Dominance and aggression in social groups of male and female rats

Behavioural Processes, 9 Elsevier

DOMINANCE

(1984)3 l-48

AND AGGRESSION

D. CAROLINE

BLANCHARD,

KEVIN J. FLANNELLY Department

IN SOCIAL GROUPS

CHANTIS

OF MALE AND FEMALE RATS

FUKUNAGA-STINSON,

LOREY K. TAKAHASHI,

and ROBERT J. BLANCHARD

of Psychology,

(Accepted

31

29 April

University

of Hawaii,

Honolulu,

Hawaii

96822

(U.S.A.)

1983)

ABSTRACT Blanchard, D. C., Fukunaga-Stinson, C., Takahashi, L, K., Flannelly, K. J. and Blanchard, R. J., 1983. Dominance and aggression in social groups of male and female rats. Behav. Proc. 9: 31-48. Two experiments were performed to examine aggression and dominance in domestic male and female Rattus norve icus living in small mixed-sex (3 males examined the development of aggression in and 3 females) groups. E-e+ A single female (alpha) within each of the six colonies tested showed females. the preponderance of attacks on male intruders placed into the home-cage when Over 12 weeks of intruder-aggression male colony residents were absent. training female alphas showed only a mild nonsignificant elevation of aggressive A comparison of aggression of male and female colony alphas tested behavior. with opponents of each sex revealed that aggression was mainly directed at like-sex opponents, and that female attack was more defensive in character than The highest intensity of aggression male attack regardless of opponent sex. Although intruders never occurred when male alphas confronted male intruders. showed offense toward male residents, 61% of intruding males showed offense in response to attack by females. Experiment 2 investigated the relationship between aggressive dominance and Alpha competitive measures of dominance within each of 10 mixed-sex colonies. stat s of male and female colony residents did not reliably predict priority of access to food or water in tests of direct resource competition with like-sex colony members. When colony males were simultaneously tested for copulation, the copulatory behavior of alpha males was signficantly greater than that of Results are discussed in relation to the role of aggression other colony males. in the reproductive strategy of male and female Rattus norvegicus.

INTRODUCTION Male rats living in mixed-sex on unfamiliar extensively domestic

adult males placed

examined

(Blanchard

with observations Bovet,

in laboratory

initiate

may approach

(Blanchard

0376-6357/84/$03.00

(Calhoun,

1962).

Science

Barnett

Publishers

are consistent

Although

typically

for the preponderance

et al., 1977b).

has been

1958) and

(Telle, 1966; Robitaille

and sniff the intruder,

0 1984 Elsevier

pattern of attack

This phenomenon

rats and these findings

under natural

conditions

attack and be responsible

interloper

show a consistent

studies of both wild (Barnett,

et al., 1975) Norway

of this species

1976) and seminatural

colony members

colonies

into their colonies.

a single male will

of attacks on the

(1958) designated

B.V.

and

several

such colony

32 protectors

as dominant

or alpha males

and distinguished

males on the basis of a number of characteristics After an often cursory piloerects

and attempts

examination

defensive escape

boxing.

(see Blanchard

is possible

wild and domestic Blanchard,

patterns

1958).

Patterns

et al., 1980).

invariant

on top of,

is clearly

of attack and defense

identical

(Takahashi

if in

and

several

females

readily fought male

Calhoun

(1962) reports

aggression

reports

indicate

if resident

instances

at least during

(1977) noted that colony

males were incapacitated.

of females

lactation

is unclear from their presentation when tests were conducted. (DeBold and Miczek,

defending

independently

against

(Erskine

observations

both juvenile

and

et al., 1978a, b). attack

if females were pregnant

Calhoun's

an area centered

Studies of maternal

housed with males

1981; Hood, 1981) indicate

(see

appears

that female rats may also

also show that the female rat will aggress

adult male intruders,

is

in larger colonies

against male and female conspecifics.

Brain et al. (1980) report that females

integrity

it seems that

in these circumstances

Lore and Flannelly

intruders

several

colonies,

share this responsibility although

For instance,

around their burrows

that the onus of insuring colony

in our small mixed-sex

Furthermore,

intruders.

intruders

but it

or postparturitional

and more recent evidence

that female

aggression

may occur

of maternity.

The present

studies therefore

attack on male and female colony of male rats on intruders generality

attack,

by contrast

1982).

to a single male

aggression

behavior

Norway rats are essentially

to be a male prerogative, attack

attack.

et al., 1977a) and it will readily flee once bitten,

several males may more equally Barnett,

intruder

To achieve this end,

of offense--lateral

The intruder's

(Takahashi

While it is almost delegated

including

of a strange male, the alpha soon

to bite the back of the stranger.

alpha uses the species-typical chase and sometimes

them from other colony

of both sexes.

of the dominance

of intruder tests.

undertook

intruders,

Several

relationship studies

to describe in comparison

and analyze female to the attack reactions

An attempt was also made to assess the observed

within

(e.g. Baenninger,

colonies

on the basis

1970; Drews and Dickey,

1977; Gage, 1978) which have previously

tried to compare different

"dominance"

a social group have yielded

with aggressive

rank within

measures

of

conflicting

results.

