Testosterone, Aggression and a Test of the Challenge Hypothesis

in Wild Chimpanzees

Authors: M. Muller and R. Wrangham2004

Presented by Valerie Butler

The Problem

How do male testosterone levels in wild chimpanzees relate to aggression, dominance, and reproductive behavior?

The Challenge Hypothesis

• Testosterone should increase during a parous female’s periovulatory period (‘POP’)

• This increase is related to aggression and not to copulation

• Testosterone levels and dominance rank are related

Terms

• Parous: Having had offspring• Nulliparous: Has not had any offspring yet• Tumescence: Degree of swelling• Periovulatory Period (‘POP’): Latter part of

estrus when ovulation occurs

Chimpanzee Anatomy

• Males: Similar to humans

• Females: During estrus, inflation of genital swelling

Female

Hierarchy

Social structure defined by dominance:

•Males: Alpha, high-ranking to low-ranking

•Females: no alpha; high-ranking to low-ranking

Methods

• Observations• Establishment of dominance levels, ranks• Urine and testosterone assays• Statistical analysis

Figure 1. T Baselines

(Gonadal)

Cir

cula

tin

g T

est

ost

ero

ne

Levels

(in

blo

od

)

Figure 2. Rates of Male Aggression

Figure 3. T and Mating

Figure 4. One Woman’s Story

Mean M

ale

Test

ost

ero

ne(p

mol/m

g C

r)

Figure 6a. DominanceLevel

Figure 6b. DominanceRank

Results Summary

• T levels, aggression increase significantly in the presence of maximally tumescent, parous females (during POP)

• Neither aggression nor T levels increase significantly when in the presence of maximally tumescent nulliparous females

• Male copulation rates the same for nulliparous vs. parous couplings

• Higher-ranking males produced more urinary T and were more aggressive than lower-ranking males

Therefore, results support the challenge hypothesis

Discussion

Cycling, parous females — testosterone increased

•Sex alone? oNo increase w/nulliparous females

•Competition more likely reason

Discussion

Higher rank — more aggression• Afternoon testosterone levels correlated to rank

(morning no correlation)o Social interaction

• Consistently higher levels of testosterone for alpha male

Effects of Testosterone

Critique and Further Research

Critique• Only one female

studied to discern her periovulatory period

• Few nulliparous females used (n = 2)

Further Research• Follow more

females to study their periovulatory period

• Further studies on morning vs. afternoon T levels

• Can high T levels be a predictor of future alpha male status?

References

Focal Article• Muller M, & Wrangham R. 2004. Dominance, aggression and testosterone

in wild chimpanzees: a test of the ‘challenge hypothesis.’ Animal Behavior. 67:113–123.

Other references• Clutton-Brock T, and Parker G. 1995. Sexual coercion in animal societies. Animal Behavior. 49;1345–1365.• Gilbert S. 2003. Developmental Biology, Seventh Edition, p. 549. Sunderland, MA: Sinauer Associates, Inc.• Goodall J. 1986. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Harvard University Press.• Goodall J. 2000 (Revised edition). In the Shadow of Man, p. 79. Houghton Mifflin Company.• Goymann W, East M, and Hofer H. 2003. Defense of Females, but Not Social Status, Predicts Plasma

Androgen Levels in Male Spotted Hyenas. Physiological and Biochemical Zoology. 76(4):586–593.

References (continued)• Muller M. 2002. Agonistic relations among Kanyawara chimpanzees. In

Behavioral Diversity in Chimpanzees and Bonobos (ed. C. Boesch, G. Hohmann, and L. Marchant), pp. 112–124. Cambridge, UK: Cambridge University Press.

• Wingfield J, Hegner R, Dufty A, Ball G. 1990. The “Challenge Hypothesis:” Theoretical implications for patterns of testosterone secretion, mating systems, and breeding strategies. The American Naturalist. 136:6;829–846.

• Wrangham R. 2002. The Cost of sexual attraction: is there a trade-off in female Pan between sex appeal and received coercion? In Behavioral Diversity in Chimpanzees and Bonobos (ed. C. Boesch, G. Hohmann, and L. Marchant), pp. 204–215. Cambridge, UK: Cambridge University Press.

• play.psych.mun.ca/~cwalsh/3750/wk4hormbehav2.ppt