Maternal aggression as a deterrent to infanticide in Peromyscus leucopus and P. maniculatus

Anita. Behav., 1985, 33, 117-123

Maternal aggression as a deterrent to infanticide in Peromyscus leucopus and P. maniculatus J E R R Y O. W O L F F

Department of Biology, University of Virginia, Charlottesville, Virginia 22901, U.S.A. Abstract. Adult male and female white-footed mice (Peromyscus leueopus) and deermice (P. maniculatus) from sympatric populations commit infanticide on neonates of either species. A laboratory experiment was conducted to determine whether aggression in maternal females could effectively deter infanticidal intruders. Females of both species exhibited post-partum aggression and dominated intruders of either sex or species in 83 of 88 trials. In the absence of the mother, neonates were attacked by intruders in 82 out of 84 trials. Maternal females were considerably more aggressive than anoestrous females. The implications for female territorially as a mechanism to prevent infanticide are discussed. The adaptive significance of infanticide has recently been demonstrated for male house mice (Mus musculus; Labov 1980), collared lemmings (Dierostonyx groenlandicus; Mallory & Brooks 1978), and meadow voles (Microtuspennsylvanicus; Webster et al. 1981), and for female collared lemmings (Mallory & Brooks 1980) and Belding's ground squirrels (Spermophilus beldingi; Sherman 1981). Males benefit by eliminating the genes of unrelated males and by causing females to return to oestrus early and then mating with them. Females benefit by eliminating competing offspring and by occupying the nest and territory of the displaced female (Mallory & Brooks 1980; Sherman 1981; Brooks 1984). If infanticide is common in a species, a selecting force for post-partum aggression in maternal females can arise as a defence against infanticidal intruders. Heightened aggression in lactating mice is well known (Rowtey & Christian 1976; Svare & Gandelman 1976; Mallory & Brooks 1978, 1980; Ayer & Whitsett 1980; Gleason et al. 1980) and probably serves this function. In this paper, I document high levels of aggression in maternal females of two syntopic species of mice, and discuss its role in the prevention of infanticide. The white-footed mouse (Peromyscus leucopus) and deermouse (P. maniculatus) occur sympatrically in the Appalachian Mountains of the eastern United States. These species eat the same foods, occupy the same habitat, are very similar in appearance, and exhibit intra- and interspecific territoriality within each sex (Wolff et al. 1983). Wolff et al. found no differences in agonistic behaviour between versus within species and concluded that the two species were behaving as one. They were not able to determine the proximate

functions of territoriality. Territories serve numerous functions including the prevention of infanticide (Sherman 1981). Here I propose to test the hypothesis that one of the functions of territoriality in female Peromyscus can be to prevent infanticide. In some rodent species, female territories are more exclusive with respect to intruders than are those of males (Bujalska 1973; Jannett 1980; Madison 1980), and this seems to be true for P. maniculatus (Metzgar 1979) and P. leucopus (Metzgar 1971; Madison 1977). To support the hypothesis that female territoriality can prevent infanticide, it must be shown that (1) intruding strangers wil! kill neonates, (2) lactating females are more aggressive than non-lactating females, and (3) lactating females are able to exclude infanticidal intruders from their nest sites. These predictions were tested by (a) pairing lactating females and their neonates with intruders of each sex and species, (b) comparing the levels of aggression of lactating versus non-lactating females when paired with strangers, and (c) allowing strangers access to neonates to see whether they would commit infanticide. As P. leucopus and P. maniculatus are very similar morphologically, ecologically and behaviourally, infanticide and associated maternal aggression might be expected to operate between as well as within species.

MATERIALS AND METHODS Experiment I A total of 18 pregnant P. leueopus and 18 pregnant P. maniculatus were live-trapped at the