EXPERIMENT

1

Experiment

1 was a comparison

the same colonies

on intruders

males was first provided

of female

to colony members

only male or female members

attack with attack by the males of

of both sexes.

were present

Extensive

of each sex.

during

intruder

experience

with intruder

During

such training

tests.

A comparison

of

33 aggressive

response

aggression

reached

toward male and female an asymptotic

intruders

level of training

was made only after

with male

intruders.

Method Subjects. female

Six rat colonies

were established

by placing

three male and three

rats, 101 to 119 days of age in each of six colony boxes.

on these rats 17 weeks intruders

later, when they were 210 to 238 days of age.

were 228 male and 24 female

colony rats, which had been housed to the study.

rats of equivalent

in individual

weight

of Psychology

The

and age to the

cages for several weeks prior

All rats were bred in the colony maintained

Hawaii Department

Tests were run

by the University

from Wistar stock originally

supplied

of

by Simonsen

Laboratories. Apparatus. outer covering the plywood.

Each colony enclosure

was a 120 x 60 x 90 cm plywood

of 2 x 2 cm wire mesh to prevent

escape through

box with an

holes gnawed

The tops were also 2 x 2 cm wire mesh. The tile flooring

colony was covered

with wood shavings

and food and water were freely Procedure.

The training

after colony formation.

which were changed

available

phase

at regular

in

of each

intervals,

at all times.

(Phase 1) was begun during

the eighteenth

week

For the next 12 weeks male and female colony members

were tested twice each week in a 8-min session

with an adult male

intruder.

On

Days 1 and 3 of each week only the colony males were present during the intruder session; members

on Days 2 and 4, only the colony females removed from their colonies

cages and returned

immediately

During testing animals.

during

behaviors

monitored

were measured:

of lateral attack, on top of, and boxing. given

in Blanchard

frequency

et al. (1977a).

of jump-attack,

other from a distance This type of attack at or just posterior Miczek

not by males.

Phase 2 of testing colony males

and females

after training

category

animal quickly

and Blanchard,

were

was recorded:

springs

involved toward

a direct intruders

at the

comparison

and

rats but

body.

of the attack behavior

of both sexes.

Beginning

colony members

Alpha males

DeBold

female

as were number of lesions

on the intruder's

male and female

intruders.

1981).

of attack by domestic

by location

was completed,

for these behaviors

behavior

of bites were also recorded,

tested with male and female

duration

all four of its legs leaving the floor.

this pattern

categorized

of the colony

in wild rats of both sexes, with attack directed

to the head (Blanchard

Frequency

on each intruder,

the behavior

The criteria

in which the observed

(1981) have observed

in holding

latency to piloerect;

One additional

of 6 cm or more,

is common

The colony

after the session ended.

two trained observers

The following

were present.

a session were placed

were separately

and alpha females

and female from each colony that showed the most aggression

of

the week

toward

(the male

intruders

34 during training

are designated

as "alpha") were tested on alternate

that the order of testing with male and female across colonies

and sex of subjects.

of 4 tests (with colony members with male and 2 with female same as those described

intruders.

Results

contact)

Measurements

including

a total

during this phase, 2

and procedures

for Phase 1 with one exception.

were the

One additional

made during Phase 1.

is the time in which resident

tail-tail

was counterbalanced

alphas received

of the same sex present)

was added based upon observations contact

intruders

Male and female

days such

This measure

and intruder were in body contact

all social

investigatory

and aggressive

measure

called (excluding

contact.

and Discussion

Phase 1 Testing.

A single female showed the majority

attacks on intruders

within each colony

the 12-week training

phase.

Chi square tests performed

the contention

that these attack behaviors

colony members

(pc.05 for each colony).

amount of lateral attack behavior this behavior was relatively typically

exhibited

This finding

male aggressive

dominance

tests during

on these measures

were not randomly

A single female

distributed

support among

also showed the greatest

in 69-70% of all tests but the occurrence

infrequent

by alpha males

of aggressive

of bites and jump-

in over 70% of aggression

among colony females

compared

(see Phase 2, "alpha" male-female

dominance

of

to that comparison).

of a single colony female does parallel

under similar circumstances,

however

(Blanchard

et al.,

1977b). A single alpha male was also readily male only tests with intruders. similar to that previously

identified

Development

few days after parturition,

that female

and gradually

postpartum.

