Testosterone, Aggression and a Test of the Challenge Hypothesis in Wild Chimpanzees Authors: M. Muller and R. Wrangham 2004 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) Circulating Testosterone Levels (in blood)

Figure 2. Rates of Male Aggression

Figure 3. T and Mating

Figure 4. One Woman’s Story Mean Male Testosterone(pmol/mg Cr)

Figure 6a. Dominance Level

Figure 6b. Dominance Rank

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) oSocial 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 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 Sexual coercion in animal societies. Animal Behavior. 49;1345–1365. Gilbert S Developmental Biology, Seventh Edition, p Sunderland, MA: Sinauer Associates, Inc. Goodall J The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Harvard University Press. Goodall J (Revised edition). In the Shadow of Man, p. 79. Houghton Mifflin Company. Goymann W, East M, and Hofer H 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 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 The “Challenge Hypothesis:” Theoretical implications for patterns of testosterone secretion, mating systems, and breeding strategies. The American Naturalist. 136:6;829–846. Wrangham R 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