Temporal association of serum progesterone concentrations and vaginal cytology in walruses (Odobenus rosmarus)

A novel method to measure steroid hormone concentrations in walrus bone from archaeological, historical, and modern time periods using liquid chromatography/tandem mass spectrometry

“…Estradiol in blubber from 2015 was below detectable limits in 20 of 22 samples (including duplicates from one 2015 female). From our results, estradiol is most likely not deposited in the blubber layer in high concentrations, possibly due to having relatively low circulating concentrations to begin with in male and female pinnipeds . Female walruses usually enter estrus twice in the year, around January and late August .…”

“…Progesterone and estradiol would be expected to be low in male walruses, because progesterone and estradiol are female reproductive hormones and only play a minor role in male reproduction . In females, estradiol does not remain elevated for extended periods of time in pregnant walruses compared with progesterone, which may stay elevated for up to 9 months throughout a pregnancy . Thus, surges of estradiol are more difficult to capture in a long‐term reservoir tissue, such as bone, compared with progesterone and could result in ND samples for modern female walrus bone samples ( n = 8 of 10).…”

“…While estradiol concentrations are discussed in more detail below, estradiol can be locally synthesized in bone, and most likely has a different reservoir time in bone compared to the other steroid hormones in this study, which could contribute to the relatively high number of ND samples. The modern female samples with below detectable concentrations of estradiol were all collected between the months of April to May, which is outside the timeframe when female walruses would go into estrus and have high estradiol concentrations . Additionally, one of the animals was a juvenile.…”

“…Additional LC/MS/MS testing could confirm testosterone and cortisol as the preferred deposited metabolites for those hormones in cortical bone, and if other hormone metabolites of progesterone and estradiol are being deposited into bone . However, progesterone and estradiol should be the targeted hormones when studying walrus reproductive physiology, because they were detected in the majority of samples and are the main hormones that initiate and sustain the female reproductive cycle in walruses …”

“…Estradiol increases in female pinnipeds before entering estrus, but all modern females used for the tissue comparison were collected in May, a time of low circulating estradiol concentrations (Appendix ). Additionally, surges in estradiol have not always been documented in walruses before estrus, but instead may be more sporadic . Hence, in 2015 there were most likely no surges in estradiol to detect in blubber or serum (Table ).…”