Since most females

the training

phase we had available

relationship

between

aggression

which did not give birth--we

aggression

returns

in the present abundant

had 7-17 training

sessions

it of

females were equally

aggressive

females were comparably

integrity

regardless

low in aggression.

of nest sites by individual

lactating

is insured by a particular

and Flannelly,

1977).

for each female

plots nor group analyses

aggression

of

on each animal while

individually

between

the

of lateral attack and frequency

bites and jump-attacks

Neither the individual

a

For all but two females--one

Duration

any relationship

within

levels by day 21

study gave birth 2-3 times during

and litter-age.

were plotted

increases

to baseline

data upon which to examine

was nursing pups l-20 days of age.

of litter age.

was

described.

Erskine et al. (I978a) reported

revealed

in each colony during the

of alpha male aggression

as a function of these data

and days postpartum.

of maternal

These findings

indicate that defense

females may be unnecessary group member,

Indeed, alpha

state while other colony

as suggested

if colony previously

(Lore

3s Table I presents weeks of training. in jump-attacks

Phase 1: males.

12-week

On-top-of

Nevertheless,

of aggression

of alpha females

training

behavior

all showed piloerection

rare and is not presented

behaviors on any in the table.

intruders was consistent

and made repeated

of lateral attack behavior

increase

Other aggressive

period did not reach significance

was extremely

attack by alpha females on male

Duration

over the 12

but nonsignificant

over the first 6 weeks of training.

across the entire measure.

various measures

All alphas showed a consistent

jump-attacks

averaged

throughout

at intruding

only a few seconds on most

TABLE 1 Aggressiona

of Alpha Females

Weeks Latency to Piloerectionb Duration of Boxing Duration of Lateral Attack Frequency of Jump Attacks Frequency of Bites Number of Lesions

aMean per session bLatency and duration

Toward Male Intruders

3-4

5-6

7-8

9-10

11-12

414.0 4.8

307.4 6.5

423.0 8.8

347.4 7.0

356.3 8.4

303.2 12.2

1.5

1.7

0.2

0.4

1.5

2.3

::; 0

::: 0

::: 0

::: .2

1":; 1.0

;:: 0

are given in seconds

Unlike males,

prior to attack; sometimes made, and it was usually intruders

with

often failed to show piloerection

did not occur until after a bite was

for only a brief period of time.

Wounding

The finding

of

appeared

that a single female will show the majority

is in accord with the recent findings

of DeBold

and

(1981).

Surprisingly, offensive

intruding

behavior,

by alpha males.

while

males often responded

intruders

The probability

by intruding males

of female training,

to female attack by displaying

never showed offense

of retaliatory

attacked

occurring

to say, that 61% of male failure

in contrast

was very rare (median of 0 per test) even though alpha females

of attack on intruders

biting)

by this behavior,

females

piloerection

sustained

to bite their opponents.

Miczek

12 Weeks of Training

1-2

tests and most attacks were not preceded normal male attack.

Across

attack

(lateral

by females was .31-.75

on an average of 6I percent

intruders

of even the most aggressive

in response

attacked

by females

of colony females

to attack

attack, on-top-of

across the 12 weeks of the tests.

retaliated to develop

That is

in kind. intensive

The attack

36 against

intruders may be explained

intruders

against

attacking

Phase 2 Testing. alpha males

in part by this retaliatory

Table 2 presents

and alpha females

the attack and defensive

sex colony animals removed

were made, these data may be directly female attack behaviors. performed

Overall, females

prior exposure

compared

to assess differences

for analysis.

alpha males were significantly

more aggresive

Male aggression

sex.

was rare, however,

and usually

low in intensity,

and Thor,

1976; 1978).

and most aggression

toward female

males toward

or receptive

intruders

intruding males was prolonged

of lateral attack, boxing, The aggression

of alpha females,

toward like-sex opponents male counterparts,

and on-top-of

by duration

as evidenced

by duration

this difference

of lateral attack.

Like their

to show piloerection

to like-

was not nearly so great as that shown by was roughly comparable

However, wounds were consistently

for male

seen only when male alphas

intruding males.

oriented,

disparity

with alpha manoeuvering

on the back (see Blanchard spend significantly strategy

jump-attacks

in severity

of attack between male and

concordance

and Blanchard,

bite it and retreat

has also been reported

with their observations,

by both males

produce fear (Blanchard

again.

typically

but usually,

et al., 1981; Takahashi

of

As noted earlier, directed

at the

(1981).

