Changes in the responses of male and female gerbils (Meriones unguiculatus) towards test pups during the pregnancy of the female

Anim . Behav., 25, 1977, 46-51

CHANGES IN THE RESPONSES OF MALE AND FEMALE GERBILS (MERIONES UNGUICULATUS) TOWARDS TEST PUPS DURING THE PREGNANCY OF THE FEMALE By R . W . ELWOOD* Department of Zoology, University of Reading

Abstract . Mongolian gerbils, maintained in monogamous pairs, were each offered a single test pup in order to assess changes in the paternal and the maternal responses during the female's pregnancy . Naive males, housed with non-pregnant females, treat the pup as food, but they respond in a paternal manner when their mate becomes pregnant . Females eat pups except during very late pregnancy, when they show maternal responses . Experienced males never eat pups regardless of the reproductive state of the female. However, experienced females respond in the same manner as do naive females . Thus both parents are brought into a `maternal' state prior to the birth of their offspring . The offering of pups to non-lactating rodents is an important method for the investigation of the onset of maternal behaviour . This has been done primarily for the rat, the mouse and the golden hamster (Noirot 1971) . Virgin female rats normally ignore pups for several days after the initial presentation . However, a small proportion may be `spontaneously maternal' (Weisner & Sheard 1933 ; Lott & Fuchs 1962 ; Roth, Richards & Lisk 1968) . Male rats will either ignore test pups or kill and devour them (Rosenblatt 1967 ; Rosenberg, Denenberg & Zarrow 1971) . Virgin female and naive male mice usually show parental responses shortly after the presentation of newborn test pups (Beniest-Noirot 1958), however, some male and female mice will kill and eat test pups (Gandelman 1972 ; Gandelman & Davis 1973) . The initial response of virgin females and naive male golden hamsters is usually one of attack (Rowell 1961 ; Richards 1966a ; Scott 1970) . Responses to test pups are influenced by the age of the pup (Noirot 1964a ; Richards 1966b) and by previous contact with pups (Noirot 1964b ; Noirot & Richards 1966) . Pregnancy influences maternal responses towards test pups in rats (Weisner & Sheard 1933 ; Rosenblatt 1965 ; Lott & Rosenblatt 1969), in hamsters (Richards 1966a) and in mice (Noirot & Goyens 1971 ; Fraser & Barnett 1975) . Male gerbils act in a paternal manner towards their own offspring (Elwood 1975a) . However, preliminary investigations suggest that male gerbils, which are living with non-pregnant

females, utilize new born, test pups as food (Elwood 1975b) . Clearly, if the male is to perpetuate his genotype, he must alter this response prior to the birth of his own offspring . This study has been made in order to investigate these changes in male behaviour during the pregnancy of the female with whom he is living . The responses of non-lactating females towards test young have also been assessed . Methods The animals were laboratory bred Mongolian gerbils, the general housing conditions of which are described in Elwood (1975a) . The animals were weaned at 35 days of age and housed in mixed-sex groups of six to eight . At 2 to 21 months, about the age of sexual maturity, the animals were caged as monogamous pairs . Naive animals were tested at 31 to 41 months of age . Experienced animals had all reared at least one litter and were between 5 and 9 months of age. Each animal was tested once, the tests being conducted between 10 .00 and 12 .00 hours. The home cage was removed from the rack and placed on the test bench . The member of the pair which was not to be tested was removed from the cage . A dark cloth was arranged in front of the cage allowing the experimenter to look through a small hole without being seen . A pup, which had been born the previous night, was placed in the centre of the cage, at least 8 cm from the nest . The activities of the adult were recorded on a check sheet using a periodsampling technique . This continued for 6 min following the first approach and sniffing at the pup . Each activity performed each 10 s was noted, resulting in a possible maximum score

