Abstract As capital breeders, baleen whale mothers fast while on the calving grounds, relying on finite energy reserves to give birth and support the rapid growth of their calves. Whereas maternal size and condition provide clear energetic advantages, the role of energy-conserving behaviours is less understood. This study aimed to examine southern right whale ( Eubalaena australis , SRW) mother-calf energy-conserving behaviours throughout the calving season. Unmanned aerial vehicles were used to collect behavioural focal follow data ( n = 342 follows), and morphometric body condition (BC) data of SRW mother-calf pairs ( n = 59) at the Head of Bight calving ground in South Australia from June to September 2021. Mothers rested on average for 33% of the time, with a greater proportion of resting occurring early in the season, when their BC was highest. This suggests that females rest more during the calf’s early development to offset the high energetic costs of lactation. Mothers rested by logging on the surface and sub-surface, and uniquely for this species, upside-down (by 25% of the mothers). Upside-down resting was associated with decreased calf nursing potential, suggesting that this behaviour functions as a mechanism to restrict access to the mammary slits and thus regulate nursing. Calves rested on average for 13% of the time, predominately when their mothers were resting, highlighting their strong dependence on their mother. Nursing was associated with maternal BC, with nursing increasing as maternal BC declined, demonstrating the importance of the mother maintaining high energy reserves throughout the season. This research demonstrates the dynamics of resting behaviours, and that resting is critical for calf development as well as maternal energy conservation.
Noort et al. (Mon,) studied this question.