Social structure and reproduction in two freelygrowing populations of house mice (Mus musculus L.)

Anim . Behav ., 1975, 23, 41 3 -424

SOCIAL STRUCTURE AND REPRODUCTION IN TWO FREELYGROWING POPULATIONS OF HOUSE MICE (MUS MUSCULUS L .) BY JAMES A. LLOYD Program in Reproductive Biology and Endocrinology, University of Texas Medical School at Houston, 6400 West Cullen Street, Houston, Texas 77025

Abstract. Changes in social structures of two populations of house mice (Mus musculus L .) were examined in relation to population growth. In one population that was regulated primarily by a decline in births, decreased births were associated with a reorganization of the territorial system . In the second population the major factor regulating further growth was mortality of infants and young adults . Mortality was associated with a sevenfold increase in aggression and redistribution of breeding females . accessible to all the mice ; the distance from the floor of the upper level to the lower level was 0 . 43 m . There were six nest-sites on each level (Fig . 1) . The nest-sites were wooden boxes that had two apertures on opposite sides for entrance and exit and a removable top that permitted nest examination with minimal disturbance to the animals . Shredded newspapers, water and Purina Laboratory Chow were provided. Sawdust was used as litter . Water bottles were situated on each level at opposite ends of the cage, and food was scattered over the entire floor of the cage on both levels . Fresh litter, nesting materials and food were added as required . This procedure reduced disturbance of the substrate of the populations . Temperature was maintained at 25°C . The populations were maintained on a daily schedule of red light (06.00 to 18 .00 hours) and white light (18 .00 to 06 .00 hours) E .S .T . Individual mice were identified by toe-clipping at 10 to 21 days . At 60 days small beads were sutured at various locations on the mice . Daily records were kept of births, deaths of young and adults, locations in which litters were found, pregnant females and nursing females . Occasionally mice died and disappeared without their remains being found . A monthly census determined the exact number of mice present, and provided opportunity to mark animals . Individual nest-sites were examined during the census by slipping a cardboard sleeve around the nest box, and lifting the entire nest out of the cage . After examining nests mice remaining in the cage were recorded . The census always was taken when activity was minimal .

Descriptions of social structures of laboratory populations of house mice exist, but there has been minimal description of the behaviour of individuals in relation to regulation of population growth. Individual investigators have concentrated on specific aspects of population biology but the consensus is that social structure (Uhrich 1938 ; Southwick 1955a, b, 1958 ; Calhoun 1956a, 1956b ; Anderson 1961 ; Crowcroft & Rowe 1963 ; Lloyd & Christian 1967, 1969 ; Vessey 1967 ; Lloyd 1973) and endocrine reproductive physiology (Christian, Lloyd & Davis 1965 ; Christian 1971a) are interrelated factors that regulate population growth . However, many aspects of the interplay between behaviour and reproduction in large populations remain undefined . The present study is primarily descriptive, presenting details of behaviour of individuals in two populations in relation to reproduction . Observed changes in social structure and reproduction are interpreted in terms of mechanisms regulating growth of laboratory populations . Methods Establishment and Management of Populations Two populations of house mice (Mus musculus L .) were established by placing two sibling 21day-old females with one sibling 21-day-old male into each of two separate cages . The mice were taken from a colony maintained in the laboratory and outbred (i.e . siblings were not mated) for a period of 25 years . The original progenitors of the colony were trapped in the basement of the Johns Hopkins School of Public Health in Baltimore . Cages were stainless steel with mesh walls, removable pans on each level and glass front doors that allowed viewing of cage interiors. Each cage was 1 . 32 m long, 0 . 66 m wide and had two levels that were

Observations of Behaviour These observations were made between 10 .00 hours and 12 .00 hours and between 13 .00 and 413

