Zoo Biology 20:375–388 (2001) © 2001 Wiley-Liss, Inc. Aggression Control in a Bachelor Herd of Fringe-Eared Oryx ( Oryx gazella callotis ), with Melengestrol Acetate: Behavioral and Endocrine Observations M.L. Patton, 1 * A.M. White, 1 R.R. Swaisgood, 1 R.L. Sproul, 1 G.A. Fetter, 1 J. Kennedy, 2 M.S. Edwards, 1 R.G. Rieches, 2 and V.A. Lance 1 1 Center for Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, California 2 San Diego Wild Animal Park, Zoological Society of San Diego, Escondido, California Aggression control is becoming an important component in the management of animals in captivity, but rigorous quantification of aggressive behavior has here- tofore been lacking. This study was done to assess the ability of melengestrol acetate (MGA) given with feed (1.54 mg/kg) to control aggression in a bachelor group of fringe-eared oryx (Oryx gazella callotis). Systematic behavioral obser- vations were conducted and fecal androgen content was measured for 42 and 90 days, respectively, before treatment, and during the 42 days of treatment. There was a significant reduction in concentrations of fecal androgen from 153 ±6.0 to 95 ±4.5 ng/g (T 66 = 7, P < 0.0001). This reduction in androgen excretion was apparent after the first week of treatment. There was measurable MGA excreted in the feces during treatment. Although treatment did not arrest all aggressive behaviors among animals, the decline in androgens and increase in MGA was accompanied by a significant reduction in several measures of agonistic behav- ior. Posturing, aggressive contact, pursuit, and submission occurred significantly less frequently after treatment, and there was also a reduction in fighting-inten- tion movements. Thus, both ritualized and nonritualized aspects of aggression were affected. Reductions in hormones and aggressive behaviors coincided tem- porally, suggestive of a potential causal relationship. Consistent with this hy- pothesis is a strong positive correlation between fecal androgen and total aggressive acts. This effect was not the result of a single behavioral element but occurred across several categories of agonistic behavior. Zoo Biol 20:375–388, 2001. © 2001 Wiley-Liss, Inc. Key words: melengestrol acetate; aggression control; antelope *Correspondence to: M.L. Patton, Center for Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, California 92112. E-mail: mpatton@sandiegozoo.org Received for publication June 4, 2001; Accepted August 6, 2001.