effect of population size on the adrenal glands and ... · paired adrenal gland weights to...

Effect of Population Size on the Adrenal Glands and Reproductive Organs of Male Mice in Populations of Fixed Size

JOHN J. CHRISTIAN

From the Naval Medical Research Institute, Bethesda, and the Division of Vertebrate Ecology, Johns Hopkins School of Hygiene and Public Health, Baltimore, Marylund

T HIS

an REPORT presents the first results investigation into the effects

of of

1. population size on the adrenal glands, and on the reproductive function of mammals. The present study is restricted to mice in confined populations of fixed sizes.

This study was prompted by the suggestion that stress was produced in the individuals of a mammalian population in proportion to the population density (I). The hierarchical social organization present in most mammalian populations should contain the necessary elements to produce a density-dependent stress reaction (2, 3). Changes in population density theoretically should produce proportional changes in the size of the adrenal gland, primarily of the cortex. Also one would predict changes in the reproductive organs in op- position to the changes in the adrenal glands (4). Such a density-dependent physiological mechanism operative in the individuals of a population would govern population growth. This mechanism would also aid in explaining the marked susceptibility of high density populations to epidemic diseases, environ- mental hardships, or idiopathic shock disease (s), depending on dose-time relationships (6) I

METHODS AND PROCEDURE

The weight of the paired adrenal glands was used as an index of the degree of stress throughout these experiments, since it is generally accepted that one of the responses to stress is an increase in the weight of the adrenal gland,. primarily by enlargement of the cortex.

The same general procedure was used for two groups of experiments: one group with albino mice and the other with laboratory raised wild house mice. Male mice were isolated shortly after weaning in individual cages for a minimum of 3 weeks. Following isolation various numbers of mice were placed for I week in small metal cages measuring IO x 14$ x 6+ inches. Simul- taneously other mice remained isolated to serve as

Received for publication January 4, 1955.

controls. Rockland mouse pellets were present in excess at all times. Two water bottles were provided for each cage, and were never allowed to become empty. All animals were kept in the same room at 72-78’F with a fixed daylength of 15 hours provided by fluorescent lights.

At the end of the 1 week experimental period the mice were chloroformed, weighed to the nearest 0.5 gm, and preserved in 10% neutral formalin with the viscera well exposed. Organ weights and measurements were determined at a later date.

The weight of the paired adrenal glands after flxation was obtained for each mouse with a Roller- Smith torsion balance.

The weights of the thymus and paired preputial glands, testes, and paired seminal vesicles, and the lengths of the latter two, were obtained for many experiments (tables I, 2 and 3). Lengths were determined to the nearest 0.5 mm.

The adrencl glands of the wild mice were serially sectioned at 5-7 p and stained with haematoxylin and eosin. The widths of the total cortex, zona fasciculata, and X-zone were measured for each adrenal with an ocular micrometer in an area of cortex with straight fasicular cords (7) in a section with maximum medullary area. The micrometer measurements were not cali- brated, since interest centered on relative changes. All measurements were made at 125X magnification.

The cortical widths for IOO female and 184 male mature mice were plotted against their respective paired adrenal gland weights to determine the relationship between cortical width and glandular weight. A regression fitted to these data had a highly significant positive slope (F = 154.86), indicating a linear relationship between adrenal weight and. cortical width in these mice. These data suggest that changes in adrenal weight were reflections of cortical changes.

The primary data used throughout these experiments were the means of the individual paired adrenal gland weights for each cage. This mean will be called the ‘cage mean.’ The cage means were also the units of measurement for the other organs measured. The cage means were used because the organ weights of individual mice in a given cage could not be considered as independent variates. In some instances, because of the limitations of time and personnel, only part of the entire population was taken from a cage to establish the cage mean value, Occasionally all of the mice in a population were not weighed due to death of an animal late in the experiment or escape at the time of sacrifice. These irregularities are self-evident in the tables. The

292

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POPULATION SIZE AND ADRENAL GLANDS 293

TABLE I.MEAN WEIGHTS INMG FORTHE PAIRED ADRENAL GLANDS OF THE ISOLATED CONTROLMALE WHITE MICE

GROUPED FOR EACH EXPERIMENTAL SERIES

Series No. of

%Eol

- -

Mean & S.E.