414

ANIMAL BEHAVIOUR, 23, 2 Location of nest boxes and water bottles in populations ~-

t

Al

1 .32 Meters NB1

NB2

NB3

NB4

NB5

NB6

0-66 Meters

1

A2

84

D B3

Upper Level

Cl

a C2

t

NB7

NB8

NB 9

NB10

NBl1

NB12

0 .43 Meters

Distance Between Levels

D4

3 D3

Lower Level

Fig. 1 . Location of nest sites and water bottles in populations. 17 .00 hours each day, 5 days a week . Every week four 15-min observations were made for each hourly interval . Observations recorded on tape included identification of individuals active, locations of individual fights and aggressive and sexual chases, copulation, manipulation of nesting material and exploratory activity . Tabulation and Analysis of Behavioural Data Distance moved during observation intervals . Figure 1 indicates distances between the various locations . The method of recording distance moved is given by the following example : if mouse 1 was seen at Al at time X and subsequently at NB 3, a distance of 0 . 16 m (from A1 to NB 1) + 0-33 m (from NB 1 to NB 2) + 0-33 m (from NB 2 to NB 3) was recorded . Since mice generally moved along the outer edge of the cage, it was assumed that a mouse moved from the first to the second location along the outside wall. For each mouse, a total distance moved during each observation interval was tabulated, and transformed to distance moved per hour. These data were subjected to computer analysis using a multivariate profile analysis (Morrison 1967), which enables one to test the significance of changes occurring from one time interval to the next . Group means assembled during each interval are treated as independent random samples . Individual scores are recorded as profiles of the measurement . Aggression. The number of attacks as well as the identity of attackers and attacked were

recorded . The number of times per hour that an individual attacked was calculated . Aggressive acts included overt attacks and threats . Each was recorded separately . If two mice fought, paused and resumed fighting, resumption of the fighting was counted as another attack . The flight of a mouse from another was counted as an aggressive act . A pause and resumption of flight was recorded as a separate act . Locations. The number of times indvidual mice were observed in designated locations was tabulated. Percentages of total times that mice were recorded in various locations were calculated for 0 per cent, 0-1 to 5 per cent, 5 to 10 per cent and 10 per cent or more . Areas in which individials were seen most frequently were then determined. Calculation of Birth and Infant Survival Rates Analyses of data from populations described herein, and from other experimental populations have shown that logarithms of cumulative births and of cumulative deaths are linear functions of time and that differences between these functions describe the growth form of these populations (Christian et al . 1971) . In the present study logarithmic plots of the cumulative number of births and of the cumulative number of infant deaths were made against the logarithm of time . Slopes of these lines were calculated, and fitted lines were calculated for the cumulative number born and the cumulative number of infant deaths . Gross birth rates, based on the

LLOYD : SOCIAL STRUCTURE AND POPULATION GROWTH IN MICE total number of mice over 21 days, and infant survival rates were calculated for 21-day intervals . The infant survival rates were back calculated to the day of birth . Smooth curves of density, birth rates and infant survival rates were derived from these data . Results In both populations there were distinctive social structures. As the population sizes increased, the social structure of each population changed . The period during which a population maintained a characteristic social structure is called a `phase' . The social structure observed in each phase is described in terms of the behaviour of individuals within each population . The two populations are separate entities with the dynamics of each being independent of the other. Territories were established in both populations . Territoriality was manifested by the defence of discrete areas encompassing one or more nest sites . Frequent fights occurred at the boundaries of territories . Territorial males moved along the edge of the cage within their own territory until encountering another territorial male . A `stand-off' often followed with both animals immobile, tails erect, ears elevated, pilo-erection and tips of noses almost touching .

Such impasses frequently were broken either by one mouse attacking the other or by one of the mice simply retreating . Non-territorial males were confined to very limited areas and moved about very infrequently. When territorial males encountered non-territorial males on the tops of nest cans, the former vigorously attacked and chased the latter . Likewise, juveniles moving about from one territory to another were attacked frequently but generally were tolerated if they remained in the territory of a single male . Although territoriality was present in both populations, each population manifested unique characteristics in terms of its social structure . The details of the behaviour and reproduction of each population therefore will be presented separately . Social Structure and Reproduction in Population A There were three distinct phases to the social structure of population A . During phase I, lasting 285 days, five territories were established on the upper level of the cage and one on the lower level (Fig . 2). These territories were established by the males born into the second, third and fourth litters that survived to weaning . Shortly after these litters reached maturity the original male became a hermit in an unused nest site . This nest was on the lower level of the

