Aggression
What is aggression? There are multiple ways that aggression can be defined
Biological - behaviors that lead to successful competition for limited resources
predatory fear induced irritable maternal
social aggression: intermale, territorial, sex-related
Psychological - behavioral sequences that could result in one animal harming another animal (+intent)
How is aggression measured?
testing environment
history of animals
sex
hormonal status (of both animals in a dydad)
-> All these factors must be considered
SEX DIFFERENCES IN AGGRESSION - varies with species and sexes in dyad
SPECIES MALE VS FEMALE SAME SEX (LEVEL OF AGGRESSION)
Humans males > females males vs male > female vs female
ex. violent crime
Apes males > females males vs male > female vs female
top-ranking always male
mouse/rat female > male male vs male > female vs female
hamster female > male female vs female> male vs male
hyena female > male female vs female> male vs male
Gerbil female > male no difference
WHY IS THERE AGGRESSION AMONG MALES OF A SPECIES??
ex. red deer
Most of the year males of this species live together in bachelor groups quite peacefully. Females are dispersed or in groups of females
Late in the summer 1) testosterone begins to rise as the gonads become active with the onset of the breeding season. 2) The antlers change from soft to hard horn. 3) Males begin to fight for control of rutting territory. The better territory that a male controls, the more females in his harem, and the more females he can mate with. The females graze on his territory during the breeding season (3-4 weeks) and mating occurs.
After the rutting season is over, the testes regress, antlers are cast and the bachelor groups reform.
Testosterone administered to deer will cause the deer to become agressive, castration prior to the rutting season prevents the harding of the horn and the onset of aggression. Thus the aggression can be shown to be dependent upon the presence of testosterone. The males that are more aggressive control larger territories and mate with more females.
HOW DOES ENVIRONMENT AFFECT TESTING FOR AGGRESSION?
Example - the dusky-footed woodrat
-solitary dwellers who live in "houses" where posession of a house is crucial for survival. It is where the food is stored, protected from predators and environment.
- dusky-footed woodrats know their neighbors, and don't fight with them. There is a clearly defined dominance relationship
- during breeding season (spring and summer) there is increased aggression between males in the wild
- in laboratory tests in which 2 males are place in a neutral arena, castrated males are as aggressive as intact males. However, if one constructs the test where males are defending houses, the intact male always emerges as the victor.
Effects of the hormonal state of the intruder and the resident can also be seen laboratory tests of rats, if the test are carried out with attention to details.
Barfield Experiments:
1) IN NEUTRAL ARENA IN TESTS WITH AN INTACT MALE INTRUDER:
TEST # RESIDENT HORMONE TREATMENT RESIDENT AGGRESSION
1) CASTRATED MALE 5
2) CAST + Testosterone for 2 weeks 20
3) CAST + hormone withdrawal 5
-> TESTOSTERONE INCREASED AGGRSSIVE BEHAVIOR IN CAST MALE RATS
2) IN RESIDENT'S HOME CAGE IN TESTS WITH AN INTACT MALE INTRUDER:
TEST # RESIDENT HORMONE TREATMENT RESIDENT AGGRESSION
1) INTACT MALE 30
2) CAST MALE 10
3) CAST + TESTOSTERONE 30
4) CAST + hormone withdrawal 10
-> CASTRATION DECREASES AGGRESSION, T RESTORES
3) BEHAVIOR OF BOTH THE RESIDENT AND INTRUDER
R=RESIDENT RAS=RESIDENT AGGRESSION SCORE
I= INTRUDER IAS=INTRUDER AGGRESSION SCORE
EAS= ENCOUNTER AGGRESSION SCORE
ENCOUNTER
ANIMALS EAS RAS IAS
INTACT R +
INTACT I 20+/- 5.4 16.6+/- 4.6 3.3 +/- 1.1
CAST R+
INTACT I 11.5 +/- 4.3 8.1 =/- 4.5 3.7 +/- 1.6
INTACT R +
CAST I 6.7 +/- 2.3 5.8+/- 1.9 0.7+/- 0.5
CAST R +
CAST I 7.3 +/- 4.5 3.2+/- 1.4 1.3 +/- 0.8
-> CAST males COULD fight but did not initiate aggressive encounters and did not evoke an aggressivce response from intruders
-> In encounters that establish dominance it is always the resident theat becomes dominant (even if CAST)
Neural basis of aggressive behavior
**Scent marking in the gerbil (Pauline Yahr)
Testosterone maintains the frequency of scent marking and the integrity of the scent gland. DHT or estradiol are as efective as T in this system (how is that possible?)
In CAST male gerbils estradiol, T or DHT implants into the anterior POA reinstate scent-marking. Testosterone in other brain regions does not reinstate scent marking.
Lesions of the mPOA disrupt sexual behavior (scent marking is intact)
Lesions of the anterior POA disrupt scent marking (sex behavior OK)
If output pathway from anterior POA to forebrain is lesioned then scent marking is disrupted.
The anterior POA is also implicated in hormonal modulation of aggression in other species (ex. whiptail lizard). It is thought that the mPOA interacts with the septum, amygdala, VMH, central gray and other brain regions to produce agggression.
AGGRESSION IN FEMALES
In some species it is the female that is the more aggressive.
Ex. spotted hyena
In the spoted hyena, the females have external genitalia that cannot be distinguished from males at a distance. The clitoris is enlarged and forms a pseudopenis with a central urogenital tract through which the female has sex, gives birth, and urinates. The vaginal labia are fused together to form a pseudoscrotum. Androstenedion is secreted by the maternal ovary, is converted to testosterone by the placenta and masculinizes the developing fetus. Androstendione is also secreted by the female throughout life.
Twins are very aggressive at birth and usually only one survives if same sex (females are less aggressive against males than against other males). Females stay in the natal group, and males disperse at puberty.
A dominance hierarchy has evolved among females to prevent fatal aggression. Females acquire the social position of their mom.
Maternal aggression
- occurs during lactational period
- 50% greater with pups present
- mediated by POA
- pituitary not necessary but prolactin facilitates maternal aggression in estrogen primed rats.
Non-maternal aggression in female rats - effects are determined by the sex and hormonal status of the intruder (more than the hormonal status of the resident).
-OVX does not affect response to intruding females but testosterone does increase a female's aggression towards males.
- I showed graphs that illustrated these points:
1) estrogen plus progesterone decrease aggression and increase sexual behavior in OVX female rats
2) Progesterone alone decreases aggression but does not increase sexual behavior
3) testosterone increases aggression in OVX females rats
4) estrogen alone does not decrease aggression in OVX females rats
-> Resident females (OVX or intact) will fight if an intact or estrogen-treated female is introduced into their cage.
PLACEMENT OF FETUSES IN THE UTERUS CAN AFFECT DEVELOPMENT OF AGGRESSION IN FEMALE MICE.
Females between 2 males (2M females) are more agressive than females between 2 females (0M females)
Active play behavi