IO

9 12

=4 10

IO

7

ii:

Mea&Fody Mcgnf;i-ed

. Gland, at,

gm mwzg

25.8 4.81 26.2 4.56 24.8 4.68 24.1 4.61

25-7 4.58 28.2 4*79 22.4 3*95 24.0 4*85 25.2 4.64

*0*3r *o.oa

Th6Y *

Paired

Pree:al .

I w

47*5 43*6

42~

58.0 48*4

b2.52

wz mm

90*4 18.7 88.3 23.0

76.5 81.1

75-o 81.6

*3-42

22.3

17.6 20.2

zto.58

Sem. Vesicle Testes

Length

1 I I

-

wt.

mg

:11.0 :os.o 107.2

*7*83

mm

7.00 7-50

7-w

7-67 7*43

+0.12

Paired wt.

w

178-9 180.3

v9*8

*7*30

Each mouse of these controls is also a cage mean value.

individual value is also the cage mean value for the isolated control mice.

In the first group of experiments albino mice of the NMRI strain were used, because of their availability of large numbers of the same sex, age and weight. These were isolated at 11-12 gm of body weight and used 3 weeks later in populations of I, 4, 8, 16 and 32 mice per cage. In the second group of experiments laboratory- raised descendants of wild house mice were used in populations of I, 3, 4, 6, 8, 9 and 17 mice per cage. Limited availability restricted the number of experiments with wild mice. The results of these two groups of experiments will be presented separately.

RESULTS

Albino Mice. Obs~rva~iouts. Severe fighting, persisting from 1-3 hours, followed introduction of the mice into the group cages. For the remainder of the week only sporadic fighting occurred. An occasional mouse died during the 1st hour in the cages of 16 or 32 mice, usually one to three per cage. These mice had a characteristic antemortem behavior pattern. A period of active running and fighting was followed suddenly by clonic convulsions lasting a minute or two. The convulsions were followed by prostration of a few minutes duration and in turn followed by another convulsive seizure. Alternating convulsions and prostration persisted for as long as an hour, terminating in a convulsive seizure. Rigor mortis followed closely upon death. A few mice became comatose without convulsing, followed by recovery within a few hours. Once prostrate, these mice were ignored by the others in the box. This syndrome is suggestive of that seen during population die-offs in the wild (5, 8, 9). No microscopic evidence of

infectious disease was found, nor can lympho- cytic choriomeningitis be implicated, since this disease was shown to be absent from this colony during the course of research on LCM in this laboratory.

It was further noted that many mice, after an hour or so in the group cage, huddled in a corner with ruffled fur, eyes half closed, and a noticeably increased respiratory effort. These mice apparently recovered fully within a few hours. This condition and the convulsive pattern appeared to.represent a response to the introduction into a cage of a large number of male mice which were previously isolated and unacquainted with each other.

A mouse which died within the first 3 hours of the experiment was immediately replaced, but the occasional mouse which died after 24 hours as a result of injuries from fighting was not replaced.

Measurements. The measurements obtained from the albino mice are summarized in tables I and 2. The results for the control mice are grouped according to experimental series for ease in presentation (table I). The results from the experimental populations are given in each case as the cage mean value (table 2).

ADRENAL GLAND WEIGHTS. Since eight series of experiments were run, it was necessary to determine the homogeneity of the various groups of control mice (table I), particularly with respect to the adrenal weights. An analysis of variance showed that the paired adrenal gland weights did not differ significantly between the several control series

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294 JOHN J. CHRISTIAN

TABLE 2. DATA COLLECTED FOR FIXED POPULATIONS OF WHITE MICE

Mice/Cage

I*

4” n = I$

Mean&S. E.

8* n = 16

Mean G.E.

16* n = 18

Mean &SE.

NO.

Mea&age

84

4 4. 4 4 3 4 4 4 3 4 4 4 4 4 4

8

7

8 8 8 8 8 4 4 4 4 4 3 4 4 8

=s

I5

16

13 4 4 4 4 4 4 4

16 16

4 16 16 16

I5

CyE;an

Adrenal Gland Wt.

gm mg

25.2 4.64 Ao.31 ho.08

25.0 4-49 24.8 49go 24.1 4.84 23.6 5*05 26.6 4-W 21.9 3.86 23*5 4-37 24*9 4.66 26.0 5-77 26.8 4.78 2543 S-40 23-3 $*I$ 22.3 4.84 20.4 4.85 22.1 $.I1