POPULATION A

A,A ° ∎ !a

A* ;1

FAMI PHASE N PHASE III KEY TO PIDIVDUAL MOUSE NUMBER MALE FEMALE UPPER ® 14 • 2 / LEVEL ° 16 • 7 LOWER ° is 31 LEVEL m 22 35 ∎ 47 0 25 o 27 Fig . 2 . Location of territories and distribution of breeding females in population A. (Dotted lines indicate the boundaries of territories ; solid lines indicate an area mice were confined to .) PHASE I

415

ANIMAL BEHAVIOUR, 23, 2

416

POPULATION A DAY 0

6

31

34

47

100

PHASE I

200

CCU

rrTl 300 PHASE II

PHASE III 400

Fig . 3 . Litters born to breeding females in population A . (Solid numbers refer to mice surviving 21 days to weaning and light numbers refer to mice dying before weaning.)

cage and had no nesting materials or food in it . Subsequently the original male was found dead in the nest. Following the death of the original male, male 14 moved into the lower level . Males 14 and 16, 18 and 22, and 25 and 27 were litter mates . In phase II, which began about day 300, the territorial system changed with male 22 extending its territory at the expense of the other territorial males (Fig . 2) . During phase III male 22 took over the entire upper level, pushing males 18, 25 and 27 into the lower right-hand corner (Fig . 2) . Phase III began at day 350 and persisted until the population was terminated at day 400 . Population A attained a size of thirty-eight mice during phase I, then declined and remained at thirty-two mice. Seven females bore all the mice with five of them producing all but two of the litters (Fig . 3). Female 2 was one of the original females ; the other original female died early. This frequently happens in populations and may account for the long latency before reproduction began in this population . Females 6 and 7 were litter mates in the first litter borne

by female 2 . Female 7 gave birth to females 31, 34 and 35, and female 31 was the dam of 47 . Due to communal nesting, it was not always possible to identify the exact female that had borne a litter, but the same group of females always was seen on the nests so it is assumed that these females produced the litters whose dams were unidentified . The birth rate fell during the latter part of phase I and remained depressed through the next two phases . The infant survival rate stabilized about mid-way through the population's growth (Fig . 4) . During the first phase of territorial stability, 66 . 6 per cent of all the litters were born and 45 per cent of the neonates survived to weaning . Only six litters were born in each of the other two phases with none of the neonates surviving in phase II and 54 per cent surviving in phase III . Territorial r--ales moved about more than nonterritorial males (Fig. 5) . However, as the territorial system changed male 22 increased his movements, and by phase III most of the previously territorial males had decreased their movements. The changes in movement were

LLOYD : SOCIAL STRUCTURE AND POPULATION GROWTH IN MICE

0

• E .~ • Z

20I

I

100

160

I

I

220 280 Days '

I

I

340 400

1 Phase I

Phase Phase II III

Fig. 4. Population density, infant survival rates and birth rates for population A . Smooth curves were derived from fitted lines calculated by using the slopes of logarithmic plots of the cumulative number born and of the cumulative number of infant deaths against the logarithms of time . The birth rates were based on the total number of animals in the population over 21 days . The infant survival rates were back-calculated to the day of birth .

found to be significant at the 1 per cent level . The analysis compares the profiles of group means from one phase to the other, and significance lies in the changes occurring from phase I to phase III in the movements of all the animals as a group . Although breeding females moved about more than the males they were not distributed randomly but associated with territorial males (Fig . 2) . As the territorial system changed, breeding females associated mostly with the dominant male 22. There was no evidence in this study of herding behaviour on the part of males toward females, nor of a given male monopolizing copulation with one or several females . Communal nesting was frequent as described previously (Saylor & Salmon 1971) . Not all nests were utilized irrespective of the size of the population. Definitive data on copulation frequency are not forthcoming from