117

118

Animal Behaviour, 33, 1

Mountain Lake Biological Station, Giles Co., in southwestern Virginia from May to July, 1983. Approximately 100 anoestrous females and adult males were live-trapped from the same area. Animals were housed individually in 20 • 30 • 15-cm stainless steel cages and provided with cotton bedding, shavings, and food and water ad libitum. Pregnant females were within 5-20 days of parturition when caught. Wild animals were used in this study to reduce the artifact of habituation to laboratory conditions. In the first part of this experiment, intruders were introduced into the cage of a dam and her neonates 24~48 h after parturition. In eight of 12 trials, intruders killed neonates within the first 10 rain. In this situation, it was not possible to determine if maternal aggression could exclude intruders, so in subsequent trials a more complex two-house system was used, in which the intruder had room to escape. Although it is impossible to duplicate field conditions in the laboratory, a system where an intruder has a choice to enter or leave another individual's cage seems more realistic than confining an intruder to another's cage and offering it no options. Within 24 h of parturition, the dam, her young and the bedding were transferred to a plastic (Habitrail; Metaframe Inc.) house. Another mouse (intruder) was placed in a second house, which was connected to the first house by a 30-cm plastic tube with two partitions 15 cm apart separating the two mice. After a 24-h acclimation period, the two partitions were removed from the tube and the mice were allowed to move between the houses and interact. Once the animals first appeared to respond to each other, the following behaviours were recorded for each individual for the next 5 min: offensive approach, attack, retreat, avoidance, touch, and wrestling (Colvin 1973; Wolff et al. 1983). Outcomes of these trials were categorized as 'resident female wins', 'intruder wins', or 'no aggression'. The animal with the greatest number of attacks and offensive approaches was declared the winner. After 5 min, the trial was terminated, and the intruder was removed from its house. The house and connecting tube were washed and deodorized, and a second mouse was placed in the house. After a 24-h acclimation period, the trial was repeated with the second animal. Four trials were conducted on four consecutive days, one each with a male and female of each species. The ordering of the trial types was random. Most trials were conducted between day 3 and day 8 post-partum. A total of 88

trials were conducted. Eight additional trials were conducted in the same way using 17-18-day-old pups that were weaned but still with their mother, in order to determine at what stage maternal aggression waned and infanticide ceased. Wildcaught adult males and anoestrous adult females weighing from 17 to 23 g, which were not related to the dam and had no prior association with her, were used as intruders in the trials. Each intruder was used just once.

Experiment II After each set of four trials in experiment I, the dam was removed and an intruder was placed in the second house for 10 rain. After 10 min, the partitions were removed and the intruder was permitted access to the natal nest. The intruder was observed until it grabbed and bit one of the neonates or for a maximum of 10 min. In the first eight trials, the intruders killed the neonates, so in subsequent trials an attempt was made to stop the infanticidal attack before the neonates were killed. Subsequent litters of neonates were exposed to intruders of each sex and species. Each intruder was used just once and in only one experiment.

Experiment III Forty-three behavioural trials were conducted as in experiment I, only using anoestrous parous females instead of maternal females paired with intruders of each sex and species. After a 24-b acclimation period, the animals were permitted access to each other and the same behaviours were recorded for 5 min. After the 5-min trial, the two animals were left together for 12 h and the nesting location of each was then recorded.

RESULTS

Experiment I A total of 88 trials were conducted involving maternal females and intruders of both sexes and species in the two-house apparatus. In all types of encounters, maternal females made more offensive approaches and attacks than intruders, and intruders made more retreats and avoidances than maternal females (Table I). Touching occurred in only two trials. The mean frequency of wrestling/5-

Wolff" Maternal aggression in Peromyscus

119

Table I. The mean (-+ sn) numbers of agonistic behaviours exhibited by P. leucopus and P. rnaniculatus mothers and intruders of each sex and species toward each other in 88 dyadic encounters (each encounter lasted 5 min; behaviours are the same as in Wolff et al. (1983), modified from Colvin (1973))

Species (sex)

N

Offensive approach

Attack

Retreat

Avoidance

Touch

Wrestle

P. leucopus(dam) P. leucopus (M)

11 1.09_+1.45 2-73_+2.33* 0.09_+0.30* 0.18___0.60 0.18_+0.60 1.82_+1-99 0 0-27_+0.65 3.36-+1.86 1.36+ 1.36 0

P. leucopus(dam) P. leucopus (F)

12 0.67_+1-23 4-00--t-_2.59" 0.08_+0.29* 0.08_+2.29 0 0.08+0-29 3.58_+2.81 0.33_+0.89