*Present address : Department of Zoology, Queen's University, Belfast . 46

ELWOOD : CHANGES IN RESPONSE TOWARDS PUPS DURING PREGNANCY IN GERBILS of thirty-six for each activity . The behavioural categories are listed below : (1) Approach : the animal approaches the pup (recorded only if the animal sniffs or makes contact with the pup) . (2) Sniff pup : nose moving over pup . (3) Lick pup : licking the pup . (4) Gather : the animal pulls the pup towards itself using the forepaws . Often the animal moves back in such a manner that the pup is rolled along . (5) Mark : mark any object, including pup, with mid-ventral gland . (6) Nest-build : gathering, shredding, or rearranging nest material . (7) Scratch : rapid alternation of forepaws aginst the floor or sides of cages . (8) Self-groom : licking or grooming own body with forepaws . (9) Eat : nibbling at food . (10) Gnaw bars : chewing on bars of cage . (11) Bite pup : adult bites and starts to eat pup . If the animal started to eat the pup the experiment was terminated and the pup was killed . If the pup was unharmed at the end of 6 min, it was removed and the other adult was returned to the cage . Only one member of a pair was tested on any particular day and test pups were only used once . The reproductive status of the female was calculated retrospectively from the date of the next parturition . If the female did not give birth within 25 days, the normal gestation period, she was regarded as having been non-pregnant at the time of testing . However, this group may contain a few females which subsequently aborted . In this manner 56 naive males, 49 naive females, 30 experienced males and 34 experienced females were each presented with a test pup . Results General Description of the Responses to a Test Pup In all cases the first approach was followed by sniffing the pup and then usually by licking the pup . A few animals, however, withdrew after sniffing and then approached the pup to sniff it again before licking . If the pup was eaten it was picked up in the mouth and carried to a corner of the cage, then held in the forepaws and eaten . These responses are similar to those seen when a piece of carrot is offered . The pups are not `attacked' as has been described for

47

hamsters (Richards 1966a), but appear to be treated as food . The latency between the first approach and the first bite varied between 10 and 80 s, but was usually between 15 and 25 s . Males housed with late-pregnant females often rolled the pup into the nest by gathering the pup with their forepaws . Once the pup was in the nest, nest-building was usually performed . Females which did not eat the pup often showed a distinct startle response after the first approach ; this occasionally resulted in footthumping and was alternated with sniffing and licking of the pup and rapid withdrawal . Several females in late pregnancy alternated between licking the pup and licking their own genital region . In these cases the activities performed were similar to those observed during parturition (Elwood 1975b) . Some females in late pregnancy retrieved the pup to the nest by carrying it in the mouth . This was never observed in the male where carrying the pup in the mouth was always followed by eating of the pup . Late-pregnant females often engaged in nestbuilding, and one female assumed a suckling position . Quantitative Results The number of animals in each group, which ate the pup, is shown in Fig . 1 . Twelve out of twenty naive males, housed with non-pregnant females, ate the test pup (Fig . 1(a)) . Two out of seven tested when the female was in very early pregnancy ate the test pup . No other naive male harmed the pup . The `non-pregnant' group were compared with each of the `pregnant' groups (Fisher exact probability test) . There was no significant difference between `non-pregnant' group and the 25-19 days group. The 'nonpregnant' group and the 18-13 days group were different in their responses (P = 0 . 002) while between the `non-pregnant' and each of the `late pregnant' groups P = 0 . 001 . Thus there was a clear difference in the responses of naive males towards test pups according to the reproductive status of the female. Naive females showed a different pattern of response (Fig . 1(b)) . All non-pregnant females ate the test pup and the majority of the early and mid-pregnant animals also ate the test pup . However, half of the 6-0 days group did not harm the pup . The non-pregnant group and the 6-0 days group are significantly different (P = 0 . 008). The data for naive males and naive females are significantly different at all re-



48

ANIMAL BEHAVIOUR, 25, 1 Response to a test pup

Inexperienced

Inoxper enced females

ales

25+ 25-19 18-13 12-7 6-0

25

Days prior to birth

(a)