In

were only rarely performed

In wild Rattus norvegicus and females,

The

(see frequency

by Oebold and Miczek

jump-attacks

to bite it

As seen in Table 2,

from a distance

attack with its jump-attacks

in the present study.

often exhibited

1977; 1981).

is contact

attempting

less time in contact with their opponents.

in Table 2), quickly

head of its adversary

Male attack

about the intruder while

is to jump at its opponent

this pattern of female

males

of alpha

like that of alpha males was mainly directed

alphas, their mode of attack was much different.

female's

showed

as evidenced

female alphas were also quicker

Aside from the overall

females

after repeated

(No females

and intense,

The number of bites on intruders

female

occurred

to

behavior.

alpha males.

attacked

previously

showed piloerection

the aggression

although

alphas.

females

By contrast,

sex intruders

and female

intruding

as has been reported

by the females.

behavior.)

than were the alpha

toward

In fact, only 2 males

mount attempts which were often rebuffed either proceptive

in male and

ANOVA was

with data from the two tests with each type of

of intruder

females

intruders.

in each case) when these tests

regardless

(Flannelly

of

to intruders

A 2 (sex of alpha) X 2 (sex of intruder)

upon each measure

intruder combined

behaviors

during tests with male and with female

Since male and female colony rats had identical (with the opposite

attack of male

females.

this type of attack is in contexts

and Blanchard,

which

1982).

by

Type of Resident Alpha Male Alpha

The Aggressive Behavior (Mean and Standard Error) of Alpha Males and Alpha Female Colony Residents to Intruders of Either Sex

Table 2

38 This female useful,since in response

attack strategy

of quick jump-attack

58% of intruding males to female

and withdrawal

showed piloerection

attack, just as observed

in Phase 1.

that male

intruders were at least as large and sometimes

females.

We should note that in hundreds

have never seen an intruder male, even when intruders never retaliated In contrast vast majority

Blanchard

were appreciably

in the present

It may be relevant

tests such as these, we

in response

to attack by an alpha Female

intruder

study. however,

alpha females

of bites to the snout, head and shoulder bit the back of intruders.

also presented

behavior

larger than the colony

larger than the alpha.

to their male counterparts,

male alphas mainly male bites,

of aggression

react offensively

is especially

and offensive

the while

The pattern of target sites for

in Table 3, is consistent

et al., 1975; Blanchard

directed

region of opponents

with previous

reports

et al., 1977a; where definitions

(see

of target

sites are provided).

TABLE 3 Target Sites of Bites by Alpha Males and Females of Either Sex Presented

as Percentage

Sex of Alpha

Male

Sex of Intruder

Male

Snout Head Shoulder Anterior Back Posterior Back Thorax Abdomen Limbs

There are several

et al., 1980).

1.51

0.00

14.01 9.03 49.26 16.45 7.18 0.00 1.04

11.11 16.67 61.11 0.00 11.11 0.00 0.00

7.51 38.42 25.60 19.99 2.98 2.57 0.76 2.17

9.15 26.37 19.68 25.45 4.55 I.77 0.76 1.27

aspects

of the interaction

lesions by female

This similarity

attack.

is very similar The Erskine

producing

any lesions,

intruders

of female bites on males

in the colony

males

and in

pattern exhibited

attack shown by wild members

et al. (1978b) findings

a similar

test (Elanchard

rats on adult males may not be a

As already noted, the jump-attack to defensive

it seems likely

We have observed

rats in a tail-shock

that attack by female

of biting adult than juvenile

of male

since females made almost as many bites on the

of these female bites is inhibited.

tests suggests

purely offensive

species.

Female

First,

pattern of bites without

females

Male

as did male alphas, but without

that the strength

tail-shock

Female

Female

other notable

and female colony rats. intruders

on Intruders

of Total Bites

that females

is consistent

by

of this

show a lower frequency

with this hypothesis.

39 It is also notable females

that the naive, male

showed piloerection

pattern,

and all the various

though these components

were

than those of colony males on male particularly consistent Blanchard asserted

experience dominant

elements

intermittant

intruders.

with intruders

Varied

are involved

Yet the present

1975; Eibl-Eibesfeldt,

finding

primarily

triggering

factors

1977), or the coordination of the behaviors

EXPERIMENT

of these behavioral

specifically

involved

1 demonstrated

a colony similar

suggests

of a

and Lore, 1975;

that, as others have elements

of male

Thus social experience

may

male attack (Lore and Flannelly, tactics

rather than the learning

in the attack itself.

the existence

of a hierarchy

to that of male colony members, Experiment

into the colony.

of dominance hierarchy

in the development

and

2

Experiment

intruders

dominant

lower intensity

social experience

19611, the individual

in rats of both sexes.

resident

of the male attack

and at a much

attack may be preprogrammed influence

which confronted

male attack on strange male rats (Luciano

et al., 1977b). (Barnett,

intruders

within

as indexed by attack on strange

2 was an attempt

a sex group to other situations

hierarchies

Experiment

2.

to extend the comparison

in which a dominance

In order to provide data on the generality

might function.

within-sex

of female rats within

across

strains,

a different

of these

strain of rats was used in

Method The ten colonies

Subjects. Long-Evans U-week

used consisted

rats placed as adults

colonies.

