The extra burden of motherhood: reduced dive duration associated with pregnancy status in a deep-diving mammal, the northern elephant seal

Abstract

The cost of pregnancy is hard to study in marine mammals, particularly in species that undergo pregnancy while diving continuously at sea such as elephant seals (genus ). We analysed the diving behaviour of confirmed pregnant and non-pregnant northern elephant seals (, = 172) and showed that after an initial continuous increase in dive duration, dives of pregnant females become shorter after week 17. The reasons for this reduction in dive duration remain unknown, but we hypothesize that increased fetal demand for oxygen could be the cause. Our findings reveal an opportunity to explore the use of biologging data to investigate pregnancy status of free-ranging marine mammals and factors that could affect pregnancy success.

Lifetime reproductive success of northern elephant seals (Mirounga angustirostris)

“…Although intermittent breeding has been documented in both northern and southern elephant seals (Huber 1987;Huber et al 1991;Sydeman and Nur 1994;Desprez et al 2018;Hückstädt et al 2018), the present study does not elucidate the prevalence and influence on fecundity. Gaps in breeding records of long-lived females suggest that females were overlooked in crowded harems.…”

“…Annual fecundity, as measured by the percentage of females present during the breeding season that give birth, is 97.5% (Le Boeuf and Reiter 1988;Crocker et al 2006;Le Boeuf et al 2011). A study of females bearing satellite tags, however, revealed that 9.9% of 172 females, tracked between 2004 and 2016, were not pregnant when sighted before or after the breeding season (Hückstädt et al 2018). It is not clear whether these females had aborted or had failed to get pregnant; they were not observed during the breeding season.…”

“…Annual fecundity, based on females of all ages, was estimated from previous studies as between 87% and 97.5% (Le Boeuf and Reiter 1988;Hückstädt et al 2018). The overall mean proportion of gaps in the records of females of age 4 and older was 28.3% ± 1.2% (N = 5270 breeding seasons).…”

Temporal changes in Weddell seal dive behavior over winter: Are females increasing foraging effort to support gestation?

“…Given that female Weddell seals have low rates of energy acquisition during their gestational foraging period (Shero, Krotz, et al, ), the scope for suppressing metabolic rates may be critical for carrying the pregnancy to term. In contrast, the northern elephant seal ( Mirounga angustirostris ) rapidly accretes tissue during gestation (gaining 70% body mass, as compared with 15% in Weddell seals; Robinson et al, ; Shero, Krotz, et al, ), and a recent study revealed that reproductive female northern elephant seals made significantly shorter dives during late pregnancy (Hückstädt, Holser, Tift, & Costa, ). This suggests that species with greater energy reserves may not require metabolic suppression and energy sparing strategies to protect the developing fetus.…”

“…Given that female Weddell seals have low rates of energy acquisition during their gestational foraging period (Shero, Krotz, et al, 2015), the scope for suppressing metabolic rates may be critical for carrying the pregnancy to term. In contrast, the north- (Hückstädt, Holser, Tift, & Costa, 2018). This suggests that species with greater energy reserves may not require metabolic suppression and energy sparing strategies to protect the developing fetus.…”