Infanticide in male laboratory mice: Effects of social status, prior sexual experience, and basis for discrimination between related and unrelated young

Involvement of Serotonergic Projections from the Dorsal Raphe to the Medial Preoptic Area in the Regulation of the Pup-Directed Paternal Response of Male Mandarin Voles

Male mammals display different paternal responses to pups, either attacking or killing the young offspring, or contrastingly, caring for them. The neural circuit mechanism underlying the between-individual variation in the pup-directed responsiveness of male mammals remains unclear. Monogamous mandarin voles were used to complete the present study. The male individuals were identified as paternal and infanticidal voles, according their behavioral responses to pups. It was found that the serotonin release in the medial preoptic area (MPOA), as well as the serotonergic neuron activity, significantly increased upon licking the pups, but showed no changes after attacking the pups, as revealed by the in vivo fiber photometry of the fluorescence signal from the 5-HT 1.0 sensor and the calcium imaging indicator, respectively. It was verified that the 5-HTergic neural projections to the MPOA originated mainly from the ventral part of the dorsal raphe (vDR). Furthermore, the chemogenetic inhibition of serotonergic projections from the vDR to the MPOA decreased the paternal behaviors and shortened the latency to attack the pups. In contrast, the activation of serotonergic neurons via optogenetics extended the licking duration and inhibited infanticide. Collectively, these results elucidate that the serotonergic projections from the vDR to the MPOA, a previously unrecognized pathway, regulate the paternal responses of virgin male mandarin voles to pups.

Olfactory exposure to late-pregnant and lactating mice causes stress-induced analgesia in male mice

In an attempt to improve reproducibility, more attention is being paid to potential sources of stress in the laboratory environment. Here, we report that the mere proximity of pregnant or lactating female mice causes olfactory-mediated stress-induced analgesia, to a variety of noxious stimuli, in gonadally intact male mice. We show that exposure to volatile compounds released in the urine of pregnant and lactating female mice can themselves produce stress and associated pain inhibition. This phenomenon, a novel form of female-to-male chemosignaling, is mediated by female scent marking of urinary volatiles, such as n-pentyl-acetate, and likely signals potential maternal aggression aimed at defending against infanticide by stranger males.

Parenting as a model for behavioural switches

Adaptability to ethologically relevant cues is fundamental for social interactions. As such, reproductive success relies on the ability of an animal to transition between parental and nonparental states. Though driven by genetically pre-programmed circuits, these instinctive repertoires are reshaped by internal state and experience, making parenting a robust model for the study of behavioural flexibility. As a functional wiring diagram for parenting emerges in mice, we are well placed to identify neural substrates and posit associated mechanisms underlying caregiving transitions. In this review, we discuss the importance of comprehensively characterising behaviour, highlight the role of shared circuit elements for behavioural malleability and explore plastic mechanisms that might guide switches between parental and nonparental repertoires.

All the Pups We Cannot See: Cannibalism Masks Perinatal Death in Laboratory Mouse Breeding but Infanticide Is Rare

Perinatal mortality is a major issue in laboratory mouse breeding. We compared a counting method using daily checks (DAILY_CHECK) with a method combining daily checks with detailed video analyses to detect cannibalisms (VIDEO_TRACK) for estimating the number of C57BL/6 pups that were born, that died and that were weaned in 193 litters from trios with (TRIO-OVERLAP) or without (TRIO-NO_OVERLAP) the presence of another litter. Linear mixed models were used at litter level. To understand whether cannibalism was associated with active killing (infanticide), we analysed VIDEO_TRACK recordings of 109 litters from TRIO-OVERLAP, TRIO-NO_OVERLAP or SOLO (single dams). We used Kaplan-Meier method and logistic regression at pup level. For DAILY_CHECK, the mean litter size was 35% smaller than for VIDEO_TRACK (p < 0.0001) and the number of dead pups was twice lower (p < 0.0001). The risk of pup loss was higher for TRIO-OVERLAP than TRIO-NO_OVERLAP (p < 0.0001). A high number of pup losses occurred between birth and the first cage check. Analyses of VIDEO_TRACK data indicated that pups were clearly dead at the start of most of the cannibalism events and infanticide was rare. As most pups die and disappear before the first cage check, many breeding facilities are likely to be unaware of their real rates of mouse pup mortality.

