Multiple ejaculations during prolonged sexual tests and lack of resultant serum testosterone increases in Male Stumptail Macaques (M. arctoides)

Abstract

The sexual behavior of four male and four female adult stumptail macaques was observed in standardized 3-hr heterosexual pair tests. The males achieved from 11 to 19 ejaculations during a single test, thus apparently exceeding any other primate studied under laboratory conditions for the ability to display multiple ejaculatory patterns in relatively short periods of time. The stage of the menstrual cycle was not found to be related to the performance of the male or female, although this variable was not analyzed in depth. Progressive increases in the interejaculatory interval (IEI) occurred for all four males for the initial two to four ejaculations of the series, and then either a plateau or a transient decrease in this measure occurred. Since the data varied considerably for IEI as well as for other quantitative measures of sexual behavior for a given pair, individual data rather than group averages are presented. Blood samples drawn immediately before and after behavioral testing as well as at comparable intervals 1 week prior to behavioral testing were analyzed for testosterone concentration by radioimmunoassay for each male. Serum testosterone values were not found to be influenced by multiple ejaculations. Instead, decreased levels of serum testosterone were encountered on the second blood sample of the day for most males, regardless of whether sexual behavior occurred. The possibility that the decreases were related to stress effects of handling is discussed. Because of this complication, it could be only tentatively concluded that repeated copulation to ejaculation over a 3-hr period did not result in appreciable changes in testosterone levels.