0 0

2.15+2-18 -

P. leucopus (dam) P. maniculatus (M)

0.20+0.63* 0 10 0.50_+1-08 4.50+3.03* 0 0.20--+0.63 4-80-+2-62 0-30_+0-67

0 0

3.20+4-13 -

P. leucopus (dam) P. maniculatus (F)

0* 0 10 0.90_+0.99 5.50+4.35* 0 0 . 1 0 _ + 0 . 3 2 6.20_+4.76 1.40_+2.01

0 0

4,40 + 5.23 -

P. maniculatus(dam) P. leucopus (M)

12 1.08___1.44 4.50-+3.15" 0* 0 0.08-1-2.29 4.25-+2.66

4.75+2.63* P. maniculatus (dana) 12 2.83+4.30 P. leucopus (F) 0 0

0 0.08_+0.292.25+1.86 0.67___1.30 0-08_+0.29

0* 0.33+0.78 6 - 7 5 _ + 5 . 3 4 1.58+_2.43

0 0

2.08 + 1.68 -

P. rnaniculatus (dam) P. rnaniculatus (M)

6.54+5.35* 0-09+0.30* 0 11 1.45+2.42 0 0.09-/-0.30 5.09___4"81 0-55-+1.51

0

4.45+4.16 -

P. maniculatus (dam) P. maniculatus (F)

5'90+3-78* 0* 0 10 0.80+1.62 0 0.10-+0.32 5.20+_4-18 1.10-+1.73

0 0

3-90+2.18 -

* Difference in means; Mann-Whitney U-test, P<0-01.

min encounter ranged from 1.82 to 4.45. Maternal females were uniformly aggressive toward intruders o f either sex or species. In the two-house system maternal females won in 83 o f the 88 trials (Fig. 1). In five trials females were n o t aggressive, and in three o f these trials intruders killed neonates. In five trials where females won intruders still killed neonates. Maternal females entered the intruder's house in 61 o f 88 trials and d o m i n a t e d intruders in their own houses in all 61 trials. In only two o f eight trials involving intruders and dams and their 17-18-day-old pups was any aggression displayed t o w a r d the pups. The pups displayed a defensive posture and nipped back at the intruder, which retreated. In the other six trials, intruders were amicable toward the pups and the d a m m a d e no offensive attacks toward the intruders.

60

~

No A~gresslon

a -Int ruc~e r Win~

_

P,,, o %~, ...... , %,~ ~-,,-, ,~ {12) P ( ~ - to r it (io1

,/ MATERNAL

P LEUCOPUS

MATERNAL

P ~4ANICULATU$

-r

Wi~

...... Intrude~ In.def. (Z'41 e,~t~

(19) prom

ANOESTROUS

Figure1. Outcomes of behavioural trials involving maternal females paired with intruders of each sex and species, and anoestrous females paired with all intruders. committed by both sexes and species against neonates o f either species. Infanticide usually occurred within the first 3 min o f the trial (J(= 2.21 min). In those two trials where infanticide did not occur, intruders explored b o t h houses, but spent m o s t of the time sitting in their own nests. Maternal females never killed their own young during any of the maintenance or testing procedures.

Experiment II In 82 o f 84 trials (98~) where maternal females were removed from their nests, intruders killed (or attacked) neonates (Table II). Infanticide was

Experiment III In 43 trials involving anoestrous females and

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Animal Behaviour, 33, 1

Table IL Number of intruders committing infanticide when the dam was removed Neonates with dam removed Intruder species (sex)

P. leucopus (N)

P. leucopus (M) P. leucopus (F) P. maniculatus (M) P, maniculatus (M)

10 12 10 10

(10) (12) (10) (11)

P. maniculatus (N) 10 10 10 10

(11) (10) (10) (10)

other anoestrous females and males, agonistic behaviour was minimal and did not differ significantly across classes of intruders, so all intruders were combined into a single sample for each species. There was no difference in the frequencies of agonistic behaviours displayed by anoestrus females toward intruders (Table III). In 24 trials conducted with anoestrous P. leucopus females, resident females won in nine trials (38%), intruders won in two trials (8%), and 13 trials (54%) ended in no aggression (Fig. 1). In 19 P. manieulatus trials, resident females won in two trials (11%), intruders won in one trial (5%), and 16 trials (84%) ended in no aggression. The proportion of wins by maternal females was considerably greater than for aneostrous females (Fig. 1). Anoestrous females were not as aggressive as maternal females (Table IV). Maternal females attacked and wrestled more than anoestrous females, whereas anoestrous females avoided and touched more than maternal females. In 19 of 24 trials, anoestrous P, leucopus females were nesting with intruders after 12 h, 10 times in the female's house and nine times in the intruder's house. In 17

of 19 trials, anoestrous P. maniculatus females were nesting with intruders after 12 h, 10 times in the female's house and seven times in the intruder's house. Co-nesting occurred with males and females of either species.