Flipping the parental switch: from killing to caring in male mammals

Killing of unrelated young by sexually naïve male mammals is taxonomically widespread, but in many species, males subsequently show paternal care or at least do not harm their own young. This dramatic and important change is due to a shift in paternal state rather than to recognition of young, the mother or the location in which mating occurred. This transition from infanticidal to paternal behaviour is timed so that the inhibition of infanticide is synchronized with the birth of their own young. Ejaculation followed by cohabitation with the pregnant female causes this transition, but the precise stimuli from the female remain elusive. However, changes in social status also cause changes in infanticide. The switch from infanticide is accompanied by physiological change in the male that can be detected by both females and pups. Hormonal changes have been implicated in the switch but establishing causal links has been difficult. Recent neuroanatomical studies show that pup odours activate the vomeronasal organ and its efferent projections to induce infanticide. The emergence of paternal care depends on the inability of the vomeronasal organ to detect pup odours. In the absence of vomeronasal input, pup odours activate a conserved parental circuit and induce caregiving behaviour. An emerging picture is of complex, antagonistic circuits competing for behavioural expression, which allow for males to commit infanticide when they may benefit from such activity but ensure that they do not damage their fitness by killing their own young. However, we stress the need for more work on the neural mechanisms that mediate this process.

Histone deacetylase inhibitor treatment promotes spontaneous caregiving behaviour in non-aggressive virgin male mice

The majority of mammalian species are uniparental, with the mother solely providing care for young conspecifics, although fathering behaviours can emerge under certain circumstances. For example, a great deal of individual variation in response to young pups has been reported in multiple inbred strains of laboratory male mice. Furthermore, sexual experience and subsequent cohabitation with a female conspecific can induce caregiving responses in otherwise indifferent, fearful or aggressive males. Thus, a highly conserved parental neural circuit is likely present in both sexes; however, the extent to which infants are capable of activating this circuit may vary. In support of this idea, fearful or indifferent responses toward pups in female mice are linked to greater immediate early gene (IEG) expression in a fear/defensive circuit involving the anterior hypothalamus compared to that in an approach/attraction circuit involving the ventral tegmental area. However, experience with infants, particularly in combination with histone deacetylase inhibitor (HDACi) treatment, can reverse this pattern of pup-induced activation of fear/defence circuitry and promote approach behaviour. Thus, HDACi treatment may increase the transcription of primed/poised genes that play a role in the activation and selection of a maternal approach circuit in response to pup stimuli. In the present study, we investigated whether HDACi treatment would impact behavioural response selection and associated IEG expression changes in virgin male mice that are capable of ignoring, attacking or caring for pups. The results obtained indicate that systemic HDACi treatment induces spontaneous caregiving behaviour in non-aggressive male mice and alters the pattern of pup-induced IEG expression across a fear/defensive neural circuit.

Sexual experience has no effect on male mating or reproductive success in house mice

The ability to learn from experience can improve Darwinian fitness, but few studies have tested whether sexual experience enhances reproductive success. We conducted a study with wild-derived house mice (Mus musculus musculus) in which we manipulated male sexual experience and allowed females to choose between (1) a sexually experienced versus a virgin male, (2) two sexually experienced males, or (3) two virgin males (n = 60 females and 120 males). This design allowed us to test whether females are more likely to mate multiply when they encounter more virgin males, which are known to be infanticidal. We recorded females’ preference and mating behaviours, and conducted genetic paternity analyses to determine male reproductive success. We found no evidence that sexual experience influenced male mating or reproductive success, and no evidence that the number of virgin males influenced female multiple mating. Females always copulated with both males and 58% of the litters were multiple-sired. Females’ initial attraction to a male correlated with their social preferences, but neither of these preference behaviours predicted male reproductive success – raising caveats for using mating preferences as surrogates for mate choice. Male reproductive success was predicted by mating order, but unexpectedly, males that copulated first sired fewer offspring.