2, 4-member

colonies,

When data collection

age and 480 g in weight.

of 3 male and 3 female

in each of the colony enclosures. 4 males

period of colony stabilization,

died, leaving

originally

3, 5-member

and 3 females colonies,

began the surviving

The surviving

females

and 5, 6-member

males averaged

averaged

During the

of these colonies

265 days of

234 days of age and

329 g in weight. The intruders

were 60 Long-Evans

males with ages and weights

those of the colony males.

These

60 days of age, then housed

individually

females

similar

of the same strain,

in adulthood were

to be used as sexual

animals

benzoate

to

Six

were ovariectomized These females

in a peanut oil suspension,

and with a .5 mg dose of progestrone

48 given

use.

Apparatus. 1.

until used in the experiment.

in age to colony females,

and 24 h prior to testing with males,

equivalent

in group cages until

lures for the colony males.

injected with 2 pg of oestradiol

5 h before

had been housed

The colony enclosures

Food and water were continuously

were identical available

to those used in Experiment

except during scheduled

deprivation

40 Access to the water was restricted

periods.

long, 4.5 cm diameter

clear Plexiglas

to one animal at a time by a 5 cm

tube, fixed to the cage wall around the

water spout. Aggression

Training.

For the first 11 weeks after colony formation,

a

strange male rat was introdu,ced into each colony once a week for a 8-min Male and female colony

session.

in like-sex

groups.

That

female colony members

rats encountered

faced an intruder

their male cohabitants

were tested.

after the session was completed. members

on alternate

Aggression males

Sessions

female colony members

sessions.

Tasks.

Duration

were given a fourth

priority

Two measures

of attack

male and

of aggression

aggression

deprivation

test, with a female

were:

conditions.

During

aggression

Male

in oestrus.

attack on a strange male

food deprivation.

between

levels were

tests.

of access to water while under water deprivation;

and 48 h intervened

were

and, number of bites.

after baseline

additional

colony

(sunned times for lateral

behaviors),

and females

of access to food, following

counterbalanced

colony

for 8 min twice each week,

Again, during these sessions

Over a 4-week period

The three tests for both males

priority

intruder

each sex group of each colony was run in three dominance

colony members

while

immediately

were given to male and female

were tested separately.

attack, chase, on top of, and jump-attack

intruder;

were removed

was returned

During weeks 12 and 13 after colony formation

for each colony adult:

obtained,

separately

days.

Baseline.

for a total of 4 baseline

Dominance

intruders

from their cage while

and colony females

Each sex-group

and females were tested with a male

recorded

these

is, colony males were removed

and,

Order of the tests was

tests to ,allow full recovery tests the same measures

from

and procedures

were used as those during baseline.

In the food-access Colony members

piece of fresh coconut dangled

on a string

gathered measuring

around this bait. approximately

placed

The bait was removed

10 x 10 mn was dropped and duration

of coconut

(opposite

(a preferred

A

food) was then in the colony

and a fresh piece of coconut

into the middle possession

of the group.

for each adult were

A second piece of coconut was then

using the same technique

and identical measures

absent colony members

extract

of all food for 18 h.

before the test began.

until all adults remaining

until the coconut was consumed.

in the colonies

members,

were deprived

immediately

saturated 'in coconut

into the home-cage

The order, frequency, recorded

test, the colonies

of one sex were removed

for gathering

the colony

wer,e taken until this portion was consumed. sex group) were returned

to the cage and ad

libitum food was provided. The water access tests were held after 10 h of water deprivation. one sex group was removed

the water

bottle was tapped

against

After

its receptacle

The

41 until all adults gathered receptacle

around the spout area.

for 8 min, and the order,

each animal was recorded. available

frequency,

After each session

The bottle was placed and duration

Twelve days after the last of the three dominance

female was placed 30-min session.

in each colony Each oestrous

case receptivity

(after colony females

A sexually

and

and ejaculations

receptive

had been removed)

female was used in three different

of the female was verified

intromissions,

mounting,

oestrous

before testing.

were recorded

for a

colonies.

female.

Two

In each

The frequency

of

for each colony male.

and Discussion

Female Dominance. and percentages

Table, 4 presents

the percentages

of attacks on intruders,

of time spent with food and water for those females

intruder

in attacks results

for

tasks just described,

performance.

days later, each colony was tested with a different

highest

of drinking

the water bottle was filled

for at least 48 h.

male colony males were tested for copulatory

Results

in the

aggression

scores.

on intruders was found

of Experiment

over 90% of intruder

1.