25-19 18-13 12-7 6-0 Days prior to birth

(b)

Experienced females

Experienced males

20

0 E C 0

0 10L C) E j c 25 . 25-19 18-13 12-7 6-0

25* 25-19 18-13 12-7 6-0 Days prior to birth

Days prior to birth (d)

(c)

Fig . 1 . The number of animals tested at each reproductive state is shown by the height of each column . The black area denotes the proportion of animals which ate the test pup. productive states except for the 25-19 days group (Table I) . None of the experienced males ate the test pup (Fig . 1(c)) . These data are different from those for the naive males only for the 'nonpregnant' groups (P = 0 .004) but are different from those for the experienced females for all

reproductive states (Table 1) . There were no differences between experienced females (Fig . l (d)) and naive females . Non-pregnant experienced females differed in their responses to those of the 6-0 days group of experienced females (P = 0 .03) .



ELWOOD : CHANGES IN RESPONSE TOWARDS PUPS DURING PREGNANCY IN GERBILS

As the majority of the females ate the pup, no further analysis was made of the other behavioural categories . The data for naive and experienced males, however, were subjected to an analysis of variance . The results of this analysis for the naive males are given in Table II, which shows that six activities were influenced by the reproductive state of the female. Naive males `approach', `sniff', `lick' and `mark' the most when their mate is in mid-pregnancy, and they `nest-build', `scratch' and `gather' the most when their mate is in the latter half of pregnancy (P = < 0 .01 for all categories) . No statistically significant results were found for the experienced males ; however, the trend for `scratch' and `nest-build' were the same as found

for the naive males, i .e . these activities increased in frequency of occurrence when the female was in late pregnancy . Discussion It has been suggested that the Mongolian gerbil lives naturally in monogamous pairs (Elwood 1975a) or in colonies (Tanimoto 1943) . If a single male were to find a pup it might be to his advantage to eat it . Should he live with a female, however, this behaviour must change prior to her parturition if he is to perpetuate his genotype . Thus it appears that the change in the male responses to pups has a strong selective advantage .

Table I. Comparisons Between Groups Using the Fisher Exact Probability Test Reproductive status Non-pregnant

Naive males/ naive females

Naive males/ exp . males

P = 0 .0107

Naive females/ exp . females

P = 0 .004

Exp . males/ exp . females

NS

P = 0 .00001

25-19

NS

NS

NS

P = 0 . 0013

18-13

P = 0 .0045

NS

NS

P = 0 .05

12-7

P = 0 .0002

NS

NS

P = 0 . 05

6-0

P = 0 . 0082

NS

NS

P = 0 .004

* NS, P > 0 .05 . Table II . Responses of Naive Males and a Test Pup . Results of Analysis of Variance Mean score F-value Activity

N.P .

25-19

18-13

12-7

6-0

df = 4/51

P-value

Approach

3 .6

6.9

10 .2

10 . 0

8.8

15 . 0

< 0 .01

Sniff pup

7.0

11 . 9

17 .2

18 . 6

13 . 7

12 . 5

< 0 . 01

Lick pup

7.8

9 .4

12 .3

12 . 1

9 .4

1 .5

NS*

Gather

1 .0

1 .9

2.1

4.9

2.4

4.8

< 0 .01

Mark

1 .0

1 .6

4 .4

4.6

2.4

4. 1

< 0 .01

Nest build

0.8

0.1

2 .1

2.7

6.4

5.3

< 0 .01

Scratch

0

0

0 .4

4 .5

2.6

4.0

< 0 .01

Self groom

1 .9

2 .1

2.4

2.9

4.3

1 .9

NS

Eat

1 .5

1 .0

1 .2

1 .1

0.7

0.8

NS

Gnaw bars

2.9

8.0

6.4

2.5

2. 6

1 .5

NS

*NS,

P = > 0 . 05.