DISCUSSION Maternal female P. leucopus and P. maniculatus exhibited post-partum aggression and dominated intruders of either sex or species. Maternal females were also considerably more aggressive than anoestrous females. Anoestrous females showed little aggression toward males or females of either species and nested with intruders in 84% of the trials. When maternal females were removed from their nests, intruders killed or attacked neonates in 82 of 84 trials. In three trials where maternal females were not aggressive and in five trials where females were aggressive and dominant over the intruders, the intruders still killed neonates. The results show that post-partum aggression in females can prevent infanticide. In single-cage experiments with lemmings (D. groenIandicus) and house mice, maternal females are apparently unable to defend their young against intruders and infanticide commonly occurs (Mallory & Brooks 1978, 1980; Labov 1980, Huck et al. 1982; V o m Saal & Howard 1982; Brooks & Schwarzkopf 1983). In this study, intruders killed pups in eight of 12 trials conducted in a single cage. In the two-house system, however, where the intruder had r o o m to retreat, maternal females were able to exclude intruders in 83 of 88 trials and to prevent infanticide in 78 of 88 trials. On two occasions intruders were left in the two-house

Table IlL The mean ( • SD) numbers of agonistic behaviours exhibited by anoestrous females and all intruders toward each other in 43 dyadic encounters

Species

N

Offensive approach

Attack

Retreat

Avoidance

Touch

Wrestling

AnoestrousP. leucopus Allintruders t P

24 0.71+_1.30 0-79-+1.47 0.54_+1.84 1.04+__l.06 0-42___1.06 24 0.17+_0.82 0.50_+1.44 1.04_+1-23 1.17+1.55 0.71+_1.16 0.13_+0.34 1"73 0"69 -- 1"11 --0'27 --0-91 0-09 0"49 0"28 0'79 0"37

AnoestrousP. maniculatus All intruders t P

19 0-05+0.23 0.26+_0.56 0.26+1.15 1.00+1.11 1-89+2.83 19 0.21_+_0.54 0.16-+0.69 0.32+-0.58 0.79+1.08 0'68+2-31 0.16-+0-50 -1-t8 0-52 --0.18 0-59 1-45 0-25 0-61 0.86 0-56 0-16

Wolff: Maternal aggression in Peromyscus

121

Table IV. The mean (_sn) number of agonistic behaviours exhibited toward intruders by maternal and anoestrons P. leucopus and P. manieulatus females Species and reproductive condition

AnoestrousP. leucopus Maternal P. leucopus t P

N

Offensive approach

Attack

Retreat

24 0.71_+1.30 0.79+1.47 43 0.79___1.19 4.14• 0.26 -5.87 0.80 0.0001

Avoidance

Touch

Wrestling

0-54+_1-83 1-04+_1-60 0-83_+1.24 0-12+_0.34 0.09+0.37 0.07+0.34 0-09+_0-43 2-86+3.56 1.18 2.94 3.06 -4.99 0,25 0.007 0.005 0.0001

AnoestrousP. manieulatus 19 0.05_+0.23 0.26+_0.56 0.26+_1.15 1.00_+1.11 2.53-+3.29 0.16_+0.50 Maternal P. manieulatus 45 1.58_+2-77 5.38+-3,79 0.02• 0.09_+0,42 0-04-+0.20 3.11___2.75 t -3.67 -8-82 0.91 3-49 3.32 -6.94 P 0.001 0.0001 0.37 0.002 0.004 0.0001

system for one hour. In one of these trials a male was killed, and in the other the intruder female was so badly bitten that she had to be destroyed. It is likely that in a natural environment intruders would retreat in response to maternal aggression. Male and female P. leucopus and P. maniculatus are intra- and interspecifically territorial within each sex. Males and females are, however, amicable toward each other in the field both within and between species, and their territories overlap (Wolff, unpublished data). Thus, females exclude females from their territories and not males, but males are known to commit infanticide as readily as females. There may be several reasons why females in the field are not aggressive toward males. Firstly, the resident (presumed stud) male, whose territory overlaps that of the female, excludes strange males from the territory, so there may be little selection for the female to be territorial towards males. Secondly, Labov (1980), Huck et al. (1982) and Mallory & Brooks (1978) have shown that stud males recognize their mates and do not kill their own offspring. Labov (1980) also showed that the longer a pregnant female lives with the stud male, the less likely he is to commit infanticide. Mate recognition has not been shown for P. leucopus or P. maniculatus, but males and females are known to nest together prior to and during the breeding season. Males, however, leave the nest but remain in the territory while the female is raising her litter (Nicholson 1941; Wolff & Hurlbutt 1982). Thus it may be adaptive for a female Peromyscus to nest with her mate for as long as possible prior to parturition to increase his confidence of paternity so he will not commit infanticide. Thirdly, strange male house mice (Labov 1980; Huck et al. 1982)