Clear dominance

of a single colony female

in 8 of the 10 colonies,

In five of these colonies

with the

confirming

the dominant

the

female made

attacks.

TABLE 4 Measures

of Dominance

Aggression

of Colony'Females

on Intruder Tests

which Scored

Highest

in

(i.e. Alpha Females)

Alpha's behavior as a percentage of total behaviora of all females in its colony

Alpha's rank within colony on competitive tests Food

Colony

Attack

Drinking

Eating

Water

lb

78.0

65.7

98.8

:

zb 3b ; 6 ;

100.0 96.2 66.5 94.1 47.0 41.9 67.5

56.0 44.6 41.5 37.7 31.4 42.5 38.5

0.0 100.0 51.2 3.5 84.2 ::o"

: 1 2 1

: 1 2.5 2

93.4 98.1

63.3 14.9

1 :

:

1:

aMeasured in seconds bColony contained only 2 females

When priority alpha females'

K

1 :

at time of testing

of access to water was used as a measure

scores were not different

from expected

of dominance,

values.

Both

the

individual

42 proportions

and the mean proportions

for the Z-female

to cluster

around the value predicted

by a hypothesis

aggressive

dominance

and priority

Variance

by Ranks performed

revealed

no significant

less evidence

of enhanced

found between

aggressive

One puzzling for the dominant

priority

for these rats.

dominance

colonies,

females

and priority

females of different

the expected

that food possession which

is--given

present

even

Again, no relationship

was

of access to limited resources task were analyzed

colonies

for the

is that individual

proportions

tended to involve either a very

of time with the disputed

value.

food item:

3 of the while 6

Only one of the 10 animals had a score This may perhaps

indicate that the first

of a food item keeps that item, or it may suggest

in female rats is strongly

the stability

time as seen in Experiment

dependent

on maternal

status,

of attack by a single female on intruders

l--independent

any event, these data provide females

colonies

spent more than 80% of total time with the coconut,

spent less than 10% with the food. approximating

of

Friedman X, (6) = 0.49, p > .05.

aspect of these data, however,

female to obtain possession

between

Analysis

for access to food for the alpha females

high or a very low percentage dominant

two-way

Friedman X,! (6) = 1.99, p > -05.

when the ranks of alpha's on the food cypetition seven three-female

groups tended

of no relationship

A Friedman

of access.

upon the scores of the three-female

effect,

The mean proportions

and 3-female

of dominance

no suggestion

are able to monopolize

in intruder

that aggressively

food resources

dominant

under conditions

over

attack.

In

colony

of mild

deprivation. Male Dominance. other dominance dominance

Table 5 presents

measures

the proportions

of one male in attacks on intruders

colonies.

In these nine colonies

two-thirds

of all attacks on intruders,

single colony male is responsible 1958; Blanchard However, males

of attack behaviors

for male rats in the different

dominance

with an alpha male, the alpha made at least confirming

previous

findings

(Barnett,

et al., 1977b).

there was no pronounced

colonies,

difference

the more aggressive

colonies,

between

high and low aggressive

male spent slightly

deprivation. less time at and in the

the alpha male spent only slightly more than l/3 of the

time that might

be expected

was no reliable

difference

(Friedman Z-way analysis colonies.

that a

for the bulk of attacks on intruders

the water spout during the 8-min test session than the other male, three-male

and Clear

was seen in all but one of the ten

in their ability to gain access to the water spout following

In the two-male

tests.

by chance.

In terms of statistical

analysis,

among colony males on the access to water test

of variance,

x: (5) = 0.33, p > .05) in 3-male

there

43 TABLE 5 Measures

of Dominance

Aggression

of Colony Males which Scored Highest

on Intruder Tests

Alpha's behavior as a percentage of total behavior of all males in its colony Colony

Attack

lb $

lob 4 5 6 7 8 9

Alpha's rank within colony on competitive tests

Drinking

Eating

Copulationa

39.8 50.6 14.0 82.0 20.1 28.0 33.0 41.2 24.6 82.2

64.4 47.6 69.8 57.9 55.6 42.3 26.7 56.0 100.0 57.8

60.0

93.4 98.9 51.1 100.0 69.2 97.3 75.0 81.2 94.1 99.5

in

(i.e. Alpha Males)

Water

Food

Sexual

1

1

44.4 63.4 41.6

2 : 1

: :

: 2

70.0 51.0 62.8

: 2

1 2

: 1

42.1 61.4 100.0

1 3 1

1 1

: 1

aCopulatory behavior scored by frequency; all others based on duration measures. bColony contained only two males at time of testing.

