled on studies of maternal behaviour in the female rat. Female parental behaviour is initiated by hormonal changes during pregnancy and parturition and by pup stimuli. Models of parental behaviour based on the neuroendocrine control of maternal behaviour are not appropriate for understanding male parental care. This paper examines seven factors which influence the snitiation and maintenance of paternal behaviour in male rodents: (1) the mating system of the species; (2) the ecological conditions under which the animals live; (3) the male's social experience; (4) sexual experience and female stimuli; (5) pup stimuli; (6) hormonal changes in adult males in response to female and pop stimuli; and (7) neonatal hormone levels. The role of stimuli from the female and pups in activating the neuroendocrine changes which may facilitate the onset and maintenance of paternal behaviour is also described. The aim of the paper is to provide an integgrative approach to the study of male parental care and to stimulate research on the hormonal and experiential factors underlying paternal behaviour in male rodents.
Ethical implications of studies on infanticide and maternal aggression in rodents
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Studies of infanticide and maternal aggression in rodents are reviewed to exemplify general ethical problems arising from research on predation and aggression. Three main areas are reviewed: (1) parental infanticide, when an experimental manipulation causes a parent to harm its young; (2) non-parental infanticide, when pups are placed with a strange adult; and (3) non-parental infanticide, when a strange adult is placed with a mother and litter and aggression between adults may occur. Field experiments are also considered. In each area, constraints on experimental design produce ethical problems; however, ways of minimizing stress are considered. Factors of ethical concern include: (1) observation and intervention to stop both killing of infants and adult aggressive interactions; (2) numbers of pups used in each test; (3) age of the pups; (4) the nature of infanticidal attack; (5) maternal aggression; (6) duration of interactions; (7) use of barriers to protect pups and provision of escape routes for defeated adults; and (8) experimental design and sample sizes. The importance of considering ethical points early in experimental studies is emphasized.
Selectivity in paternal and infanticidal responses by male mice: Effects of relatedness, location, and previous sexual partners
1991, Behavioral and Neural Biology
Male mice tend to be infanticidal to unrelated infants but parental to their own offspring. The present study examines three hypotheses that might explain this apparent discrimination. There was no evidence of selective infanticide toward infants encountered for the first time on the basis of kinship, location, or cues associated with previous sexual partners. However, males tended to direct more paternal responses toward their own unfamiliar infants than toward unrelated infants, infants encountered in the male's home cage than those in the cage of another male, and when cues associated with a previous sexual partner were present. Data suggested that the responses of females to male intruders might influence the responses of those males. It was concluded that the infanticidal responses of male mice are mediated by a particular state of the male but that males in a noninfanticidal state may vary their paternal responsiveness on the basis of direct and indirect cues concerning relatedness.

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¹: Financial support was provided by NSERC Grant A5990 to R. J. Brooks.

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Research reportFactors affecting incidence of infanticide and discrimination of related and unrelated neonates in male Mus musculus1

Physiology and Behavior

Physiology and Behavior

Animal Behaviour

Animal Behaviour

Journal of Theoretical Biology

Ethology and Sociobiology

Behavioral and Neural Biology

Behavioral Biology

Physiology and Behavior

Hormones and Behavior

Physiology and Behavior

Ecological structure and gene flow in small mammals

Zoological Society of London Symposia

Gene flow in house mice: Behavior in a population cage

Behavioral Ecology and Sociobiology

What does individual housing mean to a mouse?

Life Sciences

Comparison of agonistic behavior in individually-housed male mice with those cohabiting with females

Aggressive Behavior

On fighting in mice: Ontogenetic and experiential determinants

Journal of Comparative and Physiological Psychology

Population ecology of feral house mice

Ecology

Postparturitional reestablishment of pup cannibalism in female gerbils

Developmental Psychobiology

Research report
Factors affecting incidence of infanticide and discrimination of related and unrelated neonates in male Mus musculus1