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Calf Survival

Bowhead whales produce calves that are capable of surviving in subfreezing waters through their thermoregulation adaptations (George et al., 1999). New-born bowheads characteristically are soft, white blubbered, rotund shaped and have very small baleen (Nerini et al., 1984). They are approximately 4-4.5m in length and have a thick blubber layer at birth (Figure 13). Within the first few months they rapidly thicken their blubber layer at expense of their length growth (Corkeron and Connor, 1999, Lubetkin et al., 2012, Nerini et al., 1984). To further optimise bowhead calve survival the females have a longer gestation and lactation period and give birth in spring-summer (Nerini et al., 1984).

Copulation occurs in winter months (January to April), then there is a 11-14 months gestation period which allows the calf to obtain sufficient insulation, body mass and length for survival (Corkeron and Connor, 1999, George et al., 2004a, Nerini et al., 1984). Females give birth between March and August but births peak in May (Corkeron and Connor, 1999, George et al., 2004a, Nerini et al., 1984). This is a behavioural adaptive mechanism for mother lactation and calf weaning because lactation is an energy costly process females have uniquely adapted to coincide their onset of lactation with the feeding season (Nerini et al., 1984). Lactation may extend for 12-14 months and optimum calf weaning time coincides again with the next years feeding season (Nerini et al., 1984). Within the first year the calves growth and baleen length increase rapidly with the body length increasing to approximately 8.5m and the baleen growing from 20cm to 70cm (Lubetkin et al., 2012, Nerini et al., 1984). After the first year there is a 2-3year growth of body mass and length pauses to divert all energy resources to the continues growth of the baleen plates allowing for more efficient filter feeding (George, 2009). Body growth (slowly) continues again at 4 years old (Lubetkin et al., 2012). Sexually mature females have calves at 3-6 year intervals (Corkeron and Connor, 1999, George et al., 2004a, Nerini et al., 1984).

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Figure 13: A Bowhead whale and her calf in the Arctic. (Photo source: Corey Accardo (NOAA), Permit No.782-1719 )

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