The results

of the food access test were suggestive.

males did spend somewhat food deprivation, coconut)

this priority

food data approached priority males

more time in possession

but except for one colony

(Friedman

an acceptable

2-way analysis

When a sexually

receptive

the six 3-male colonies although

Statistical

level of statistical

of variance,

p < .OlI. provided

female was placed

also ranked first

Flannelly

of the

when

is consistent

limited or indirect evidence

in the P-male

three

of

males

in

in copulatory

colonies

for statistical

failed

to do SO.

analysis,

higher for alphas CxF (5) = 9.01,

with several

previous

that alpha males

their own colony

priority

The most aggressive

in their colonies

activity was found to be reliably

access to females within

significance

in the colonies

apparent.

two of the four males

This finding

analysis

X, = 6.33, p = .052).

Using only the data from the 3-male colonies copulatory

piece during

in the six co120nies which each contained

access of alpha males was more clearly

activity,

of the coconut

(in which the alpha male ate all the

of access was not marked.

of access was assessed

Eight of the ten alpha

(Barnett,

studies which have

rats may have priority

1958; Calhoun,

of

1962;

and Lore, 1977; Thor and Carr, 1979).

For male colony rats, then, dominance strange males tive females

(alpha status) and perhaps

appears

ranking

as measured

to be related

food, but are unrelated

in attacks on

to access to sexually

to access to water.

recep-

This

44 finding

agrees with DeFries

sire a disproportionate previous

and McClearn's

19811 indicate that differential observed

in the present

it is unclear just what factors

the last ejaculation1 different

males

that exclusive

may determine

copulating

agreement

with Baenninger's

dominance

and aggressive

for food, however,

Others

preferred

(e.g.

access' to‘ water foy,t,thealpha males

.

(1970) failure

dominance,

observed

and the separation

by'G&-tner

Lore and Flannelly,

may perhaps

receptive

as Table 5 suggests,

between

this relationship

attack-based

female with other colony monopolized

the relationship

either the food or between

and percentages

dominance

of dominance

water, though they do suggest that rats winning females

a

dominance.

Thus,

the actual

of utilization

even though there

and utilization

is far from overwhelming.

little support for the validity

food or receptive

lab

that the alpha male of almost every colony did in fact

of attack by one animal on intruders,

resources,

be the present

rather than standard

of either the food or the female, were not impressive.

provide

failed to

by a single male or female within

Only one alpha male of the ten completely

is some relationship

and

results

while Baenninger

to aggressive

share both food and access to a sexually

percentages

The present

1977) have found that food, especially

food items, may be monopolized

Moreover,

between water

of water hierarchies

et al. (1981).

of dominance,

colony but did not relate these results

the female.

that

is also in

to find a relationship

food item, the coconut morsel,

It is clear, however,

of young sired by

to assure the likelihood

One factor 'in,this difference

use of a much favoured

achieving

it is evident from these studies

is not necessary

do suggest, an effect

find such an effect.

of offspring

of ejaculations;

proportion

as

of a co.l,ony'syoung.

The liack of differential

hierarchies

the relative

et al.,

among colony members,

of the paternity

(e.g. number

with a single female,

access to females

1980; GErtner

performance

study, is highly predictive

alpha will sire the majority

copulatory

and Hartung,

copulatory

Although

males.

born in a colony and with

studies on rats noted above.

Two recent studies on rats (Dewsbury

chow.

(1970) report that dominant male mice

number of the offspring

of some

These figures

therefore

tests based on access to

disputes

are more likely to be dominant

over access to preferred in other situations

as

well.

In terms of interpretation E. norvegicus area (Barnett, reference

are colonial

1975).

of these data it should also be noted that wild

animals,

to access to resources

territories.

defending

a group area and foraging

It may well be that more extreme occur

In fact, the continued

same area would be almost impossible

in species which males maintain

presence

of numbers

if the dominant

to food and water to these animals during periods

in that

forms of dominance

of adult males

animal totally

of deprivation

with

individual in the

denied access

equivalent

to

45 those used here.

Access to receptive

effect of aggressive group-living

dominance

species.

of young may make

GENERAL

that other colony males

it worthwhile

on the availability

and a stronger

a disproportionately

are able to sire a percentage

for them to hold a low status

of other resources,

even for

in a given group,

rather than to venture from it.

DISCUSSION

The present attack,

is another matter,

activity might be adaptive

While the alpha male may father

larger number of offspring,

depending

females

on sexual

results enable

based on previous

a preliminary

classification

alpha and defensive

attack

differences

the adaptive

between

analysis

(Blanchard

in male rats.

