Breaking diapause: Using minimally-invasive ultrasonographic techniques shows intra-specific variation in the timing and probability of pregnancy in the Weddell seal
The timing of fetal development, and the length and costs of gestation influence reproductive success necessary for maintaining stable populations. Fetal growth has been characterized in many pinniped species, but until recently, such studies required sacrificing both the adult animal and the pregnancy. We utilized the Weddell seal (Leptonychotes weddellii) as a model to test the feasibility of using minimally-invasive ultrasonographic techniques. Weddell seals give birth in Oct/Nov each year, followed by the breeding season in Dec. In Jan/Feb 2014-2015, we used transrectal ultrasonography to characterize the reproductive tract, detect early pregnancy and estimate the length of embryonic diapause in Weddell seals. We handled mature skip-breeding female Weddell seals that did not give birth during the study year (SB; n = 13), females that gave birth early (EB; October; n=13), and late (LB; November; n=13) in the pupping season. Twelve seals were captured for a second examination for longitudinal measures of embryonic growth. Pregnancy was defined as detection of a fluid-filled chamber (embryonic vesicle) within the uterus, with or without an embryo proper and heartbeat. Embryonic vesicle measurements were incorporated into growth curves and mixed model analysis to assess the influence of reproductive history on the timing of conception.
Overall, 77% of SB, 92% of EB, and 62% of LB seals were detectably pregnant in Jan/Feb. Embryonic vesicle size was greater in SB seals (15.9±3.4cm2) compared to EB (8.4±1.5cm2; P = 0.031) and LB seals (4.8±1.5cm2; P < 0.001), with no effect of age, parity, total body mass, or blubber mass of the female. The wide range in embryonic vesicle sizes observed in this study indicates that diapause is brief and does not serve to synchronize embryo development among reproductive groups. Instead, embryo size was more strongly correlated to days post-partum than to calendar date. Preliminary results of pregnancy outcomes (2014 data) suggest that embryos that were smaller than expected, based on days post-partum residuals, appeared less likely to be brought to term. Females that had larger embryos (earlier end to embryonic diapause) also tended to give birth earlier the next spring, which may have implications for pup survival. This study demonstrates that ultrasonography can be used to address intra-specific variation in reproductive success and phenology in a wild phocid.