HORMONES AND BEHAVIOR, 6, 9-17 (1975)

Aggressive Behavior of Pregnant Mice Toward Males ELIANE NOIROT, JACQUELINE GOYENS and MARIE-CHRISTINE BUHOTl Laboratoire de Psychologie Exp&imentale,

IO50 Bruxelles, Belgium

Aggressive behavior toward male intruders was compared between pregnant, pseudopregnant, and virgin females during a series of successive daily encounters. The pregnant and the pseudopregnant females obtained significantly higher scores than the virgins. Changes in ovarian activity may be a determinant of the female aggression toward males.

Aggressive behavior seems to be related to gestation in the female mouse. During previous experiments pregnant mice were presenteddaily with an intruder, alternately male and female. Several scoresof aggressivebehavior rose progressively with the advance of gestation and reached a maximum toward the time of parturition. During the subsequent lactation period a progressivedecline occurred2 unlessthe female had been fertilized postpartum and was thus pregnant again. In such casesaggressivebehavior remained at a high level (Beniest-Noirot, 1958, 1959; Noirot, 1969, 1974). An intriguing change in the behavior of the pregnant female is the intense aggressiondisplayed toward males, which is rare or absent at other times (Beniest-Noirot, 1958). However, since the animals had been confronted alternately with a male and a female intruder, it was possible that part of the increasing hostility toward the males had been caused by the intermittent encounters with females. The present experiment further investigated the behavior of the pregnant mouse toward males with these possible effects of female encounters eliminated. Preliminary attempts to understand physiological correlates of the change in the behavior were made by comparing the results from pregnant 1Present address: C.N.R.S.-I.N.P. 9 13274 Marseille - Cedex 2, France. 2Gandehnan (1972) reported similar observations on the behavior of lactating female mice with intruders. His analysis of the data did not permit one to detect a possible progressive change but the percentage of females attaching male and fernate intruders was higher during the fust part (O-13 days postpartum) of the lactation period than during the second part (14-20 days postpartum). 9 Copyright @ 1975 by Academic Press, Inc. AII rights of reproduction in any form reserved.

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females, presented daily with a male intruder, with those from virgin and pseudopregnantanimals similarly treated. METHODS Residents

Adult femalesfrom the colony (housing, feeding, and breeding described elsewhere;Noirot, 1964) served as the subjects or residents. They had been reared in litters culled to six pups: three males and three females,whenever possible. At weaning on Day 25, they were housed with their female littermates in living cagesas described elsewhere(Goyens and Noirot, 1974). The females were 6 wk-2 mo old when the present experiment began. They were randomly assignedto one of the following four groups: P (pregnant), PP (pseudopregnant),VI (virgin, condition l), and V2 (virgin, condition 2). Shortly before the experiment the females from groups P and PP were housed alone. Vaginal smearswere taken each morning and analyzed.When a female was judged in heat, it was marked by a color spot dyed on its fur and replaced in its living cage either with a normal male (group P) or with a vasectomizedmale (group PP). The next morning the male was removed and the female was housed in a standard observation cage(described previously; Goyens and Noirot, 1974). It was then tested daily with a male intruder, over 19 days if it belonged to group P, or over 13 days if it belonged to group PP. Fifteen females were observed in each condition. During the experimental period no smearswere taken in condition P since this might have interrupted gestation. All the females from this group gave birth on Day 20. In condition PP vaginal smearswere taken daily in order to check for the pseudogestational state of the females.In most of these animals (1 l/ 15) pseudogestationlasted 10 days. One animal was pseudopregnant for 9 days only and the three remaining femalesfor 11 days. Two control groups of 15 virgin females each were used. The animals from the first group (Vl) served as controls for group P. They were tested during 19 successivedays without vaginal smearsbeing taken. The females from the second group (V2) served as controls for group PP. Daily vaginal smearswere taken and analyzed for about 10 days before the beginning of the experiment when the females were placed in an observation cage. Testing started the same day and was continued over 13 days. Daily taking and analysisof vaginal smearswas pursued during this period. Intruders

Adult male mice, reared from birth till weaning under the same conditions as the female residents, served as intruders. All the males were

AGGRESSIVENESS IN PREGNANT MICE

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sexually experienced but not actually living with a female during the experiment. They were housed singly in small living cages,20 X 12 X 12 cm in size, made of transparent plastic and provided with a metallic roof containing a food hopper and a water bottle. Intruders were never used more than once a day and they never encountered the sameresident female twice during the experiment. Procedure

An encounter started when the experimenter placed a male in a resident female’s cage. From the moment of contact onwards, i.e., when the female touched the male with the snout, the animals were left together and observed for 3 min. The occurrence and the duration of the following items were recorded on an event recorder: Attacking: the female pushes,scratchesand/or bites the male; “impulsive attacking”: described previously (Beniest-Noirot, 1958) consists of attacking without any previous investigation or other response toward the intruder; submissive posture by the intruder: the male displays the upright submissiveposture as describedby Grant and Mackintosh (1963).

RESULTS The data are representedin Figs l-3; the results of statistical two-by-two comparisons between groups appear in Table 1. The numbers of subjects attacking (Fig. l), those of subjects displaying impulsive attacks (Fig. 2), and the mean time spent attacking (Fig. 3) were systematically higher in the

is

days

Fig. 1. Numbers of pregnant (P), pseudopregnant (PP), and virgin females (Vl and V2) attacking a male intruder during successivedaily encounters. (n = 15 in each group).

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NOIROT, GOYENS AND BUHOT

l

P

0 PP

Fig. 2. Numbers of pregnant (P), pseudopregnant (PP), and virgin females (Vl and V2) displaying impulsive attacks with a male intruder during successivedaily encounters.

15

days

Fig. 3. Mean durations of attacks on a male intruder by pregnant (P), pseudopregnant (PP), and virgin (Vl and V2) females during successive daily encounters.

pregnant and pseudopregnantfemales than in their respectivevirgin controls. All of these six. differences were significant (see Table 1A). Submissive postures by the intruders occurred regularly although not very frequently (= 2115 per day on average)during encounters with pregnant femalesbut never in the other groups. The difference between the scores obtained by the pregnant females and by the virgin control females(Vl) was significant (Table 1A). However, analysis of the individual data showed that only two pregnant females were mainly responsible for this finding since they elicited most (25/33) of the abandon postures during tests with many different male intruders.

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AGGRESSIVENESS IN PREGNANT MICE TABLE 1 Results of Comparisons Between Groups by the Median Test (see Text for Explanation)

Comparison

Nb Ss attacking

Nb Ss giving impulsive attacks

Time spent attacking ~___

Nb encounters eliciting submissive posture by the male

A P(19 days)-Vl(l9 days) PP(l0 days)-V2(10 days)

p

Aggressive behavior of pregnant mice toward males.

HORMONES AND BEHAVIOR, 6, 9-17 (1975) Aggressive Behavior of Pregnant Mice Toward Males ELIANE NOIROT, JACQUELINE GOYENS and MARIE-CHRISTINE BUHOTl L...
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