24.0 4.86 ho.48 &O.II

22.5 4.26 23*4 4.21 24.0 5*7= 27.6 4.65 25-9 5~6 26.0 S-07 24-7 5.95 25.I S-85 23*3 5.02

25.4 4*35 =9*3 40 77 25.6 5-09 24-4 4.52 19.6 5-37 2$*I 5-47 25*5 4.86 24.2 5.02

+0*55 =to, 14

27*3 4.89

30*3 4*92 20.6 5.22

29*9 5.69 24*g S-30 19.8 4.69 19.8 4.58 23*4 565 26.6 5.36 2$*0 5.06 24.3 4.16

25-7 5*3o 20.7 $937 25.6 5.60 24.6 4*55 22.8 5.00 22.8 4.85 22.7 4-57 24*3 $904

j=o* 73 zko.10

Cage Mean Thymus Wt.

mg mg

4894 81.6

h2.52 *3*42

20.2

ho.51

w 107.2

A7.8;

mm

7.43 Zt0.1:

34*2 27.0

3496 2494

70.6 68.3

101.8

$396 98.6 78-7 48-4 65.6

73.2 k6.80

16.8 17.0

18.0

17.0

7.88 7.67 7.00 7-13

29.6 $494 34*0

j=4*39

ES.0 16.3 16.7

Ao.35 r7*3 19.0 rg.0 19.0

[I$*5 [33-6 81.3

94=4 ro6.2

ZtII.$S

36=5 78.1

48.0 69.2

45-o 68.8 33.4 65*5

7975 7*25 7*4$

zto.15

5.88

7-43 7.25 7*38

~8.0 15.8

7*56 7m5o

40*7 *3*45

39*9 39*2

68.0

69*9 A2.14

$586 77-o

c8.0 *a.$3

[9*5 20.3 t7+8 [g*o

:01.7

:01.7 7*33 zto.og

7-33 7.20

5.63 7.62

49.5 $7.0 !0.7 1.32 28.5 46.0 :5*0 7.28

59.6 49-g 40.8

43*9 &3*78

73*6 61 .g 53*2 $4*1 59*8

h3.74

1594 :3.2 :4* I

17.2

*0*95

:02.4

73*6 64.1 67.2 76.8

*8* 75

7 .06 7.06 7 .40 7.21

zto.09

caj;ik$n Pre utial Gd. Wt.

Seminal Vesicles Testes

Cage mean paired wt.


w

179.8 =j=7*30

189.8 r77*2 176.2

165-7 177

*4*93

164.4 164-4

193.4 [59*3 [49-4 c6g.2 167.8

=t;9*45

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POPULATION SIZE AND ADRENAL GLANDS

TABLE 2. (Continued)

295

Mice/Cage

32* = n 9

Mean IJ=S.E.

No. Mea&age

32 31 29 32 32 16 16 16

3*

capp,eiry;an Adrenal

Gland Wt.

24-9 537 27.8 534 23*3 4*46 28.6 5.04 28.2 4*93 27-3 4.49 27.1 4-90 27-3 4.86 22.8 4.69 26.4 4-90

zto.72 zto.11

Cage Mean Thymus Wt,

mg 43*6 29*3

45-7

39*5 =tS*IS

Cage Mean Faire+

Pcfi”BLlt$l l .

Seminal Vesicles Testes

Cage mean C;f;gy;an paired wt.


mg

53*6 494

48-7 x8.7

lg.6 16.7 I4*4 19.2

s7*3 52.3 =7*7

hr.99 zto.9’;

-- mg mm

7*38 7*32 7.21

6.97

mg

Results are given as cage means TOT all experImenta populations and summarized as the means of the cage means with its standard error for each population size.

* Controls grouped for the purpose of tabulating, otherwise treated individually as cage means for statistical analyses.

(P > 0.20) ; consequently all were combined tions of 4 through 32 mice per cage approxi-

into a single group of experiments (table 2). mately parallels the adrenal weight curve. All experimental findings were thus compared Although these data are suggestive, the changes with a single group of 84 isolated control mice in thymus weight with population size were no

(table I). greater than could be explained by the There was an increase in adrenal weight variation between individual cages.

over the isolated control value for every BODY WEIGHTS. There was a decline in me.an population size (table 2, fig. I).

