Changes in incidence of infanticidal and parental responses during the reproductive cycle in male and female wild mice Mus musculus

Anon. Behav., 1988, 36, 1275-1281

Changes in incidence of infanticidal and parental responses during the reproductive cycle in male and female wild mice Mus musculus VICTORIA SOROKER & JOSEPH TERKEL* Department of Zoology, Tel Aviv University, Ramat Aviv, Israel

Abstract. Most studies of infanticide in rodents have been carried out on laboratory animals. In the

present study, pairs o f infanticidal male and female wild mice were observed for changes in their responsiveness to alien pups during the reproductive cycle. Females killed pups during pregnancy, adopted alien pups during the lactation period, and killed pups again a month after the weaning of their offspring. In males, the transition from infanticidal to parental responses occurred as early as the first half of their mates' pregnancy, and only half of them resumed infanticide a month after the weaning of their litters. Subsequent experiments showed that two independent factors, the experience of copulation and cues from the pregnant female, accounted for the cessation of infanticide and onset of parental care in the male. The adaptive value of the co-existence of multiple mechanisms underlying the cessation of infanticide in male and female house mice is discussed with respect to the social life of this species.

In a number of rodent species, the male assists his female partner in caring for the offspring (see Elwood 1983 for a review). However, non-parental female and male rodents may show a strong tendency to kill alien, unweaned young (golden hamsters, Mesocrieetus auratus, Richards 1966; Marques & Valenstein 1976; Mongolian gerbils, Meriones unguiculatus, Elwood 1977, 1980; collared lemmings, Lemmus torrquatus, Mallory & Brooks 1978, 1980; meadow voles, Mierotus pennsylvanieus, Webster et al. 1981; deer mice, Peromyseus californieus, Wolff 1985; house mice, Mus museulus, Svare & Mann 1981; Jakubowski & Terkel 1982). This differential behaviour of parental and non-parental rodents may reflect a difference in response to related versus unrelated young, as has been shown in lactating wild Norway rats (Rattus norvegieus, King 1939). Alternatively, it may reflect a general transition from infanticidal to caretaking behaviour, a transition that occurs around the time that the animal gives birth to her offspring. In the female, the transition from infanticide to caretaking behaviour is believed to be governed mainly by hormonal changes associated with gestation and parturition (Peters & Kristal 1983). In the male, inhibition of infanticide appears to be related to social interactions associated with pair

* To whom all correspondence should be addressed.

bonding, but there is contradictory evidence as to the direct cause for this change in behaviour. According to Elwood (1985), both copulation and subsequent cohabitation with the pregnant mate are required for inhibition of infanticide in male mice of the CSI strain. Several authors have reported that the act of copulation alone is sufficient to inhibit male laboratory mice from killing young (vom Saal & Howard 1982; Brooks & Schwarzkopf 1983; vom Saal 1985). Other investigators have argued that it is cohabitation with the pregnant female, rather than copulation itself, that effectively suppresses infanticidal tendencies in male laboratory mice of the CS1 strain (Elwood & Ostermeyer 1984), as well as in other domesticated rodents (Elwood 1980; Brown 1986). The fact that the incidence of infanticide varies greatly among different stocks of laboratory mice (Labov et al. 1985) might partially account for the discrepancy in the literature as to the mechanisms of inhibition of infanticidal tendencies in the male. Since wild house mice show a much stronger infanticidal tendency than do laboratory mice (Jakubowski & Terkel 1982; vom Saal 1985), we undertook in the present study to reappraise the relative contribution of copulation and cohabitation with a pregnant female to the male's response to young in wild rather than laboratory house mice. Part of the results reported in this article were presented at the Annual Meeting of the Zoological Society of Israel, Oranim, 1984.

