Characterization of the estrous cycle in female beluga ( Delphinapterus leucas) using urinary endocrine monitoring and transabdominal ultrasound: Evidence of facultative induced ovulation

Abstract

Recent, successful application of assisted reproductive technologies in captive beluga has resulted from the extensive study of male beluga reproductive biology. Optimization of assisted reproduction requires additional detailed knowledge of the female estrous cycle. Our specific objectives were to: (1) validate urinary immunoassays for use in this species; (2) elucidate annual ovarian cycle dynamics through the combined use of hormone excretion patterns and transabdominal ultrasound; and (3) establish whether ovulation in this species is spontaneous or induced by male factors. Ovulation was observed in four of 15 estrous cycles monitored in four adult female beluga maintained in a single-sex group. After introduction of a breeding male, ovulation was observed in six of seven estrous cycles. All estrous cycles occurred from March through June. For spontaneous ovulations ( n = 4), the inter-estrous interval was 34 d (range 33–35 d), with a follicular phase length (FPL) of 25 ± 8 d (mean ± SD). For all ovulatory estrous cycles (with and without a breeding male), urinary estrogen conjugates (EC, 15.3 ± 7.9 ng/mg Cr) and ovulatory luteinizing hormone (ovLH, 17.1 ± 6.6 ng/mg Cr) concentrations both peaked on Day 0, and EC concentrations returned to baseline 8 ± 7 d later. For non-conceptive cycles, urinary progestagen (Pg) concentrations increased on Day 0 (3.5 ± 1.7 ng/mg Cr), peaked on Day + 19 (19.7 ± 17.1 ng/mg Cr), and were elevated above baseline for 27 ± 4 d. Preovulatory follicular diameter and circumference on Day −2 ± 2 (range: Day −4 to −1) from peak EC were 2.5 ± 0.7 and 7.8 ± 1.3 cm, respectively. The FPL in non-ovulatory estrous cycles ( n = 11) lasted 24 ± 10 d and EC concentrations gradually declined to baseline over a 21 ± 10 d interval following the EC peak (27.8 ± 28.8 ng/mg Cr). Non-ovulatory estrous cycles were characterized by the absence of an ovLH surge and no concomitant increase in Pg concentrations above baseline excretion; the mean follicular diameter at or near peak EC was 3.1 ± 0.8 cm on Day 2 ±3 d from peak EC (range: −1 to +5 days from peak EC). Overall, these data confirm that captive beluga exhibit reproductive seasonality and demonstrate that the species is a facultative-induced ovulator.