Estradiol and creatinine values of captive female bonobos

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
four years for estradiol and creatinine concentrations (n=117). While E2
was significantly higher in pregnant vs. non-pregnant 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.