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

An increase in estradiol facilitates the onset of paternal behavior in the dwarf hamster (Phodopus campbelli)

In the dwarf hamster (Phodopus campbelli), activational effects of testosterone (T) and estradiol (E₂) in the regulation of paternal behavior have been repeatedly rejected because peripheral concentrations of E₂ do not change across the reproductive cycle of males. Further, castration no affected paternal behavior despite that both T and E₂ concentrations decreased significantly. However, the role of these hormones has not been evaluated in models of castration and hormonal replacement in virgin males. Here, we analysed the effects of E₂ and T in paternal behavior in virgin male dwarf hamster (Phodopus campbelli). Thirty paternal (PAT) males were bilaterally castrated; of them, 10 were implanted with T, 10 with E₂ and 10 males received no treatment. Other 10 PAT males underwent sham-castration. Seventeen aggressive (AGG) males were also bilaterally castrated; of these, 10 AGG received E₂ replacement, 7 were not treated. Other 7 AGG males were submitted to sham-castration. Following treatments, paternal behavior tests were conducted again. T and E₂ levels in plasma were quantified by radioimmunoassay (RIA). The results showed that the treatments did not affect the paternal behavior of males that were initially paternal. Neither castration nor sham-castration surgery affected the behavior of AGG males. However, when these males were treated with E₂ and the concentrations of this hormone increase significantly they became paternal. Our data suggest that an increase in E₂ levels shifted infanticidal behavior to paternal behavior in dwarf hamster.

A reduced propensity to cooperate under enhanced exploitation risk in a social mammal

Conditional adjustment of cooperativeness to the expected pay-off might be a useful strategy to avoid being exploited in public good situations. Parental care provided towards all offspring in a communal nest (containing offspring of several females) resembles a public good. Females indiscriminately caring for all young share the costs equally, but the pay-off may vary depending on their contribution to the joint nest (number of own offspring). Females with fewer offspring in the joint nest will be exploited and overinvest relative to their contribution. We experimentally created a situation of high conflict in communally nursing house mice, by using a genetic tool to create a difference in birth litter sizes. Females in the high conflict situation (unequal litter sizes at birth) showed a reduced propensity to give birth as part of a communal nest, therefore adjusting their cooperativeness to the circumstances.

Socially mediated polyandry: a new benefit of communal nesting in mammals

In many species, females have evolved behavioral strategies to reduce the risk of infanticide. For instance, polyandry can create paternity confusion that inhibits males from killing offspring they could have sired. Here, the authors propose that females could socially obtain the same benefits by nesting communally. Singly sired litters could be perceived as a large multiply sired litter once pooled together in a single nest. Long-term data from a wild house mouse population showed that monandrous litters (singly sired) were more common in communal than in solitary nests and 85% of them were raised with litters sired by different males hence becoming effectively polyandrous (multiply sired). These socially polyandrous litters had significantly higher offspring survival than genetically or socially monandrous litters and reached a similar survival to that of multiply sired litters raised in solitary or communal nests. Furthermore, the number of sires within nests significantly improved offspring survival whereas the number of mothers did not. These results suggest that the survival benefits associated with communal nesting are driven by polyandry and not communal defense. This socially mediated polyandry was as efficient as multiple paternity in preventing infanticide, and may also occur in other infanticidal and polytocous species where the caring parent exhibits social behavior.

Maternal aggression in rodents: brain oxytocin and vasopressin mediate pup defence

The most significant social behaviour of the lactating mother is maternal behaviour, which comprises maternal care and maternal aggression (MA). The latter is a protective behaviour of the mother serving to defend the offspring against a potentially dangerous intruder. The extent to which the mother shows aggressive behaviour depends on extrinsic and intrinsic factors, as we have learned from studies in laboratory rodents. Among the extrinsic factors are the pups' presence and age, as well as the intruders' sex and age. With respect to intrinsic factors, the mothers' innate anxiety and the prosocial brain neuropeptides oxytocin (OXT) and arginine vasopressin (AVP) play important roles. While OXT is well known as a maternal neuropeptide, AVP has only recently been described in this context. The increased activities of these neuropeptides in lactation are the result of remarkable brain adaptations peripartum and are a prerequisite for the mother to become maternal. Consequently, OXT and AVP are significantly involved in mediating the fine-tuned regulation of MA depending on the brain regions. Importantly, both neuropeptides are also modulators of anxiety, which determines the extent of MA. This review provides a detailed overview of the role of OXT and AVP in MA and the link to anxiety.