The adrenal weight increases from one through 16 mice per cage, then declined to a value at 32 mice per cage which wag between the adrenal values for cages of four and eight mice. A maximum increase of 8% in adrenal weight was found in populations of 16 mice per cage. The moderate decline in weight from 16 to 32 mice per cage was probably signifkant in view of the results of experiments with wild mice (below).

An analysis of variance showed that the differences in the weight of the adrenal glands between population sizes were significant (P < o.001). The changes in adrenal gland weight did not result from differences in the age or size of the mice, since the shape of the body weight curve is approximately the reverse of that for the adrenal glands. On the basis of body weight the changes in adrenal weight were still more pronounced (cf. fig. I).

TECYMUS GLAND WEIGHTS. Themeanthymus gland weight decreased for all population sizes when compared with the control values. The curve for the thymus weight for popula-

body weight as the population increased from one to four mice per cage and -femained at approximately this level through populations of 16 mice per cage. There was an increased body weight of 5 % greater than the control values in populations of 32 mice per cage. The variation in body weight with population size was not significant.

PREPUTIAL GLAND WEIGHTS. The paired preputial gland weight declined steadily as the population size increased. The smallest mean preputial size was at 32 mice per cage with a mean weight of 64 % of the control values. The variation in cage mean paired preputial gland weight with changes in population size was significant (P < 0.01). Histological examina- tion showed a decreased amount of secretion in the lumens of the preputial glands.

TESTIS LENGTHS AND WEIGHTS. The data for the lengths and weights of the testes suggested a slight decline with population size (fig. I)

but in neither case were the changes significant. SEMINAL VESICLE LENGTH AND WEIGHT. The

seminal vesicle weight declined from I through

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TABLE 3. DATA COLLECTED FOR FIXED POPULATIONS OF WILD MICE

NO.

Meas/Cagf

51” 3 3 3 3 3

Mean 4

4

4 Mean

6 8 8 8 8

Mean

9 9

Mean

=7 =7 17

Mean

m 23-35 26.5

20.8

28.0

29-3 23*7 257 24.8 21.6

26.4

24*3

24.7

3**5 21.4

21.2

21.6

23*9 25.4 19.0

22.2

22.5

23.6 22.1

22.7

capg;i?rn Adrenal

Gland Wt.

4*73 5.81

4*90 s-85 S-30 4.85 534 S-37 5.12

563 5-37 5-55 599 5935 5.49 s-70 5.63 6.27

s-*9 s-73 5.86 5.2I

5.61

5-56

- -

Cage Mean Thymus Wt.

mg

49-18 357 s-3 13*3 32*7

33*3 18.8

3997

29*3

20.6 32.6 26.6 28.0

28.0

ca$;Y$‘tn Preputial

Gland Wt.

c;yrn:y Vesicle Length

caFes;rn Lengih

Cage Mean Fasciculata

Width Micro meter Units

Casgezy;t

Width Units

mg

8530

mm

16.5

13.0

9*3 20.0

22.3

mm

7-90

46*3 $2.0

137-7

7*50 8.00

7-67

17.2

22.7 20.5

19*5 17.8

1.34 0

vi 0.8

0.3

78*7 16;2 7=72 20.1 0.4 37*0 IO*5 8.00 19.3 1.0

81.7 14.0 7-25 IS.3 0.8

59*4 16.8 18.8 o-9

53*1 13.6

43*2 14.0

4144 12.7

459 13.4 80.9 19.1

47*2 14.4 64.1 16.8

58-5 I9.1

7-63

7.00 7.80 7.00

7-27 7*83 7.00

7-42 7*74

7-25 7*so

20.2 0

=9*5 0

=7*5 0

19.1 0

21.8 3.1 20.0 2.1

20.9 2.6 20.6 o-9

39*0 48.8

12.4 IS.8

18.9 0

19.8 0.5

Results are given as cage means and summarized as the mean of the cage means for each population size. * Controls grouped for the purpose of tabulating, otherwise treated individually as cage means for statistical

analyses.

32 mice per cage, but the decline, although suggestive, was not significant.

The decrease in seminal vesicle length with changes in population size was greater than could be explained by variations between cages (P < 0.05). The seminal vesicles of the experimental groups averaged smaller than those from the isolated control animals.