41 7

this study . Fighting frequently occurred inside nests and often was followed by the flight of mice . It was not always possible to determine the exact sequence of events in such conflicts nor to discern if the attacks were initiated by parturient females . Breeding females moved about more than non-breeding females and, as a group, increased their movements by phase III (Fig. 5) . Non-breeding females were relatively inactive, joining in large piles of mice along with non-territorial males huddled in the corners of the cage or on the tops of nests . These animals were active very infrequently . In phase I most fighting occurred between territorial males with males 16 and 22 being the most aggressive (Fig . 6) . During phase II male 22 was most aggressive and maintained this position throughout phase III . Territorial mice became less aggressive as they became less active . Overall aggression fell in the population from phase I to phase III (Fig . 6) . Juveniles and non-territorial males received a high proportion of attacks in all three phases, and increased fighting occurred between nonterritorial males and juveniles in the last two phases (Fig. 6) . In summary, population A progressed through three phases . A stable territorial system persisted throughout phase I, became unstable during phase II and was reorganized in the third phase . Breeding females associated with territorial males. As territoriality diminished, the birth rate fell. Territorial males were more active than non-territorial males, and breeding females were more active than non-breeding females . As the territorial system changed, males became less active and aggression decreased in the population. Social Structure and Reproduction in Population B In population B two phases were evident in the social structure . In both phases territoriality was maintained with changes in distribution of territories occurring between phase I and phase II (Fig. 7) . The distinctive qualities of the two phases lay in the different levels of aggression and in the distribution of females . In phase II aggression increased almost two and a half times in population B (Fig . 8) with a sevenfold increase on the lower level . During phase II juveniles and non-territorial males bore the brunt of constant aggression . Juveniles were pursued and attacked in `gauntlet like' fashion as they fled from one territory to another . Although females associated with territorial



ANIMAL BEHAVIOUR, 23,

418

POPULATION

2

A KEY TO INDIVIDUAL MOUSE NUMBERS FEMALES MALES 2 ® 14 L .16 i 7 A 31 0 18 1 35 m 22 ∎ 47 0 25 m NON-BREEDING 0 27 FEMALES' 0 NONTERRITORIAL MALES'

•

MALES

p < 0.01'

IH L ED ot30000

8

12 10 8

w

6

F 4 2

0

PHASE I

PHASE II

PHASE III

Fig. 5 . Mean distances moved in metres per observation hour for males and females in population A . ( 1) Levels of significance using multivariate statistical analysis. (*) All non-territorial males and all non-breeding females were grouped and these represent mean values .

males during phase II, the breeding females abandoned the lower level of the cage as aggression rose (Fig . 7) . Although aggression was high on the upper level, it was not as high as on the lower level (Fig . 8) . Phase I lasted for 130 days, and phase II lasted until day 250 when the population was terminated . Population B attained a size of sixty mice during phase I and fell to forty-six mice during phase II remaining at that number until termination . Eight females produced all the litters (Fig . 9). Females 2 and 3 were the original females. Females 6, 7, 8 and 9 were in the first litter delivered by female 3, and females 25 and 30 were in the first litter borne by female 6. At the end of phase I the birth rate was declining and early in phase II the infant survival rate fell (Fig. 10). During phase I, 48 per cent of all the litters were born with 91 per cent of the neonates surviving to weaning . In phase 11 52 per cent of the litter were born, but only 8 per cent survived to weaning . There was also mortality among young adults and maturing juveniles . Territorial males were more active than non-territorial males, and breeding females

were more active than non-breeding females (Fig . 11) . Movement increased among both males and females from phase I to phase II . The changes in movement among males were significant at the 0 . 1 per cent level and among the females at the 5 per cent level . In summary, population B's social structure was characterized by a stable territorial system early in the population's history . Subsequently, aggression increased greatly, introducing instability to the social structure . The increased aggression in population B during the second phase was associated with a redistribution of breeding females and with precipitous mortality of infants, juveniles and young adults . Discussion It is hypothesized that in the populations examined in this study reproduction was related directly to changing social structures. Populations may be regulated by a decline in births, an increase in mortality, or by a combination of both (Christian et al . 1965 ; Christian 1971a) . In the two populations discussed here one was regulated more by an increase in mortality than