Why do female mice mate with multiple males?

Females often show multi-male mating (MMM), but the adaptive functions are unclear. We tested whether female house mice (Mus musculus musculus) show MMM when they can choose their mates without male coercion. We released 32 females into separate enclosures where they could choose to mate with two neighboring males that were restricted to their own territories. We also tested whether females increase MMM when the available males appeared unable to exclude intruders from their territories. To manipulate territorial intrusion, we introduced scent-marked tiles from the neighboring males into males' territories, or we rearranged tiles within males' own territories as a control. Each female was tested in treatment and control conditions and we conducted paternity analyses on the 57 litters produced. We found that 46 % of litters were multiply sired, indicating that multiple paternity is common when females can choose their mates. Intrusion did not increase multiple paternity, though multiple paternity was significantly greater in the first trial when the males were virgins compared to the second trial. Since virgin male mice are highly infanticidal, this finding is consistent with the infanticide avoidance hypothesis. We also found that multiple paternity was higher when competing males showed small differences in their amount of scent marking, suggesting that females reduce MMM when they can detect differences in males' quality. Finally, multiple paternity was associated with increased litter size but only in the intrusion treatment, which suggests that the effect of multiple paternity on offspring number is dependent on male-male interactions.

Does it pay to delay? Flesh flies show adaptive plasticity in reproductive timing

Life-history plasticity is widespread among organisms. However, an important question is whether it is adaptive. Most models for plasticity in life-history timing predict that animals, once they have reached the minimal nutritional threshold under poor conditions, will accelerate development or time to reproduction. Adaptive delays in reproduction are not common, especially in short-lived species. Examples of adaptive reproductive delays exist in mammalian populations experiencing strong interspecific (e.g., predation) and intraspecific (e.g., infanticide) competition. But are there other environmental factors that may trigger an adaptive delay in reproductive timing? We show that the short-lived flesh fly Sarcophaga crassipalpis will delay reproduction under nutrient-poor conditions, even though it has already met the minimal nutritional threshold for reproduction. We test whether this delay strategy is an adaptive response allowing the scavenger time to locate more resources by experimentally providing supplemental protein pulses (early, mid and late) throughout the reproductive delay period. Flies receiving additional protein produced more and larger eggs, demonstrating a benefit of the delay. In addition, by tracking the allocation of carbon from the pulses using stable isotopes, we show that flies receiving earlier pulses incorporated more carbon into eggs and somatic tissue than those given a later pulse. These results indicate that the reproductive delay in S. crassipalpis is consistent with adaptive post-threshold plasticity, a nutritionally linked reproductive strategy that has not been reported previously in an invertebrate species.

Modeling dad: animal models of paternal behavior

In humans, paternal behaviors have a strong influence on the emotional and social development of children. Fathers, more frequently than mothers, leave the family nucleus, and/or become abusive, leading to offspring that are more likely to grow under stressful conditions and greater susceptibility to abnormal health and social outcomes. Literature on parental behaviors, human or animal, has primarily focused on the interactions between mothers and offspring, with little research directed at understanding paternal behavior. In animal studies, experimenters correlate paternal behaviors with those seen in rodent or primate mothers, often under situations in which behaviors such as nest protection, huddling, pup grooming, and retrieval are artificially induced. In humans, the majority of the studies have looked at paralleling hormonal changes in fathers with those occurring in mothers, or observed paternal behaviors in populations with specific anthropological backgrounds. These studies reveal commonalities in parental behaviors and their underlying neural circuits. However, this work highlights the possibility that paternal behavior has components that are strictly masculine with unique neurobiological mechanisms. This review summarizes this information and provides a current view of a topic that needs further exploration.

