Fecal estrogen, progestagen and glucocorticoid metabolites during the estrous cycle and pregnancy in the giant anteater (Myrmecophaga tridactyla): evidence for delayed implantation.

Background: Declining numbers of wild giant anteaters highlight the importance of sustainable captive populations. Unfortunately, captive reproductive management is limited by the lack of external physical indicators of female reproductive status and the aggressive behavior of males. We examined the endocrinology of the estrous cycle and pregnancy, and whether delayed implantation is a gestational strategy for giant anteaters as described for other xenarthrans.

Methods: Feces were collected from seven captive females 3-5 times weekly and mating was recorded. Concentrations of estrogen (estrone-glucuronide, E1, and estradiol-17β, E2), progestagen (20-oxo-progestagens, P4), and glucocorticoid (GC) metabolites were examined in fecal extracts by enzyme immunoassay.

Results: Estrous cycles for nulliparous females (6 cycles, n = 2) compared to the multiparous female (6 cycles, n = 1) were shorter (47.3 +/- 4.3 days versus 62.5 +/- 2.6 days) with relatively lower luteal phase concentrations of P4 (49.4 +/- 2.9 ng/g versus 136.8 +/- 1.8 ng/g). The four remaining females had unclear ovarian activity: two females exhibited apparent luteal activity but unclear fluctuations in estrogens, while the other two females had parallel fecal P4 and estrogens concentrations. Pregnancy ranged 171-183 days with females returning to estrus post-partum as early as 60 days (n = 3, 1.8-4 years of age at mating). Delayed implantation was indicated by a biphasic elevation in fecal P4 metabolites: the initial 4-fold increase occurred for 81-105 days and was followed by a 26-fold secondary rise in P4 metabolites lasting 66-94 days prior to parturition. Fecal GC was correlated with fecal estrogens and greatest during estrus, late pregnancy, and six days prior to parturition (estrous cycle GC, 14.4-62.8 ng/g; pregnancy GC, 13.6-232.7 ng/g).

Conclusions: Estrous cycles of giant anteaters occurred year-round, but were shorter and more intermittent in younger nulliparous animals compared to a multiparous female. A pronounced elevation in fecal P4, estrogen, and GC occurred during late gestation after an initial post-mating delay providing evidence for delayed implantation. Adrenocorticoid activity indicated impending parturition. Differences in estrous cycle characteristics with age and the protracted but variable gestation length must be considered to improve reproductive success and neonatal survival in giant anteaters.