Old age and sexual exhaustion in male rhesus macaques

The capacity for multiple ejaculations was measured in three groups of rhesus macaques whose ages ranged from 9 to 30 years. Each male was given a test of sexual exhaustion with each of five ovariectomized, estrogen treated females. Sexual exhaustion was defined as a 45-min period without a mount when the males were paired with a female. None of the very old males (25-30 years, N = 4) achieved more than two ejaculations before reaching sexual exhaustion. Old males (19-20 years, N = 5) displayed as many as six ejaculations and fully adult (9-15 years, N = 5) males achieved a maximum of five ejaculations before meeting the criterion of sexual exhaustion. Although very old males mounted as often as younger males they achieved fewer intromissions and the latency to intromission and ejaculation was longer. Mean mount and ejaculation latencies and the postejaculatory interval increased after ejaculation in all age groups. The females were not equally attractive sexually to the males and very old males were especially selective, copulating with only three of the females.