Monitoring the ovarian cycles of Pan troglodytes and P. paniscus: A comparative approach

Abstract

Females of both species of chimpanzee, Pun troglodytes and P. paniscus, exhibit conspicuous anogenital sex swellings that are useful external markers for assessing ovarian cyclicity. A new, quantitative method for describing these swelling cycles was refined during laboratory studies of four P. paniscus and nine P. troglodytes and was used to analyze variability in swelling patterns. Clear-cut anatomical changes of the perineum were found to have both interindividual and interspecific reliability. The swollen phase of the cycle was defined as the period when the medial surfaces of the labia minora are pressed against each other so the aperture leading to the vagina appears as a slit (labial occlusion). When defined with this precision, comparisons within and between the species revealed two types of cycle for both species; swollen phases of P. puniscus were about 24 or 39 days in duration, on average, whereas those of P. troglodytes were about 14 or 18 days. Urine samples from the P. troglodytes were collected and analyzed for luteinizing hormone (LH) (34 cycles), estrone glucuronide (E,G), and pregnanediol-3-glucuronide (PdG) (18 cycles). When the two types of cycle were separated in analysis, it was found that the day when labial occlusion occurred served as a predictor of the preovulatory LH peak in the cycles with a long swollen phase. Levels of E,G correlated closely with swelling scores in multiparous females regardless of phase lengths, suggesting that the new swelling scores are a reliable indicator of E,G excretion in multipara. Cycles with long swollen phases had relatively high levels of PdG during the luteal phase, whereas cycles with short swollen phases had relatively low levels of PdG and a shorter luteal phase. The marker of labial occlusion and the duration of the preswollen phase defined by the marker appear to be potentially useful in studies of reproduction in Pan, for monitoring female fertility, and in artificial insemination procedures. If it is confirmed that swelling pattern predicts progesterone levels during the luteal phase, it would make interventions possible in the case of low levels so that a potential pregnancy could be maintained to term.

Menopause occurs late in life in the captive chimpanzee (Pan troglodytes)

“…The present results refute this claim by revealing that swelling cycles occurred in all eight of the old chimpanzees that ovulated but did not occur in the single old animal that did not ovulate. Indeed, anogenital swelling has an underlying physiology precisely regulated by fluctuations in ovarian steroids (Dahl et al 1991;Deschner et al 2004;Emery Thompson 2005;Graham et al 1972;McArthur et al 1981;Nadler et al 1985). This high degree of association between anogenital swelling and ovulation reinforces our previous conclusion that ovulation in chimpanzees continues into advanced old age.…”

Ovarian hormone concentrations and genital swelling patterns in female chimpanzees with Norplant implants

“…A second estrogen peak occurs during the luteal phase of the cycle concurrent with the peak in progesterone. However, this rise in estrogen is not usually associated with an increase in the size of the genital swelling [Clark and Birch, 1948;Graham et al, 1972;Nadler et al, 1985;Dahl et al, 1991].…”

“…Genital swelling data were analyzed using three categories: no swelling, partial swelling, and full swelling. A full swelling was defined as maximal swelling of the labia minora and perianal tissues resulting in labial occlusion, a deeply recessed anus and a shiny taut appearance [following Wallis and Lemmon, 1986;Dahl et al, 1991]. Three facility employees were responsible for the daily recording of genital swelling data; however, one person recorded 75% of these data.…”

The context of female dispersal in Kanyawara chimpanzees

“…We used Kaplan-Meier survival analyses to calculate time interval data, such as the age of emigration or the interval between first maximal sexual swelling (defined as the first unambiguous observation of tumescence of both vaginal and perianal tissue without occurrence of wrinkling (Dahl et al, 1991) and emigration. This method increases statistical power by including censored datapoints (e.g., females who had experienced a swelling but had not yet emigrated).…”