Wild House Mice. Observation. The wild male mice fought violently immediately upon introduction into the boxes and continued fighting for about an hour. After this period the conflict decreased and, except for an occasional sporadic fight, ceased after 3 or 4 hours. Deaths with convulsions were noted more frequently than in the albino mice. One or more deaths occurred in most cages of eight or more mice. In one instance three deaths occurred in a single cage of eight mice. The dead mice were replaced with others if death occurred while vigorous fighting persisted, but they were not replaced after this time. Deaths occurred in several cages 3 or 4 days

after introduction, probably due to injuries from fighting. The cause was not investigated, however, since it would have interfered with the living populations. These cages were left in their original population classification. At autopsy these mice showed no gross or microscopic evidences of an infectious or neoplastic disease.

All of the mice other for several

were hyperreactive to each hou .rs after introduction.

Their fur became ruffled, respiration increased, and they appeared to be in a state of marked tension. Each mouse attempted to avoid the others except for one or two very aggressive individuals, but withdrawal was unsuccessful for more than a few minutes.

Measurements. The data from the experimental cages of wild mice are tabulated (table 3) along with the mean value for each population size. These values are plotted in figure I.

The data for 51 isolated control male mice are tabulated as their means, but are used as individual values for statistical treatment. The

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seminal vesicle lengths and preputial gland weights were obtained for 37 of the control animals. Other measurements were made on all of the controls. As with the albino mice, the cage mean value is the experimental unit of measurement.

ADRENAL GLAND WEIGHT. The weightofthe paired adrenal glands of the wild mice increased with increases in population size to reach a maximum at nine mice per cage. The mean adrenal weight in populations of 17 mice was below the value for nine mice per cage and approximated the mean weight for populations of six mice. The mean paired adrenal gland weights were greater in all experimental populations than the control value. The variation in cage mean adrenal weights between population sizes was significantly greater than can be explained by the variation between cages (P < 0.05).

The adrenal glands of the wild mice were initially heavier, and increased in weight at twice the rate of the albino mice with respect to population size. The slopes of the adrenal weight on the logarithms of the population sizes were 0.335 and 0.966 for the white and albino mice respectively. Both slopes were positive and significant (P < 0.01). The break in the adrenal weight curve for the wild mice came between 9 and 17 mice per cage rather than between 16 and 32 per cage as for the albino mice (fig. I).

ADRENAL ZONE WIDTHS. When the mean widths of the zona fasciculata were plotted against population size the resulting curve paralleled very closely the adrenal weight curve (fig. I). The width of the X-zone (IO) was negligible (table 3), because these were all mature male mice.

THYMUS GLAND WEIGHTS. The weight of the thymus gland declined from the control level to 9 mice per cage, followed by a slight increase to 17 mice per cage. The change in thymus weight with population size was significant (P < 0.05)*

BODY WEIGHTS. Although there was some variation in body weight (table 3, fig. I>; the differences in body weight were not significant. The differences in adrenal weight cannot be attributed to differences in the size of the mice, as the body weight curve is roughly the reverse of the adrenal weight curve.

PAIRED PREPUTIAL GLAND WEIGHTS. The weights of the preputial glands were con- sistently less than the control weights for all population sizes. There was an apparent progressive decrease in preputial weight with increasing population size, however, this variation in preputial gland weight with population size was not significant (P < 0.20). A regression fitted to the preputial weights on the logarithms of the population sizes had a negative slope significantly different than zero (P < 0.01). More importance may be attached to the decline in preputial weight in these experiments in view of the results with the albino mice.

SEMINAL VESICLES. No significant changes were observed in the lengths of the seminal vesicles of the wild mice.

TESTIS LENGTHS. The lengths of the testes, like the preputial glands, did not vary significantly with population size. A regression fitted to the cage mean testis lengths on the logarithms of the population sizes had a negative slope significantly different than zero (P < 0.01). Graphically there is a steady decline in mean testis size with increasing population size.

DISCUSSrON

The mean weight of the adrenal glands of highly inbred albino mice increased with population size over a limited range. Direct competition for food and water was eliminated by keeping both in excess. The mice were un- disturbed during the experimental period except for routine watering. Temperature and day length were kept constant for all experimental populations and the control mice. The adrenal glands probably increased in weight in response to increased population (or social) pressures. The exact nature of these pressures is obscure. It would be desirable to know the constituent physical, psychological and physiological parameters of these pressures.

The peak adrenal weight in the albino mice was followed by a decline as the population further increased. A possible explanation is a break-down of social organization due to a failure of individual recognition in very crowded situations such as has been observed previously in mice (I r). Our observations have in fact indicated that as the populations

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NUMBER OF MICE PER CAGE

‘MEAN PAIRED n ;

ADRENAL GLAND WEIGHT. I

Mg. f 3.8.