LLOYD : SOCIAL STRUCTURE AND POPULATION GROWTH IN MICE 0

8 Q r-

N

a

O w

® O

0

O O 0 e -- O Q

oC

0

O e Q 4 ®

G O

m

Q

N 0---o

U U

N _.- 0 --- 0 0 O - 0 0 --- o - O - ° - N 0 - 0 -0 -0

0 N

O

N

w

-0- -0

- N

-0

0 w

- O

-0

q

O - e -0-0-0 Q -o -o -

Q

N

-- O

0

O

va

e a

0

Q J a R

od N0 ^ - ~ b 0 .--0O O - cf Q w-F->y -N 40-~"ro aC -N-m ® -~+ - p -0 0 -- -'-< O e - o 0 0 -N G ®

I

-86

v0 4O w

~ ®I 4'

I I ' ~ 'a e oII o®I ' a E E >

O l

O --o ---- 0

ed - p --0 -O ® 0 - N m - a - 0 O e-fR-O Q 0 N ® - O

a~ a s u>

'4 Cp - O ® 0 O 0 - 0 0% O 0 9 0 -0 Q-0-0

m ,0 Q C O ,~ is

4

- 0

t9 0

toe

41 9

420

ANIMAL BEHAVIOUR, 23, 2

POPULATION

B

;• o

o

00 o .,\

o • ;i /`° 0 0

∎ 0~

•

o

®

~

0

a

0 , 0

i i PHASE I

PHASE II KEY TO INDIVIDUAL MOUSE NUMBER

UPPER LEVEL

/

\

LOWER LEVEL

MALE

FEMALE

0 1

• 2 • 6

® 4

O 5 14

∎ 7 ' 8

O

16

• 9

0

20

O

25

0 30 d 3

Fig. 7. Location of territories and distribution of breeding females in population B. Dotted lines indicate the boundaries of territories .

by a decline in births . Similar populations have been described previously (Southwick 1955a, b ; Lloyd & Christian 1967) . In both populations A and B territoriality sustained breeding. The relationship of territory to breeding in house mice has been well established by Reimer & Petras (1967), and Rowe & Redfern (1969) have shown the importance of family groups in populations of mice . This study provides further evidence of the importance of the relation between territoriality and breeding, in that the trend of the territorial systems in both populations toward instability was associated with decreased reproduction . It is possible that both populations were moving toward hierarchical systems as has been previously observed in house mouse populations (Young, Strecker & Emlen 1950 ; Calhoun 1956a, b ; Davis 1958 ; Anderson 1961 ; Crowcroft & Rowe 1963 ; Anderson & Hill 1965 ; Vessey 1967) . It is possible that if population B had been maintained for a longer time, one individual may have assumed more control as in population A . It is postulated that changes in

territorial systems may possibly reflect endocrine response to increased social pressure . The inhibition of reproduction in assymptotic populations has been amply documented (Christian et al . 1965 ; Lloyd & Christian 1969 ; Christian 1971a) . Recent studies further indicate that higher ranking mice and more active mice in populations may have higher levels of androgen activity than lower ranking mice and less active males (Lloyd 1971, 1973). Others have shown that aggression may suppress gonadotropins and that lower ranking animals in groups show depressed levels of FSH (Bronson 1973 ; Bronson, Stetson & Stiff 1973). In this study lower levels of activity among non-territorial males suggest lower androgen levels-in these mice . Such lower androgen levels may be due to increased social pressure as population size increased . This is an area needing further investigation, and measurements of testicular and plasma levels of testosterone in territorial and non-territorial males are currently underway in our laboratory . Aggression also may be considered indicative of the social organization of a given population .

LLOYD : SOCIAL STRUCTURE AND POPULATION GROWTH IN MICE

N

a-%-t3 ®-»z-2

0-0-0 O-o-m ®- 4-0-0 O - 0 - o ®-0-0 9-0-0

£~

0- O 4 .- - o ®-0-o C, 8- O- O

O

o

H

O O w

_O O cd O'E N>

co O - e9 4-0- .0 0-0-0 ®

q.

9

- o - o

44

m

® - r4 - A

4-

~(

0-0-0 - m -0-0--a

CTj d >

G- 0

-

- O

O v ti y i

g-co - o pq oG-~-~

;j o

0

49

p~cd

®a®oaoo®-

°a.2

r- - N ® - a-a

O y N

O - N-- -N O - o - 0

4- •- - ,0-0-0 e-¢-0 O-~-12 9 - O - 0 - w

4--0--a <>- 0 - 0 0-0-0

I HE

Q C ; 06 OIl„0,,'C7 O

4 21 .