From here to paternity: neural correlates of the onset of paternal behavior in California mice (Peromyscus californicus)

In a minority of mammalian species, including humans, fathers play a significant role in infant care. Compared to maternal behavior, the neural and hormonal bases of paternal care are poorly understood. We analyzed behavioral, neuronal and neuropeptide responses towards unfamiliar pups in biparental California mice, comparing males housed with another male ("virgin males") or with a female before ("paired males") or after ("new fathers") the birth of their first litter. New fathers approached pups more rapidly and spent more time engaging in paternal behavior than virgin males. In each cage housing two virgin males, one was spontaneously paternal and one was not. New fathers and paired males spent more time sniffing and touching a wire mesh ball containing a newborn pup than virgin males. Only new fathers showed significantly increased Fos-like immunoreactivity in the medial preoptic nucleus (MPO) following exposure to a pup-containing ball, as compared to an empty ball. Moreover, Fos-LIR in the bed nucleus of the stria terminalis (STMV and STMPM) and caudal dorsal raphe nucleus (DRC) was increased in new fathers, independent of test condition. No differences were found among the groups in Fos-LIR in oxytocinergic or vasopressinergic neurons. These results suggest that sexual and paternal experiences facilitate paternal behavior, but other cues play a role as well. Paternal experience increases Fos-LIR induced by distal pup cues in the MPO, but not in oxytocin and vasopressin neurons. Fatherhood also appears to alter neurotransmission in the BNST and DRC, regions implicated in emotionality and stress-responsiveness.

Hormonal changes related to paternal and alloparental care in common marmosets (Callithrix jacchus)

The physiological mechanisms of parental and alloparental care in cooperatively breeding nonhuman primate species such as the common marmoset (Callithrix jacchus) are poorly known. In this study, we examined prolactin and cortisol plasma levels of fathers and older offspring of both sexes, with and without previous experience in infant carrying, around parturition and during infant carrying. Blood samples were collected from fathers and older offspring and prolactin and cortisol were measured by RIA and EIA, respectively. Prolactin levels of both caretakers were not influenced by infant's birth, previous experience or proximity to parturition. However, prolactin levels increased in both caretakers while in physical contact with infants and also with the number of infants being carried in older offspring. These findings suggest that increased prolactin seems to be mainly due to physical effort rather than a physiological trigger of paternal and alloparental care in common marmosets. Cortisol levels were higher for experienced fathers shortly before parturition which could act to reinforce affiliative bonds between breeding males and females at this time or in the ability of males to detect the proximity of the parturition or both.

Variability in "spontaneous" maternal behavior is associated with anxiety-like behavior and affiliation in naïve juvenile and adult female prairie voles (Microtus ochrogaster)

Juvenile female prairie voles (Microtus ochrogaster) are spontaneously maternal, while virgin adult females show significant variability in their response to first pup exposure, ranging from infanticidal to full maternal behavior. In the present study, we investigated whether differences in anxiety-like behavior and affiliation are associated with juvenile-adult and adult individual differences in the response to pups. Forty juvenile (19-20 days) and 42 adult (60-90 days) female prairie voles were exposed to pups for the first time and tested for maternal behavior, anxiety-like behavior (elevated plus maze, open field), and affiliation toward age-matched, same sex conspecifics. Juveniles displayed less anxiety-like behavior, were more affiliative to unfamiliar conspecifics, and interacted with pups more positively than adults. Adults that displayed maternal behavior spent less time immobile, made more crosses through the center of the open field arena, and were more affiliative than adults that attacked the pups. This suggests that lower locomotion or exploration in a novel environment and poor affiliative behavior are negatively associated with maternal responsiveness in female prairie voles.

Primer effects by conspecific odors in house mice: a new perspective in the study of primer effects on reproductive activities

Half a century has passed since the first report of the influence of odors on mice. Odors are known to influence behavior (signaling effect) and affect the physiology (primer effect) of mice. This review focuses on summarizing the primer effects found so far in female and male mice. Odors from conspecifics of the opposite sex had the tendency to enhance reproductive activities, whereas odors from conspecifics of the same sex diminish them. Only 2,5-dimethylpyrazine, one of the odor components identified in group-housed females, has been reported to have a suppressive influence on both males and females by lowering reproductive activities. Studies showed progress from the discovery of phenomena to the identification of odor components that cause such changes in reproductive organs and related neuroendocrinological changes. Compared to studies on the mechanisms of primer effects in females, the mechanisms in males are not yet clarified, and detailed studies on effects on the reproductive organs are still in primitive stages especially for males. Hypotheses on the influence of changes in the concentration of testosterone, estrogen, and prolactin on spermatogenesis and sperm maturation after exposure to odors are discussed.