49



50

ANIMAL BEHAVIOUR, 25, 1

Pup-killing in male and female mice and in male rats has been shown to be enhanced by testosterone treatment (Gandelman 1972, 1973 ; Rosenberg & Sherman 1975) . However, the influence of testosterone is blocked in female mice during pregnancy (Gandelman & Davis 1973) . Pregnancy is marked by an increase in the progesterone level (Velardo 1958) which has been shown to act antagonistically with testosterone in birds (Erickson et al . 1967) and in mice (Erpino 1975) . This antagonism between testosterone and progesterone may account for the findings of Gandelman & Davis (1973) . It may also provide a mechanism by which a male's response to pups changes during the female's pregnancy . The level of progesterone or progesterone metabolites in the female urine will increase during her pregnancy . This may be ingested by the male when he licks the females' genital region during grooming and this may subsequently influence his behaviour . Other mediating mechanisms may be hypothesized . The mid-ventral gland of the female increases in size, and the marking rate increases during pregnancy (Wallace, Owen & Thiessen 1973) . Substances which inhibit pup-killing may be produced at this time . Another possibility is that copulation itself may decrease pup-killing in males ; however, this seems unlikely as the effect is not fully apparent until over 7 days after copulation . Further investigation is required in order to elucidate the mediating mechanism. The changes in the responses to a test-pup by female gerbils during pregnancy are similar to those shown by golden hamsters (Richards 1966a) and by some mice (Gandelman & Davis 1973) . These changes occur later in the female than in the male gerbil . In the natural environment, where the animals are less confined, these changes may be better synchronized . The responses of the females do not change with experience and seem to be mediated by a specific physiological state . Experienced males, on the other hand, do not eat pups even when they are housed with a non-pregnant female . This difference may be due to age differences but it seems more likely that the previous exposure to a pregnant female is responsible . However, it is possible that changes brought about by exposure to a pregnant female may only be temporary . Permanent changes in the male behaviour may require actual experience with pups, but this has yet to be determined .

Acknowledgments I would like to thank Dr D . M . Broom for his help and encouragement . Thanks are also due to Dr P . Laming and Mr P . Miller for their helpful criticism of the manuscript . REFERENCES Beniest-Noirot, E. (1958). Analyse du comportment dit maternal chez la souriz . Monogr, Frac . Psychol., 1 . Elwood, R . W . (1975a) . Paternal and maternal behaviour in the Mongolian gerbil . Anim . Behav., 23, 766772 . Elwood, R. W. (1975b) . Paternal and maternal behaviour of the Mongolian gerbil and the development of the young. Unpublished Ph .D, thesis, University of Reading . Erickson, C . J., Buder, R . H ., Komisaruk, B . R . & Lehrman, D . S. (1967) . Selective inhibition by progesterone of androgen-induced behaviour in male ring doves (Streptopelia risoria) . Endocrinology, 81, 39-44. Erpino, M . J . (1975) . Androgen-induced, aggression in neonatally androgenized female mice . Inhibition by progesterone . Horm . Behav ., 6, 149-157 . Fraser, D . G . & Barnett, S. A . (1975) . Effects of pregnancy on parental and other activities of laboratory mice. Horm . Behav ., 6,181-188 . Gandelman, R . (1972) . Induction of pup-killing in female mice by androgenization . Physiol. & Behav ., 9, 101-102. Gandelmen, R. (1973) . Reduction of maternal nestbuilding in female mice by testosterone proprionate treatment. Dev. Psychobiol., 6, 539-546 . Gandelman, R . & Davis, P . G . (1973) . Spontaneous and testosterone-induced pup-killing in female Rockland-Swiss mice : The effect of lactation and the presence of young . Dev. Psychobiol ., 6, 251-257 . Lott, D. F . & Fuchs, S . S. (1962) . Failure to induce retrieving by sensitization or injection of prolactin . J. comp . physiol. Psychol ., 55, 1111-1113 . Lott, D . F. & Rosenblatt, J . S . (1969). Development of maternal responsiveness during pregnancy of the rat . In : Determinants of Infant Behaviour, 4 (Ed . by B . M . Foss) . London : Methuen . Noirot, E. (1964a) . Changes in responsiveness to young in the adult mouse . II . The effect of external stimuli . J. comp . physiol. Psychol., 57, 97-99 . Noirot, E . (1964b). Changes in responsiveness to young in the adult mouse . IV . The effect of an initial contact with a strong stimulus . Anim . Behav., 12, 442-445 . Noirot, E . (1971) . The onset of maternal behaviour in rats, hamsters and mice : a selective review . In : Advances in the Study of Behaviour (Ed. by D . S . Lehrman, R . A . Hinde & E . Shaw) . New York Academic Press . Noirot, E . & Goyens, J. (1971) . Changes in maternal behaviour during gestation in the mouse . Horm . Behav ., 2, 207-215 . Noirot, E. & Richards, M . P . M . (1966). Maternal behaviour in virgin female golden hamsters : changes consequent upon initial contact with pups . Anim. Behav ., 14, 7-10 .

