Urinary Estradiol in Captive Bonobos: Variation With Reproductive State and Sexual Swelling

Estradiol is known to have a variety of biological and behavioral effects, but monitoring its function is complex given the many factors influencing its variation. This necessitates large sample sizes which are challenging in captive and wild situations. This study validates the use of opportunistically collected urinary estradiol levels (E2) for use in reproductive monitoring and behavioral research in bonobos (Pan paniscus). We analyzed frozen urine samples from four Columbus Zoo adult females over 4 years for estradiol and creatinine concentrations (n = 117). While E2 was significantly higher in pregnant versus nonpregnant females (F = 66.30, df = 1, p < 0.001) it was not significantly different between lactating and regularly cycling females (F = 0.40, df = 1, p = 0.5304). Among the regularly cycling females, there was a significant positive regression between E2 and sexual swelling size (F = 4.43, df = 1, 81, p = 0.0384). No differences in E2 variation were detected between individuals in this study. Specifically, when the amount of variation in estradiol due to sexual swelling was statistically controlled for, there was no significant effect of age (n = 83, r = 0.08059, p = 0.4689) or rank (n = 83, r = 0.1361, p = 0.22) on estradiol variation. Overall, these findings indicate that opportunistically sampled urinary estradiol can be paired with visual observation to help detect changes in reproductive status. The shift from lactational amenorrhea back to estrogen cycling may be less clearly defined than expected, and uneven sampling may exacerbate difficulty in detecting some of the more subtle shifts in estradiol levels. While it is known that extended maximal tumescence in bonobos may function to obscure the exact date of ovulation, we did confirm that ratings of visual tumescence still provide useful information regarding relative estradiol levels. By publishing more methodologies and results of this kind, we hope to promote the continued study of estradiol in bonobos as it is relevant to both health monitoring and behavioral research goals.