Central vasopressin administration regulates the onset of facultative paternal behavior in microtus pennsylvanicus (meadow voles)

Pharmacological experiments have implicated a role for central arginine vasopressin (AVP) in regulating paternal behavior in monogamous prairie voles. Although nonmonogamous meadow voles exhibit appreciable paternal care when housed under winter, short day lengths (SD), no research has examined whether the same neurobiological systems are involved in regulating paternal behavior in a nonmonogamous species when it behaves paternally. The goal of these experiments was to determine whether central administration of AVP, but not cerebrospinal fluid (CSF), affected the suppression of pup-directed aggression and/or the onset of paternal behavior in meadow voles. Data from experiment 1 implicated a role for AVP in facilitating changes in male behavior: central administration of 1 ng of AVP (but not 3 ng or CSF) inhibited pup-directed aggression in previously pup-aggressive males, and 3 ng of AVP (but not 1 ng or CSF) induced paternal behavior in previously nonpaternal males. In contrast, AVP (1 and 3 ng) did not enhance paternal behavior in already paternal males. Experiment 2 tested the specificity of AVP. Previous research indicated that 24 h of unmated cohabitation with a female reliably induced paternal behavior in SD males. Hence, experiment 2 examined whether administration of a V(1a) AVP antagonist (AVPA), but not CSF, prior to 24 h of unmated cohabitation would block the onset of paternal behavior. Males that received CSF displayed paternal behavior faster and engaged in more investigatory and paternal behaviors than males that received AVPA. Thus, pharmacological experiments support the hypothesis that AVP likely regulates paternal behavior in both facultatively and consistently paternal vole species.

MHC-mediated fetal odourtypes expressed by pregnant females influence male associative behaviour

Mice can recognize one another by individually characteristic phenotypic body odours (odourtypes) that reflect their genetic constitution at the highly polymorphic major histocompatibility complex (MHC) of genes on chromosome 17. We have shown previously that MHC-determined odours are produced by fetuses: house mice, Mus domesticus, can be trained to discriminate between genetically identical pregnant females carrying 9-18-day-old fetuses of differing MHC type. Theoretically, it should be possible for a mouse to determine the MHC type of the sire based on the odourtype of the pregnant female. In the current study we investigated whether untrained male mice show spontaneous discrimination between such pregnant mice. In experiment 1, sexually inexperienced male mice spent more time near pregnant females that carried fetuses most genetically different from the males themselves. Experiment 2, designed to evaluate possible experiential effects on this preference, tested males that were cohabiting and had impregnated a female that was either genetically identical to the test male (excepting X and Y chromosomes) or differed from him only at the MHC. Males in the former case performed virtually identically to those tested in experiment 1. In contrast males in the latter group did not display this preference. These studies reveal that among untrained male mice, fetal MHC type influences choice behaviour presumably via fetal odourtypes expressed in maternal secretions/excretions and that previous housing and/or mating experience modulates male choice. Copyright 2000 The Association for the Study of Animal Behaviour.