ELWOOD : CHANGES IN RESPONSE TOWARDS PUPS DURING PREGNANCY IN GERBILS Noirot, E. & Goyens, J . (1971) . Changes in maternal behaviour during gestation in the mouse . Horm . Behav ., 2, 207-215 . Noirot, E . & Richards, M . P. M . (1966) . Maternal behaviour in virgin female hamsters : changes consequent upon initial contact with pups . Anim . Behav ., 14, 7-10. Richards, M . P . M . (1966a) . Maternal behaviour in the golden hamster : responsiveness to young in virgin, pregnant and lactating females . Anim . Behav ., 14, 310-313 . Richards, M. P . M . (1966b) . Maternal behaviour in virgin golden hamsters : the role of the age of the test pup . Anim . Behav., 14, 303-309 . Rosenberg, K . M ., Denenberg, V . H. & Zarrow, M . X . (1971) . Effects of neonatal castration and testosterone on the rat's pup-killing behaviour and activity . Physiol. & Behav., 7, 363-368 . Rosenberg, K. M . & Sherman, G. F . (1975) . The role of testosterone in the organization, maintenance and activation of pup-killing in male rats . Horm . Behav ., 6, 173-179 . Rosenblatt, J . S . (1965) . The basis of synchrony in the behavioural interaction between the mother and her offspring in the laboratory rat . In : Determinants of Infant Behaviour. 3 (Ed . by B . M . Foss) . London : Methuen .

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Rosenblatt, J . S . (1967) . Nonhormonal basis of materna behaviour in the rat. Science, N.Y., 156, 15121514. Roth, L . L ., Richards, M. P . M . & Lisk, R . D . (1968) . Effects of oestrogen and progesterone on maternal behaviour in virgin rats. Am . Zool., 8, 748 . Rowell, T. E . (1961) . Maternal behaviour in nonlactating golden hamsters . Anim . Behav., 9, 11-15 . Scott, J . L . (1970) . Studies of maternal and pup behaviour in the golden hamster . Unpublished Ph.D . thesis, University of Cambridge. Tanimoto, K . (1943) . Studies on mammals in relation to bubonic plague in Manchuria . Tokyo Zool. Mag., 55, 117-127 . Velardo, J. T. (1958) . The anatomy and endocrinology of the female reproductive system . In : The Endocrinology of Reproduction (Ed . by J . T . Velardo) . New York : Oxford University Press . Wallace, P ., Owen, K . & Thiessen, D. D. (1973) . The control and function of maternal scent marking in the Mongolian gerbil . Physiol . & Behav., 10, 463-466. Weisner, B . P. & Sheard, N. M . (1933) . Maternal Behaviour of the Rat . Edinburgh : Oliver & Boyd . (Received 24 November 1975 ; revised 13 March 1976 ; MS . number : 1490)