and lemmings (Mallory & Brooks 1978) that have not previously copulated with the mother commit infanticide. Brooks & Schwarzkopf (1983) have shown that copulating with any female 3 weeks prior to parturition tends to inhibit infanticide in some strains of mice. To prevent infanticide from other males, female Peromyscus may mate promiscuously with neighbouring males so they will recognize her later and not kill her offspring. Multiple insemination is known to occur in P. leucopus (Dewsbury 1976) and P. maniculatus (Birdsall & Nash 1973; Dewsbury 1981). This experiment has shown, however, that if a strange male enters the nest area of a maternal female she is able to drive him away. Mallory & Brooks (1978) proposed that maternal female lemmings exhibit post-partum aggression around the nest to protect their young from infanticide. During post-partum oestrous, however, females will accept males away from the nest but not near it. During this study I conducted eight behavioural trials in the field in a 32-cm-diameter plastic area in the home ranges of lactating females. All trials involved adult male and lactating female P. leucopus, using the method of Wolff et al. (1983). In six of these trials, females were amicable toward males and displayed no aggression. In the laboratory, lactating females were very aggressive and never amicable toward males. These results support the observations of Mallory & Brooks (1978) that post-partum aggression may be site-specific and occur only around the nest area (see also Paul et al. 1980). The lack of aggression displayed by intruders toward 17-18-day-old pups suggests that infanticide may occur during a critical period (between

Animal Behaviour, 33, 1

122

days 1 and 12-15) when the pups are most vulnerable and defenceless. After weaning, pups fight back and infanticide becomes more 'costly' to the intruder. Secondly, if intruders recognize young because of the odour or presence of their mother and not because of any characteristic of the offspring ( L a b o r 1980), intruders would not recognize free-ranging juveniles and therefore should not take the chance of killing potential relatives (see also Brooks & Schwarzkopf 1983). These results are very different from Sadleir (1965) and Healey (1967) who found that adult Peromyscus were aggressive toward juveniles. The adaptive advantage of infanticide to unrelated males seems obvious (Mallory & Brooks 1978, 1980; Hrdy 1979; Labov 1980; Huck et al. 1982; Vom Saal & Howard 1982; Brooks 1984). Although in this study males were not given the opportunity to mate with the mothers of their infanticide victims, in a natural setting this might well occur. It is not known if infanticide is c o m m o n on the field or whether mothers abandon their nest and territory after the loss of litters, as in lemmings (D. groenlandicus; Brooks & Banks 1973; Mallory & Brooks 1978, 1980) and Belding's ground squirrels (Sherman 1981); but if they do, infanticidal intruders would benefit by taking over the territory. Infanticide may also benefit intruding females by directly eliminating young animals which might attempt to establish themselves in the surrounding area. As these two species are so similar ecologically, interspecific infanticide is just as adaptive as intraspecific infanticide. A similar relationship of intra- and interspecific aggression occurs between collared and brown lemmings (Dicrostonyx groenlandicus and Lemmus trimueronatus; Banks et al. 1979). Thus these data support the hypothesis that territoriality in female P. leucopus and P. man&ulatus can function to prevent infanticide, but this does not necessarily exclude complementary or alternative hypotheses. Although laboratory studies do have their limitations and one should use caution in extrapolating to field conditions, the results of this study are consistent with predictions made from previous field studies.

ACKNOWLEDGMENTS I thank V. J. Fitzgerald for her assistance in the laboratory and R. J. Brooks, D. Krohne, A. Bulger, D. Hornbach and J. J. Murray for helpful comments on the manuscript. This work was

conducted at the M o u n t a i n Lake Biological Station, University of Virginia, and supported by N S F Grant 81-05177.

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(Received 3 November 1983; revised 14 February 1984; MS. number: A4191)