MEAN THYMUS

28-

MEAN BODY 26- WEIGHT, Gms.fs.e.

MEAN PAIRED PREPUTIAL GLAND

f RL MEAN TESTIS -‘-I _._-__. _ _ -- -- -C**nA

LENGTH, MM f s.e 76

74t L--L T

ftsffa

LENGTH

MEAN SEMINAL VESlCLE LENGTH

MM & s.e

MEAN PAIRED TESTIS WEIGHT, TESTIS

Mg. + s.e 16Oc - WEIGHT I

$I SEMINAL

LVEENSK ;

MEAN PAIRED SEMlNAL VESICLE WEIGHT, Mg.ks.e.

SE MlNAL VESlCLE WElGHT

A

FIG. I. Means of cage mean values for the organ measurements from albino and wild male mice plotted against the logarithm of the population size. The standard error of the mean is given for the values from the albino mice.

increased beyond a certain limit (16 mice per fixed populations above certain sizes; it does cage for albino mice; g per cage for wild mice) not appear to occur in freely growing labora- pursued mice escaped more readily from their tory populations (12). The populations of 32 aggressive cage mates, and chases were not albino mice had a mean adrenal weight above sustained. In smaller populations the dominant that for four mice per cage; so that in spite of a mice persistently pursued their subordinates. moderate decline in adrenal weight, the The bulk of the mice in these larger populations persistence of a relatively high level of stress got along amicably, once the initial fighting was indicated. subsided, without detectable signs of hier- Since the thyrnus gland is involuted by archical situations in most instances. This cortical hormone (13), cortisone (14), and break in the adrenal weight curve occurs in androgens (IS), a decrease in thymus weight

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would be expected to follow increased adrenocortical function. The mice in the present experiments were mature males presumably with a high level of circulating androgens and minimal thymus size. The decline in thymus weight of the albino mice, although suggestive, was not conclusive. A decrease in the level of circulating androgens would tend to obscure involution of the thymus by corticosteroids.

The variations in body weight were suggestive of an inverse relationship between body weight and adrenal gland weight, and consequently population size, but such a relationship was not established in these experiments.

The weights of the preputial glands, testes and seminal vesicles, and the lengths of the latter two were used to detect alterations in reproductive function. The preputial glands were selected as indicators of androgenic activity since androgens are primarily responsible for stimulating their growth and activity (IS, 16). ACTH also has a stimulating effect on these glands, producing a moderate weight increase in the absence of androgens (17-19). Therefore, a decrease in preputial gland weight in male mice with adrenal hypertrophy should reflect especially well a decrease in androgenic activity.

There was a significant continuous decrease in preputial gland weight of the albino mice with increasing population density. These results indicate a progressive decline in androgenic activity through the highest population size, although the adrenal glands of the largest populations had declined from their maximum weight .

The length of the seminal vesicles of the albino mice decreased significantly with increased population size. The decline in length was paralleled by a decline in weight. The reduction in vesicular size with increasing population parallels that of the preputial glands.

The decreases in the mean weights and lengths of the testes paralleled the changes in the secondary sexual organs. Until there are additional data, it is reasonable to believe that testicular size declines with increasing population size.

The results of the experiments with wild mice were in general similar to those from the albino mice. The adrenals of the more ag-

gressive wild mice exhibited more precipitous and greater changes than did those of the albino mice. For example, the maximum adrenal weight was seen in populations of smaller size (9 per cage), and the break in the adrenal weight curve occurred between populations of 9 and 17 mice per cage.

The width of the adrenal cortex in the wild house mice was shown to increase linearly with glandular weight. In the experiments with male wild house mice changes in the width of the zona fasciculata paralleled the glandular weights. Since the adrenal cortex of mature male mice consists almost entirely of the zona fasciculata, changes in the adrenal gland weight result primarily from changes in the amount of fascicular tissue. It is generally assumed without proof that cortical hypertrophy is indicative of an increased cortico- steroid production.

The significant decline in thymus weight in wild mice may be attributable to a more active adrenal cortex in the wild than in the albino mice. These results suggest an inverse relationship between thymus weight and population size which is probably mediated through the adrenal cortex.