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METHODS Animals The subjects were male and female house mice, 3-5 months old, which were F 1 - F 9 (primarily F3) offspring of individuals trapped in the field (Hazeva, Arava District, Israel). They were bred and maintained on a 14:10 h LD lighting schedule (lights on at 0500 hours) in a temperature-controlled room (23 + 1°C) and housed in plastic cages (28.5 x 19 x 13.5 cm) with wood shavings and wiremesh lids. Food pellets and water were supplied ad libitum. Animals were reared by both parents, weaned at the age of 3 weeks, and thereafter housed in same-sex groups (five to seven mice per cage) until the beginning of the experiment.

ing and crouching was based on the knowledge that mouse pups (1) have limited locomotor ability, (2) are incapable of independent urination and defaecation, and (3) are poikilothermic. Hence, retrieving was scored when the pups were present in the shelter of the test animal 24 h after introduction; licking was scored when bladder content (assessed by gently pressing the pup's lower abdomen to express urine) was estimated as 'little' 24 h after its introduction; crouching was scored when the pups were found to be warm 24 h after their introduction, even if the animal itself was not attentive to the pups at the time of checking. Thus, the most obvious evidence for occurrence of parental behaviour was the presence of warm pups, depressions in the nest, or a covered-over nest. Detailed Experimental Procedure

General Procedure During the experiments, the animals were housed either individually or in pairs in plastic cages (see above) divided in half by a diagonal wiremesh partition. Each compartment was provided with an inverted clay pot for shelter. Animals were either confined in separate compartments or permitted free access to both sides, using a movable door in the middle of the partition. An initial study was carried out to confirm that both parents of our present stock of wild mice exhibited parental care. All animals, except those used in the initial study, were screened for spontaneous pup-killing. The screening test (designated test 0 with respect to subsequent tests) consisted of placing a single pup aged 2-7 days diagonally across from the nesting pot; spot checks were made 30 min, 1 h and 24 h after introduction of the pup, which was removed immediately if it was found injured. Animals that did not show spontaneous infanticide (29% of the males and 48% of the females) were omitted from the study. Responsiveness to unrelated young was subsequently tested by exposing each infanticidal mouse to a single pup aged 2-7 days, with the test being concluded immediately if the pup was bitten or killed. If the pup remained unharmed for 15 min, another one or two pups were introduced, and testing continued for an additional 15 min. If no maternal activity was scored during this 15-min period, the pups were left in the cage for 24 h and the occurrence of maternal activities was verified indirectly. Indirect verification of retrieving, lick-

In an initial study, nine breeding pairs with no previous parental experience were tested for their parental responsiveness between days 2 and 5 postpartum. Males and females were tested separately in the two compartments of the cage, with the door in the mesh partition closed. Each parent was exposed to three to five of their own offspring which were introduced into the compartment corner most distant from the clay pot. The latency to retrieve the first pup and the occurrence of other components of parental behaviour (crouching, nest building and licking) were recorded in a 30-min observation period. Additional spot checks were made 1 h and 24 h after introduction of the pups. The behaviour towards unrelated young was monitored throughout pregnancy and lactation in 12 males (group 1) and 12 females (group 2) that had been screened as infanticidal and maintained as pairs throughout the reproductive cycle. Vaginal smears were taken daily from the day of pairing until the day of mating (confirmed by sperm or vaginal plug), which was designated as day 0 of pregnancy. The mates, kept separately in the two compartments of their home cage, were tested simultaneously for their responses to young. Testing was conducted on days 10-12 and days 15-16 of pregnancy (tests 1 and 2, respectively), days 4-5 and days 8-9 postpartum (tests 3 and 4, respectively), and on day 30 after the weaning of the young (test 5). In addition, four control groups were used" males paired with female partners (group 3, N = 13) or with male partners (group 4, N = 14) while separated from their cage mates by a wire-mesh

