Measurement of scrotum and testis size of unrestrained captive cotton-top tamarins (Saguinus oedipus oedipus).

Reproduction is restricted to a small number of animals within both captive and wild groups of callitrichid monkeys (marmosets and tamarins). Sexual development of nonbreeding group members might differ from that of active breeders. Measurements of physical growth typically involve animal handling. However, capture and restraint of callitrichid monkeys can disrupt natural behaviors and physiology. This study introduces a method for estimating size of external genitalia by measuring frontal scrotum width of unrestrained cotton-top tamarin males. We examined whether scrotum width was a reliable indicator of testis size, and whether the method provided information useful for comparisons of growth and development among individuals. Animals were lured to a standardized position, and maximum width of the scrotum was systematically compared to a card with a series of rectangles graded in 1 mm increments. Card measures were validated with caliper measurements of scrotum width and testicle dimensions. Regressions of testes dimensions measured by caliper on scrotum width measured by card were between 82 and 93%, suggesting that measures of frontal scrotum width provide consistent estimates of relative testis sizes. The period of rapid growth ended by roughly two years of age, after which scrotum size did not differ from that of breeding males.