Male infanticide in captive plains zebra, Equus burchelli

On the assumption that infanticide exists in plains zebra, as reported for horses, Equus caballus, we tested the following hypothesis. Introducing a new zebra male into a herd of breeding females should increase foal mortality in comparison with herds in which the sire of the foals is still present. The younger the foal, the more likely infanticide should be. We collected data from five herds in two zoological gardens in the Czech Republic. We found nine records of infanticide in plains zebra and three cases of abortions that were probably induced by forced copulation. We analysed additional indirect data to investigate the possibility of introduced males causing other abortions. Abortions were three times more likely in herds with introduced males than with only fathers present. Postnatal mortality of the foals was four times greater with introduced males than with fathers. No indication of a sex preference was observed for infanticide by a new male for either abortions or postpartum deaths. When we combined all records involving introduced males, the probability of foal death was greatest when the new male joined the herd just after conception and decreased with increasing time between conception and date of the new male introduction (the chance of a foal surviving was less than 5% just after conception and more than 50% at the time of delivery). Mortality of foals did not depend on whether the new male was introduced before or after the foal was born. Survival increased to more than 60% after the foal reached 1 month of age. Our results suggest that captive plains zebra show the highest occurrence of infanticide reported among ungulates. Copyright 2000 The Association for the Study of Animal Behaviour.

Effects of experience on the parental responses of male Mongolian gerbils

We examined responses of adult male Mongolian gerbils to nest site and young during the hours preceding and days following birth of a litter. We found that (a) male attendance at the nest site was markedly reduced for several hours following the birth of pups; (b) this lack of contact by males with nest and pups did not result from active exclusion of males from the nest by their mates; (c) males lacking previous experience of pups, but not those familiar with pups, avoided contact with pups on the day of their birth; and (4) 3-day-old gerbil pups were attractive even to males encountering young for the first time.

Genetic mediation of infanticide and parental behavior in male and female domestic and wild stock house mice

Infanticide is a reproductive strategy found in many mammals, especially rodents. The proportion of male and female house mice (Mus domesticus) that are either infanticidal or noninfanticidal is strain specific and varies widely from stock to stock. Male house mice also show strain-specific variation in the behavioral mechanisms that inhibit infanticidal individuals from killing their own offspring. The adult offspring generated from reciprocally crossed CF-1 and Wild stock house mice were tested for their behavior toward newborn pups. In male CF-1 x Wild hybrids, the proportion of infanticidal and noninfanticidal males matched with their maternal phenotype, whereas female CF-1 x Wild hybrids exhibited a proportion of behaviors typical of the CF-1 phenotype, regardless of their mother's genotype. Our results suggest three conclusions: first, that infanticide is a highly labile and heritable behavior in both sexes; second, that there is a sex difference in the genetic substrate that regulates the inheritance of infanticidal behavior; and third, that selection pressures in male mice may operate independently on the mechanisms that promote spontaneous infanticidal behavior versus the mechanisms that inhibit infanticide.

Selectivity in paternal and infanticidal responses by male mice: effects of relatedness, location, and previous sexual partners

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.

Individual variation in the neural timing of infanticide and parental behavior in male house mice

In male house mice (Mus domesticus and M. musculus), the act of coital ejaculation provides a fail-safe neural signal for timing the birth of their offspring. A unique aspect of this phenomenon is the extraordinary latency that can occur between the stimulus (ejaculation) and its adaptive neural response (male mice cease killing pups and behave parentally toward them). Thus the inhibition of infanticide is routinely time-delayed for many days after mating. In the absence of mating, cohabitation with a female will not inhibit infanticide in CF-1 stock males (M. domesticus), whereas the birth of pups in the male's home cage will inhibit infanticide. But with regard to the ejaculatory phenomenon, which also includes the spontaneous reemergence of infanticide 50-60 days after mating, this entire behavioral cycle toward pups can occur in the total absence of regular time cues from a light/dark cycle following ejaculation. However, exposure to photoperiodic (L:D 12:12) or constant light (LL) accelerated the transition time from infanticide to parenting after ejaculation, while in constant dark (DD), the transition time to parenting was significantly prolonged. The time interval between ejaculation and the inhibition of infanticide, which varied among individuals first mated at 6 months of age, was repeatable when the same males were remated at 9 months of age; however, when males were again mated at 18 months of age, the time interval between ejaculation and parenting was dramatically prolonged. In general, coital ejaculation triggers a neural timing system that cannot be explained by any presently known physiological mechanism. Our results do suggest, however, that the neural timing variation observed among individuals is influenced by sex steroid exposure during late fetal development.