Soroker & Terkel: Infanticide in wild mice partition; virgin females (group 5; N = 11) that were similarly housed with female partners; males (group 6; N = 15) housed individually in one compartment with no partner on the other side of the mesh partition. In these groups, testing was conducted at intervals corresponding to those employed in groups 1 and 2. To evaluate the effect of copulatory experience per se on infanticidal tendencies of the male mouse, two further groups were used. In one group (group 7, N = 12), each male was housed with a female which was then removed from the cage as soon as mating had been confirmed. In the other group (group 8, N = 12), each male was housed with a female which was removed on the day of proestrus, thus preventing mating. These two groups were tested on days corresponding to days 10, 16, 22, 26 and 69 after mating, a schedule paralleling the one employed for groups 1-6. To evaluate the effect of cohabitation with a pregnant female on infanticidal tendencies of sexually naive males, two groups were used. In one group (group 9, N = 9), each male was paired with a female which had been impregnated by another male 7 days earlier, a time delay necessary to prevent pregnancy failure, i.e. the Bruce effect (Parkes & Bruce 1961; Chipman & Fox 1966). The other group (group 10, N = 11) consisted of males paired with females which had mated with other males 7 days earlier, but had failed to become pregnant. Group 9 was tested on day 12-13 and again on day 16-17 of their mate's pregnancy, and group 10 was tested at corresponding intervals. During testing, the male and female were separated by the mesh partition.

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as low as possible (Still 1982). Particular attention was paid to the following. There was no way of predicting from the behaviour of the adult mouse prior to insertion of the pup into the cage whether infanticide would occur. Therefore, we could not run the study without having a minimum number of pups killed. To minimize the number of infanticides, every pupkilling mouse received only one pup per day. In all cases where infanticide occurred, it took place as soon as the pup was noticed by the adult mouse. The attack was swift, a sharp bite to the head, and death was generally instantaneous. In the rare cases that it was still alive, the pup was removed immediately and anaesthetized. Since wild mice that ceased killing young behaved paternally immediately after the last day of infanticide (Jakubowski & Terkel 1982), pups that were not attacked received continuous paternal care by the experimental animals throughout the 24-h test period. The pups were left unattended for only short periods of time. Room temperature was warm (average 23°C) and at no time did the pups produce distress calls that might indicate discomfort due to cold. Pups were returned to their biological mothers at the end of the 24-h period for immediate nursing (Crnic 1980). The weight of the test young 48 h after the test did not differ from their litter-mates that had nursed uninterruptedly, thus indicating that a 24-h separation period from the mother does not have a long-term effect on weight gain. No other differences in behaviour were observed between the test and non-test pups. RESULTS

Data Analysis The raw data were analysed using the chisquared test. In cases of 2 x 2 contingency tables in which an expected frequency was less than 5, the chi-squared test with Upton's correction was used (Upton 1982). The level of significance was set at 0"05 and all tests were one-tailed. Ethical Considerations In this study, we took care to minimize the suffering of the young, according to the Guidelines of the Association for the Study of Animal Behaviour (Anim. Behav., 29, 1-2). Taking into consideration the needs of the study, the number of animals used in each experimental group was kept

Parental Behaviour in Male and Female Mates Male parents used in the initial study showed parental care at a rate comparable to that of their female partners. Retrieving, crouching and nest building were shown by 89% of the males and by all of the females; licking of pups was shown by 22% of the males and 55% of the females. The only parameter that differed between males and females was the latency to retrieval of the first pup. While all nine females retrieved pups within 0-4 min 0(+SE=1"33+0-50), six out of nine males retrieved pups within 0-17 min (4"83+2'64; P<0"05), two males retrieved with a latency greater than 30 min, and the remaining male did not retrieve at all.

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Animal Behaviour, 36, 5 (o)

=

0

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Fig bree Closed bars: (group 2,

Cycle During the reproductive cycle, the temporal changes in responses to unrelated young differed between male and female mates (Fig. 1). Towards the end of the gestation period, all breeding females (group 2) were still killing alien pups, whereas the majority of their male partners (group 1) were either parentally responsive (58%) or indifferent, leaving the pups unharmed (25%). During the lactation period, both parents showed parental behaviour towards their own offspring; similarly, the majority also showed parental behaviour towards unrelated pups. One month after the weaning of their young, all females, but only half of the males, resumed killing pups (Fig. 1). A sex difference was also evident among the control mice in which some 30-40% of the males (groups 3, 4 and 6), but none of the females (group 5), ceased killing pups with repeated testing (Fig. 2). This nonspecific decline among the control males was significantly smaller than the marked decline observed among the breeding males (group 1) during the pregnancy and lactation of their mates (Figs 1 and 2).