These

functions

of the form of female

& Blanchard,

results

1977; 1981) of

also suggest

of male and female

some

attack

in rat

alpha attack and defensive

attack

colonies. For male rats the distinction patterns history

of the animal making

consequences 1977).

between

may be made on the basis of the situation

with reference

This distinction

et al., 1977a)

(Blanchard

especially

(Blanchard

and Blanchard,

defensive

rat (Blanchard

promote distance

somewhat

and especially

Female

patterns

head of the intruder a pattern

teeth

female

than domesticated

(Blanchard

attack appears

rats did approach,

back site

with considerable

These

as previously

of wild trapped

investigate,

of alpha males. as do attacking "jump-attacks"

described

behaviours

animal's

back

a mixture

of

and ultimately

they tended to show and back-biting

Alpha females

did not

males--initiating

each

were directed

by DeBold

males

R. norvegicus,

rats (Takahashf

to constitute

lateral attack,

of piloerection,

instead.

of defensive

et al., 1977a).

However,

(alpha) males.

is seldom seen in domesticated

is very typical

is aimed

may be interpreted

or serve to remove the defensive

close contact with opponents

bite from a distance,

alpha male,

attack

to this favored

et al., 19781, while the majority

than those which are so characteristic maintain

Alpha

animal.

as do attacking

different

and Blanchard,

by the resident

Bites on the back are vicious,

of the opponent's

the two male patterns.

(Blanchard

attack bites are made only to the snout of the

In terms of this analysis,

bite intruders

and its

1981; Takahashi

is attacked

the

and Blanchard,

in both domesticated

for the two animals.

from the opponent

from the vicinity

(Blanchard

back, and many attack components

1981).

to the bitten

In contrast,

animal

and Blanchard,

access to the opponent,

tissue damage

attacking

overlap

at the opponent's

as promoting

clear

in which an intruder

with almost no behavioural

the form of the attack,

to the attacked

is extremely

and feral

1982) rat colonies

the attack,

in which attack occurs,

and Miczek

toward the (1981).

Such

under similar circumstances,

but

which are much more defensive

and Blanchard,

1982).

The jump-attack

thus

46 appears

to be an attack manoeuver enabling

siveness, jump away.

the attacker

characterized

by high levels of fear or defen-

to come in quickly

from a distance,

bite, and

This type of attack is also seen in male and female wild rats

confronted

by a predator

or human

(Blanchard

et al., 1981a; Blanchard

et al.,

1981b). Characterization further

of female rats' attack as mixed offensive

strengthened

often inhibited,

resulting

any actual weakness wounding

by the finding

that bites by the females

in no wound.

of the smaller

This lack of wounding

similar

produce wounds on a male rat (Blanchard, female rats do show some tendency but this tendency 1980).

is more marked

This inhibition

aggression

to those

et al., 1980).

In fact, both male and

in the case of females

of defensive

(Blanchard

the general

sex, which clearly

of

in which their bites fail to

to inhibit the strength

may, in part, reflect

toward the opposite

is

were

does not reflect

since they are quite capable

females,

a cat under circumstances

and defensive on intruders

inhibition

operates

bites,

et al., against

in both sexes (Brain

et al., 1980). The present

results

also suggest that this mixture

attack may be functional of wild-living

-R. norvegicus.

and this animal serves conspecific

for female rats in the context

males.

and an opportunity

Wild rat groups normally

as principal

when a strange

feature

reference

appears.

This adaptive

females

male,

strange

with both a risk

The stranger,

male or

of a sexually

suggests

mature male

to intruders would

strategy

involve consistent

however,

counter-attacks, An additional

(but not female)

success of the female.

that the optimal

if it does

feature must be balanced,

if the stranger

are more likely to do than are strange females.

the future reproductive

of this situation

conspecific

to the female herself

is that the presence

to promote

have a dominant

Thus some form of attack will be adaptive

tend to drive off the intruder.

larger males

and defensive interactions

male, can pose a threat to young pups of the female

(Erskine et al., 1978al.

by the risk of danger

of social

of the home area against

The absence of such a male provides

female but especially

which

defender

of offensive

is likely

A cost-benefit

hostility

to strange

females,

attack toward strange males when the female has a litter or is pregnant, warm welcome

when she is not.

1) adult females

Even this last clause might be modified

are almost always either

and 2) it may be beneficial

to females

pregnant

offspring.

or with a litter, or both,

At any rate, the present

over time, of dominance

by one colony female with reference

strange

it clear that this attack propensity

intruders

dependent

makes

on the maternal

and a

because

to attempt to drive off strange males on

the basis that any strange male which could be driven off by a female choice to father potential

analysis

for female rats with

status of the female.

is a poor

consistency,

to attacks on is not totally

Thus the female pattern

appears

47 to be based on a mixture protection female

of an existing

herself

of offensive litter),

attack

(advantageous

and defensive

behavior

primarily

in

(needed to protect

the

from counterattack).

ACKNOWLEDGEMENTS This research IMental Health

was supported

awarded

by grant MH-29163

to D. C. Blanchard

from the National

Institute

of

and R. J. Blanchard.

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