Inhibition of infanticide in male Swiss mice: behavioral polymorphism in response to multiple mediating factors

The socio-sexual factors mediating the inhibition of pup-killing in previously infanticidal Swiss Webster male mice (Mus domesticus) were examined. As reported in other studies, postmating co-habitation (i.e., physical contact) with a female during pregnancy suppress pup-killing but the present experiments also showed that several factors are implicated in this phenomenon, namely: 1) Postcopulatory sensory contact (i.e., behind a wire-mesh partition) with the pregnant mate was sufficient to inhibit infanticide virtually in all the subjects. Copulation seems to function as a "primer," but cues, most likely of an olfactory nature, emitted by pregnant mate induce paternal behavior in the stud male. In fact, either copulation followed by sensory contact with a nonpregnant mate (abortion was induced) or sensory contact without copulation with a pregnant female did not suppress pup-killing in the majority of cases. 2) Mating, per se, is capable of inhibiting infanticide in a minority of males. 3) Physical contact with a parturient female (impregnated by another male) at the time of pup delivery inhibited infanticide in approximately 40-50% of males. The data essentially show that, in a house mouse population, there is a behavioral polymorphism in response to the coexisting multiple mechanisms which mediate the inhibition of infanticide.

Olfactory recognition: a simple memory system

Mice have an olfactory (pheromone) recognition memory located at the first relay in the sensory system. It is acquired with one-trial learning, contingent upon norepinephrine activation at mating, and lasts for several weeks. The mechanism involves Hebbian (association-dependent) changes in synaptic efficacy at dendrodendritic synapses in the accessory olfactory bulb. As a result of this memory, males made familiar by mating are recognized by the females, thereby mitigating pregnancy block. Such a memory function is biologically important to the female, as it is required to sustain pregnancy in the presence of her stud male's odors.

The relationship between infanticide and pregnancy block in mice

Newly pregnant mice are more likely to exhibit pregnancy block when females are exposed to infanticidal males rather than to noninfanticidal males. These data suggest that females are able to assess the risk to their future infants and adjust their reproductive tactic. Those females that did not exhibit pregnancy block in the presence of infanticidal males suffered a greater loss of pups than those in the presence of noninfanticidal males, demonstrating the advantage of pregnancy block. Females served by infanticidal or noninfanticidal males showed no difference in pregnancy block, but if the litter was produced, there was a greater loss of pups sired by infanticidal males.

Social dominance and reproductive success in pregnant and lactating golden hamsters (Mesocricetus auratus) under seminatural conditions

Pairs of female hamsters were acclimated to seminatural enclosures and permitted to interact for brief periods on days 12-14 of gestation or on days 2-4 of lactation. In one experimental paradigm, food was provided in excess of the daily requirements while in a second, food rations were restricted. Control females were similarly housed and fed but were not exposed to another animal. Paired females readily established dominant-subordinate relationships. Dominant females successfully removed food from the subordinate females and, as a result, accumulated larger food hoards. Subordinate females gave birth to fewer pups than either dominant or subordinate animals when social interactions occurred late in pregnancy and maintained smaller litters when interactions occurred during lactation. Reductions in the fecundity of subordinate females were more pronounced when the food ration was restricted. Under ad lib feeding conditions dominant females also maintained fewer young than control animals, but when food was restricted they were more successful than the control group. Subordinate females successfully defended their burrows against intrusions by dominant animals; maternal cannibalism accounted for most reductions in litter size. These results suggest that a defensible burrow and the acquisition of a food hoard are essential for successful reproduction in this species.

Infanticide in rats: male strategy and female counter-strategy

The present series of experiments addresses the question of whether mating governs infanticide in the male rat and, in addition, asks whether the female rat has available an effective counter-strategy to male infanticide. With regard to the first question, we found that mating provides a safeguard against the killing of own young. That is, mating induces a general inhibition of infanticide coincident with the birth of the male's young and a recrudescence of infanticide synchronized with their weaning. The particular gain realized in killing alien young depends on the age of young and consequently on whether the mother had reached postpartum estrus. We also found that the pregnant female has an effective strategy to counter male infanticide which she employs before the young are born. The data show that this counter-strategy involves the synthesis of a chemosignal of low volatility emitted during pregnancy. The possible role of the male's vomeronasal system in the reception of this chemosignal is discussed.