I00

.:5 50

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13 0

I00

50

0

J

2 3 Test no.

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Figure 2. Incidence of infanticide on alien pups in male and female wild mice during six repeated tests. (a) Closed bars: unisexuaUy paired males (group 4, N= 14); open bars: females (group 5, N= 11). (b) Closed bars: males housed individually (group 6, N= 15); open bars: males housed with females behind a wire-mesh partition (group 3, N= 13). (c) Closed bars: males with mating experience (group 7, N= 10); open bars: males without mating experience (group 8, N= 12). For testing schedule, see Methods. The asterisks indicate tests in which significant differences occurred between the matched groups. *P<0.05; **P<0.01.

Effect of Copulation Mating experience per se effectively inhibited infanticidal tendencies in male mice (Fig. 2). However, the difference between sexually experienced (group 7) and sexually naive males (group 8) reached significance only at test 3 (day 22 postcoitum; Fig. 2), rather than at test 1 (day 10 postcoiturn), as in the case of breeding males that remained

with their mates after copulation (Fig. 1). At the fifth test (69 days postcoitum), the percentage of sexually experienced males committing infanticide returned to a level similar to that scored in sexually naive males. Virtually all males showed parental behaviour in tests in which they did not kill the young.

Soroker & Terkel: Infanticide in wild mice I00

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Figure 3. Incidence of infanticide on alien pups during three repeated tests in male mice that cohabited with females that were pregnant by other males (closed bars: group 9, N=9), or males cohabiting with females that copulated with other males but failed to become pregnant (open bars: group I0, N= 11). *P<0.05.

Effect of Interaction with Pregnant Females Sexually naive males that were permitted to cohabit with pregnant females (group 9) showed a significant decline in the incidence of infanticide compared with males that cohabited with nonpregnant females (group 10; Fig. 3). Pregnancy Effect versus Copulatory Effect Although the pregnancy effect that was achieved by the time of the second test (Fig. 3) appears to occur earlier than the copulatory effect, which was established only by the third test (Fig. 2), both effects do in fact appear simultaneously, as evidenced by the similar height of the black histograms for both Figs 3 and 2. The reason for the apparent slight difference in timing is the slower rate of fall in infanticide in males cohabiting with females that had copulated with other males but were not pregnant (Fig. 3).

DISCUSSION The level of infanticidal tendencies of different inbred strains of laboratory mice is quite complex. Not only is it a consequence of the genetic quirks of the animal, but housing and testing conditions, as well as social factors (Elwood 1986), also influence this behaviour (for a review see Labov et al. 1985). The spontaneous expression of parental beha-

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viour, common among non-parental males and females of many laboratory strains of mice (Noirot 1972), probably represents an artefact resulting from intensive inbreeding. Artificial selection exerted during the development of various strains of laboratory mice is believed to have acted in favour of parental responsiveness among nonparental mice, and against the infanticidal tendency which is very uniform and widespread among non-parental wild mice (Jakubowski & Terkel 1982). We have shown here that the occurrence of parental responsiveness in spontaneously infanticidal wild mice is generally restricted to reproductive periods. Most parental male and female wild mice showed caretaking behaviour not only towards their own offspring, but also towards alien pups. In both sexes, the transition from infanticidal behaviour to parental responsiveness occurred after formation of the pair bond, and both sexes showed the reversed transition (from parental responsiveness to infanticidal behaviour) after weaning of young and breakdown of the family unit. However, males differed markedly from females in the temporal pattern of these behavioural changes, suggesting that the mechanisms underlying the transition from infanticide to parental responsiveness are sex specific. Female wild mice killed young during the entire period of their pregnancy, and were ready to adopt alien pups only while they were nursing their own offspring. This implies that the reciprocal disappearance of infanticide and onset of maternal responsiveness in the wild female mouse bears a relationship to physiological (perhaps hormonal) changes occurring around the time of parturition. This supposition is compatible with the recent implication of prostaglandin and oxytocin in the cessation of infanticide and onset of maternal behaviour in wild female mice at the time of parturition (McCarthy et al. 1986). In confirmation of previous results (Jakubowski & Terkel 1982), female mice killed pups with greater persistence than did males. Among non-breeding mice, 30-40% of the males, but none of the females, ceased killing young with repeated testing, and among breeding mice, all females, but only half of their male partners, resumed infanticide a month after the weaning of their offspring. In view of the reappearance of infanticide among non-lactating primiparous females, it is likely that the maintenance of postpartum maternal responsiveness depends on continuous feedback from young, as in