422

ANIMAL BEHAVIOUR, 23,

2

POPULATION B

7

2S

30

W1Ya

nPHASE

I

100

200

KIM

KIM

©-

KIM

win

-n

0

©w

-LI

LO

w

©I1I

S I

C ti

61

-

-I I I

L

.BOG

Fig . 9. Litters born to breeding females in population B . Solid numbers refer to mice surviving 21 days to weaning and light numbers refer to mice dying before weaning .

Brown (1953), Crowcroft & Rowe (1963) and Southwick (1955b) concluded that in populations with stabilized hierarchies fighting may be minimal, while Crowcroft (1963) observed less aggression at high densities . The entrance of young maturing males into the population may create instability and may increase aggression. Increased aggression has been observed among maturing Microtus (Christian 1971b) while young male house mice were attacked more quickly by adult males as the young approached sexual maturity (Lloyd 1974 ; Mugford 1974) . The relationship of age composition, the proporuon of young maturing males and aggression to social structure also merit further study . Varying age composition may be a factor producing divergent levels of aggression in populations of similar size. This was a tentative conclusion reached in an earlier study by Lloyd & Christian (1967) . In population B, increased aggression associated with maintenance of territories appeared to have adversely affected reproduction by contributing to the high mortality of neonates during phase II . The increased fighting, particularly among the males on the Fig. 10. Population density, infant survival rates and birth rates for population B.

• 1 .6

cc• 1 .2s t. 0.8m 0 .40-

; •

0 .8-

_

0.40-

M %1.-

- r-

40

60y

40-

• `° -Q

= E 20• c 0' . z Q 40

I

100

I

160

,

I

220 280

Days t t i Phase I Phase II



LLOYD : SOCIAL STRUCTURE AND POPULATION GROWTH IN MICE

423

POPULATION B MALES P < 0.001' KEY TO INDIVIDUAL MOUSE NUMBERS FEMALES MALES 01 02 A6 °4 07 05 1114 + 8 016 ∎9 o 2O 025 0 NOW 000 TERRITORIAL A NON-BREEDING MALES * FEMALES'

m°

A ®# ∎O GA 'PHASE I

PHASE II

Fig . 11 . Distances moved per observation hour for males and females in population B. (') Levels of significance using multivariate profile analysis . (*) All non-territorial males and all non-breeding females were grouped and these represent mean values for the groups . lower level, apparently disrupted the social order sufficiently to result in the movement of breeding females from the lower level of the cage, further disrupting the entire population . Bailey (1969) has demonstrated that disruption due to movement within a population may adversely affect population growth . Aggression has been implicated by other investigators as a factor affecting infant survival in populations of house mice (Brown 1953 ; Southwick 1955a, b) . The indications here are that the added disturbances attendant on the movement and redistribution of females in population B possibly contributed

to increased mortality in this population during phase II. In summary, it is concluded that in the two populations studied reproduction was adversely affected as the territorial system became unstable, with one population being regulated by a decline in births associated with reorganization of territories and the other being regulated by increases in mortality associated with increased aggression. Acknowledgments This work was supported by P.H .S . Service Grant HD 00096 .



