Can blubber steroid hormone measurements reveal reproductive state in narwhals?

Hormone measurements from blubber samples have been used to assess reproduction in cetaceans and are a widely applicable technique, as blubber samples are often collected from necropsied individuals and biopsies are readily collected from live, free-swimming cetaceans. Many studies have assessed reproduction in cetaceans based on blubber hormone concentrations but few have validated their findings with individuals of known reproductive state. The objectives of this study were to use a unique dataset of paired female narwhal (Monodon monoceros) reproductive tracts and blubber samples to: (1) determine narwhal reproductive state based on ovarian analysis; (2) evaluate progesterone, estradiol, testosterone and corticosterone concentrations in paired blubber samples to validate the use of blubber hormone measurements as a technique to assess reproductive state in narwhals; and (3) determine narwhal reproductive rates using reproductive tract and hormone analyses. Female narwhals with complete reproductive tracts or known ages (n = 13) were categorized as: pregnant (fetus or placenta present; n = 5), active (at least one corpus luteum present; n = 2), resting (at least one corpus albicans present; n = 3) or immature (absence of corpora lutea and albicantia or age <8; n = 3), and eight individuals were classified as unknown due to incomplete reproductive tracts. Estradiol, testosterone, and corticosterone concentrations were not useful for assessing reproductive state; however, progesterone concentrations were higher in pregnant narwhals (432.66 ± 182.13 ng/g) than active (1.57 ± 0.42 ng/g), resting (1.52 ± 0.87 ng/g) and immature (1.44 ± 0.71 ng/g) individuals, validating the use of blubber progesterone concentrations in determining pregnancy in narwhals. Using a progesterone threshold for pregnancy, determined in this study, we were able to classify three individuals with incomplete reproductive tracts as pregnant and identify a potential impending pregnancy loss. The results from this study suggest that blubber progesterone concentrations are useful for assessing pregnancy and can help inform reproductive rates of narwhal populations.