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the case of the rat (Rosenblatt & Lehrman 1963; Jakubowski & Terkel 1986). In male wild mice, the transition from infanticidal to parental responses appears to be governed by extrinsic social cues rather than intrinsic hormonal changes as in the female. Like male Mongolian gerbils (Elwood 1977), male mice ceased killing pups and responded parentally to pups earlier than did females, i.e. within the first half of their mates' pregnancies. Subsequent experiments have shown that two factors accounted for this transition in behaviour, namely, the experience of copulation and sensory cues from the pregnant female. The influences of these factors, which were independent of each other, were comparable in terms of the proportion of males that eventually ceased killing pups, yet, in the present study, the pregnancy effect seems to be established at about the same time as the copulation effect. It is still possible that the pregnancy effect could have been established even earlier, as early as in males that were permitted both to copulate and to cohabit with their pregnant mates, had the sexually naive males been paired with the pregnant females soon after impregnation rather than 7 days later (a procedure which was necessary in order to prevent pregnancy failure). Thus, our results concur with those of vom Saal & Howard (1982) and vom Saal (1985) that the copulation effect is achieved with a latency corresponding to the duration of pregnancy, but we did not find that the pregnancy effect is achieved with a much shorter latency as reported by Elwood (1985, 1986) and Elwood & Ostermeyer (1984). The coexistence of both these effects accords with Labov (1980) and Elwood (1985, 1986), while being at variance with other authors who have argued that only one of the factors, either copulation (vom Saal 1985) or pregnancy (Elwood & Ostermeyer 1984), is responsible for the suppression of infanticide in the male mouse. The multiplicity of mechanisms underlying the transition from infanticidai to parental responses in male and female wild mice must be considered with respect to the social life of this species. Under natural and seminatural conditions, house mice live in stable family groups, each consisting of a single dominant male, several subordinate males, and several breeding females and their offspring (Reimer & Petras 1967; Berry 1970; Lund 1975; Bronson 1979). In the female, hormonal changes at parturition guarantee that the transition from infanticidal to maternal responses takes place at the

appropriate time. The readiness of the lactating female to adopt unrelated pups may be advantageous to the offspring, as it provides the basis for communal nursing (Southwick 1969) which has been shown to boost litter growth (Sayler & Salmon 1969). The pregnancy effect exerted by the female on male companions other than the dominant male may provide a mechanism eliminating infanticidal tendencies (Elwood 1986). For the dominant male, however, mating experience apparently provides a sufficient basis for switching from infanticidal to paternal responses, since he is likely to be the parent of all young born in the group. However, the process of transition from infanticidal to parental behaviour by the male may vary according to the type of stimuli provided by the pups, as was recently shown for CS-1 mice (Elwood & Ostermeyer 1986).

ACKNOWLEDGMENTS We gratefully acknowledge the help of Dr M. Jakubowski in the preparation of the manuscript, Ms N. Paz for editing and typing the manuscript and A. Shoob for the photography. This research was conducted in partial fulfilment of the M.Sc. degree at the Faculty of Life Sciences at Tel Aviv University.

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(Received 14 January 1987; revised 6 January 1988; MS. number: 2953)