424

ANIMAL BEHAVIOUR, 23,

REFERENCES Anderson, P . K . (1961) . Density, social structure, and nonsocial environment in house mouse populations and the implications for regulation of numbers . Trans. N.Y. Acad. Sci., 23, 447-451 . Anderson, P . K . & Hill, J. L. (1965) . Mus musculus : experimental induction of territory formation . Science, N.Y, 148, 1753-1755. Bailey, E. D. (1969) . Immigration and emigration as contributory regulators of populations through social disruption. Can . J. Zool., 17, 1213-1215 . Bronson, F . H. (1973) . Establishment of social rank among grouped mice : relative effects on circulating FSH, LH, and corticosterone . Physiol. & Behav ., 10, 947-951 . Bronson, F . H ., Stetson, M . H . & Stiff, M . E . (1973) . Serum FSH and LH in male mice following aggressive and nonaggressive interaction . Physiol. & Behav., 10, 369-372. Brown, R. Z. (1953). Social behavior, reproduction and population changes in the house mouse (Mus musculus) . Ecol. Monogr ., 23, 217-240 . Calhoun, J. B. (1956a) . A comparative study of the social behavior of two inbred strains of house mice . Ecol. Monogr ., 26, 81-103 . Calhoun, J. B. (1956b) . Behavior of house mice with reference to fixed points of orientation . Ecology, 37, 287-301 . Christian, J. J. (1971a) . Population density and reproductive efficiency . Biol. of Reprod., 1, 248-294. Christian, J . J . (1971b). Fighting, maturity, and population density in Microlus pennsylvanicus. J. Mammal 52, 556-567. Christian, J. J ., Lloyd, J. A. & Davis, D . E. (1965) . The role of endocrines in the self regulation of mammalian population . Rec . Prog. Hor. Res ., 21, 501-578 . Christian, J . J ., Lloyd, J . A ., Goldman, D . E . & Davis, D . E . (1971) . An empirical formula for the growth of some vertebrate populations. Currents in Modern Biol., 4, 26-34 . Crowcroft, P. & Rowe, F . P . (1963) . Social organization and territorial behavior in the wild house mouse (Mus musculus L .) . Proc . zool. Soc. Lond., 140, 517-531 . Davis, D . E . (1958). The role of density in aggressive behavior of house mice. Anim. Behav., 6, 207-210. Lloyd, J . A. (1971). Weights of testes, thymi, and accessory reproductive glands in relation to rank in paired and grouped mice (Mus musculus). Poc. Soc. Exp . Biol. Med., 137, 19-21 .

2

Lloyd, J . A. (1973) . Frequency of activity and endocrine response among male house mice (Mus musculus) in freely growing populations . Proc . Soc. Exp . Biol. Med., 142, 784-786 . Lloyd, J . A . (1974) . Maturation of juvenile male house mice (Mus musculus) and latency to attack by adult males . Proc. Soc . Exp . Biol. Med., In press . Liovd, J . A. & Christian, J . J . (1967). Relationship of activity and aggression to density in two confined populations of house mice (Mus muscidus) . J. Mammal., 48,262-269 . Lloyd, J . A . & Christian, J . J . (1969) . Reproductive activity of individual females in three experimental freely growing populations of house mice (Mus musculus) . J. Mammal., 50, 49-59 . Morrison, D . F. (1967) . Multivariate Statistical Methods . New York : McGraw-Hill . Mugford, R. A . (1974). Androgenic stimulation of aggression eliciting cues in adult opponent mice castrated at birth, weaning, or maturing . Hor . & Behav., 5, 93-102. Reimer, J. D . & Petras, M . L . (1967). Breeding structure of the house mouse (Mus musculus) in a population cage . J. Mammal., 48, 88-89 . Rowe, F. P . & Redfern, R . (1969) . Aggressive behavior in related and unrelated wild house mice (Mus musculus L .) . Ann. Appl. Biol., 64, 425-431 . Saylor, A . & Salmon, M . (1971) . An ethological analysis of communal nursing by the house mouse (Mus musculus) . Behaviour, 40, 62-85 . Southwick, C . H. (1955a) . The population dynamics of confined house mice supplied with unlimited food . Ecology, 36, 212-225 . Southwick, C . H. (1955b) . Regulatory mechanisms of house mouse populations : Social behavior affecting litter survival . Ecology, 36, 627-634. Southwick, C . H. (1958) . Population characteristics of house mice living in English corn ricks : Density relationships . Proc . zool. Soc . Lond., 131, 163-175 . Uhrich, J. (1938) . The social hierarchy in albino mice . J. comp . Psychol., 25, 373-413 . Vessey, S . H. (1967) . Effects of chlorpromazine on aggression in laboratory populations of wild house mice . Ecology, 48, 367-376 . Young, H ., Strecker, R. L . & Emlen, J . T ., Jr (1950) . Localization of activity in two indoor populations of house mice . Mus musculus . J . Mammal ., 31, 403-410. (Received 20 March 1973 ; revised 15 April 1974 ; second revision 3 September 1974 ; MS. number : A1434)