Characterization of Cycling in a Hoolock Gibbon (Hoolock leuconedys)

Here we examine the patterns of reproductive hormones (progesterone and estrone-3 glucuronide, or E1G) in one female hoolock gibbon (Hoolock leuconedys) housed at the Gibbon Conservation Center, throughout the maturation period. Three hundred forty-five fecal samples were collected from the individual over a 5-year period (2012-2017) beginning at the age of 6 years and ending at the age of 11. The average measured progesterone concentration increased from 19.572 ± 1.706 ng/g feces in 2012 to 107.922 ± 12.094 ng/g feces in 2016/17 (p < 0.00001). The average measured estrogen value increased from 1.234 ± 0.063 ng/g feces in 2012 to 2.783 ± 0.274 ng/g feces in 2016/17 (p < 0.00001). This was accompanied by the emergence of a clear hormonal cycling pattern in the 2016/17 samples that was absent in all earlier samples. These data are consistent with the known sexual maturation period for other gibbon species, which typically occurs between the ages of 6 and 8 but shows some variation. To our knowledge, this is the first hormonal study and first data on cycle length for a hoolock gibbon.