Female long-tailed macaques with scrotum-like structure.

Area of Science:

• Primatology
• Reproductive Biology
• Genetics

Background:

• Female long-tailed macaques (Macaca fascicularis) typically display anogenital sexual swellings near ovulation.
• In some populations, these swellings are confined to the tail base.
• A novel inguinal sexual swelling, resembling a male scrotum, has been observed in pubertal females north of the Isthmus of Kra, Thailand.

Purpose of the Study:

• To investigate the sex and hormonal status of long-tailed macaques exhibiting scrotum-like inguinal swellings.
• To determine if these unique swellings are geographically restricted.

Main Methods:

• Chromosomal sex determination using polymerase chain reaction (PCR) for SRY and AMEL genes.
• Measurement of plasma sex-steroid hormone levels (testosterone and estradiol).
• Histological examination of the swelling tissue.

Main Results:

• PCR assays indicated the absence of Y-linked SRY and AMEL loci, and presence of X-linked AMEL, consistent with an XX chromosomal makeup.
• Testosterone levels were comparable to ordinary females and lower than males; estradiol levels were higher than both ordinary males and females.
• The scrotum-like swellings were primarily composed of adipose tissue.

Conclusions:

• The observed inguinal swellings in these long-tailed macaques are characteristic of XX females.
• The phenomenon appears geographically restricted to the Indochinese region north of the Isthmus of Kra.
• Further research is needed to understand the evolutionary and ecological factors influencing this trait's geographical distribution.