The Sichuan snub-nosed monkey(Rhinopithecus roxellanae), a species on the First Category State Key Protection List, is an endangered species endemic to China. This paper examines seasonal changes in urinary testosterone (T) levels in this species. Urine samples were collected by injectors from the cement floor of the monkey’s enclosures in the early morning, then stored in a freezer at -24 ℃. Urinary testosterone (T), was detected with radioimmunoassay (RIA) after being extracted by dichloromethane.The results show that the levels of urinary T varied widely over time and displayed seasonal variation that corresponded exactly with the monkey’s sexual behavior. The mean value of T was 0.576-2.359 ng/mgCr in the nonreproductive season and 36.483-47.889 ng/mgCr in the reproductive season. The latter was 14.84-83.14 times as high as that in the nonreproductive season. Peak T values in the nonreproductive and in reproductive season were 0.82-10.60 ng/mgCr and 237.841-326.231 ng/mgCr respectively. Peak values in the reproductive season were 30.78-290.05 times that of the nonreproductive season. This seasonal change in T levels may be the main factor determining seasonal differences in sexual motivation, copulation and conception rate in these monkeys. In addition, co-habitation with females in the breeding season produces a sharp increase in T level in male monkeys; there is a causal relationship between sexual activity and T secretion .
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