The inconclusive changes in body weight of the wild mice with population size add little to the information already gathered from the albino mice.

The sizes of the preputial glands and testes of the wild mice decreased with increasing population size, parallelling similar decreases in the reproductive organs of the albino mice.

The declines in the preputial weights and seminal vesicle lengths of the albino mice, for which there were adequate data, were significant. Changes in the weights of the preputial glands and lengths of the testes of the wild mice with population density had regressions with significant slopes. The curves for the mean testis weights and lengths of the albino mice and mean testis lengths of the wild mice closely paralleled those curves for which significance was demonstrated. The linear measurements of the testes were probably not sufficiently precise. The limited weight data indicate that weight is a better measurement. The data for the reproductive organs indicate that androgen production was suppressed in proportion to population size. The degree of

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reproductive suppression was also related to the adrenal cortical size, and was believed to have resulted from a decreased gonadotrophic function with increased adrenocorticotrophic function of the anterior pituitary (4).

Finally, the increase in adrenal weight and decrease in reproductive function suggest that a degree of stress proportional to the population size was produced in both white and wild male house mice in confined populations of fixed size. Changes in the size of the adrenal glands, thymus gland, and organs of repro- duction reflected responses on the part of the mice to the stress of population density. The exceptions were in the largest populations used in which the adrenals declined from their peak weights, although they still remained well above the control weights.

SUMMARY AND CONCLUSIONS

Increases in adrenal weight were produced in albino and wild male house mice in confined populations of fixed size, and within a limited range adrenal weight was proportional to the population density. The size of the adrenal glands was used as one measure of the response of the animals to density-dependent stress. Adrenal weight changes were correlated with the amount of cortical tissue, primarily of the zona fasciculata. The nature of the population pressures producing stress are unknown. In the lower population sizes there was a direct relationship between the logarithms of the population sizes and the weights of the adrenal glands. Declines in adrenal weights from the preceding peak values were noted in cages of 32 white and 17 wild mice. These declines may have resulted from a degeneration of the social structure. The adrenal weights were still well above the control values in these populations. The rate and magnitude of adrenal weight change in response to population density was greater in the wild than in the albino mice. Adrenal glands of wild mice attained peak

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REFERENCES

CHRISTIAN, J. J. J. 2M‘am?nal. 31: 247, 1950. SCOTT, J. P. Ann. New York Acad. Sc, 51, art. 6: 1001, 1950.

BROWN, R. 2. EcoZ. Monogs. 23: 217, 1953. SELYE, H. Endocrinology 25: 615, 1939.

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ZWEMER, R. L. Am. J. Path. 12: 107, 1936. HAMILTON, W. J., JR. J. A@. Res. 54: 779, 1937. FRANK, F. Zoo,?. Jahrb. 82: 95, 1953.

HOWARD, E. Am. J. Anat. 49: 251, 1927. RETZLAFF, E. G. Biol. Generalis. 14: 238, 1948, CHRISTIAN, J. J. Unpublished observations. SELYE, H. Stress. Montreal: Acta, Inc. 1950. INGLE, D. J. J, Clin, Endocrinol. IO: 1312, 1950.

BURROWS, H. Biological Actions of the Sex Steroids. Cambridge: Cambridge Press. rg49. HESS, M., 0, HALL, C. E. HALL AND J. C. FINERTY.

Proc, Sot. Exper. Biol. & Med. 79: 290, 1952. JACOT, B. AND H. SELYE. Proc. Sot. Exper. Biol. & Med. 78: 46? 1951. ASLING, C. W., W. 0. REINHARDT AND C. H. Lr. Endocrinology 48: 534, 195 I.

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weights at lower population sizes than those of albino mice, and also increased in weight by twice the amount. Adrenal gland weight was related linearly to the width of the cortex, which consisted almost entirely of the zona fasciculata. The sizes of the thymus and preputial glands, seminal vesicles, and testes were inversely related to population size. The reproductive changes were attributed to suppression of gonadotrophic function with increased adrenocorticotrophic function of the anterior pituitary with a resultant decline in androgen output and testicular activity. The decline in thymus weight was attributed to adrenocortical activity.

The author wishes to express his indebtedness to H. G. Ward, D. L. Clark, and J. I. Thomas for tech- nical assistance; Drs. D. E. Davis and E. P. Vollmer for valuable advice and guidance; and Dr. P. Meier for advice in the statistical analyses.

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