Ontogenetic changes in energy expenditure and resting behaviour of humpback whale mother–calf pairs examined using unmanned aerial vehicles

Abstract

Context Baleen whale calves rapidly increase in size and improve locomotion abilities, while on their low-latitude breeding ground, allowing them to undertake a successful migration to high-latitude feeding grounds. Aims We investigated energy expenditure and resting behaviour of humpback whale (Megaptera novaeangliae) mother–calf pairs in regard to changes in calf length on an undisturbed breeding/resting ground off Exmouth Gulf, Western Australia. Methods Data were collected from August to October in 2018 and 2019 on lactating mothers that were predominantly resting on the surface with their calf. Focal follows on mother–calf pairs (n = 101) were conducted using an unmanned aerial vehicle to obtain detailed video of behaviours and respirations (23.7 h). Body length measurements of individual whales were calculated from aerial still frames. Key results Results on calves ranging in length from ~4–8 m demonstrated that calf respiration rate decreased with an increase in calf length and increased with presence of activity (P < 0.001). Calf inter-breath intervals became longer in duration with an increase in calf length (P < 0.01). Calf activity level and resting behaviour remained constant, with calves logging for 53% of the time their mothers were logging. Maternal respiration rate remained low and did not differ with respect to maternal or calf length. Conclusions Results highlighted the importance of resting grounds for energy preservation, which benefits the calves’ rapid growth before migration to polar waters. Implications Findings from the present largely undisturbed population serve as a baseline for understanding the impacts of anthropogenic disturbance on resting behaviour and energy expenditure in humpback whale mother–calf pairs globally.

Ontogeny and synchrony of diving behavior in Humpback whale mothers and calves on their breeding ground

“…Our findings are consistent with the previous studies done on humpback whales calves in their breeding grounds (Cartwright & Sullivan 2009, Tyson et al 2012, Ejrnaes & Sprogis 2021. Of the 29 studied calves, we found that 4 out of the 10 analyzed dive metrics varied significantly with age.…”

“…However, since we could record dives only on two C1 calves, great care should be taken in generalizing the present results. Nevertheless, these findings suggest that older calves showed higher swimming and breathing abilities, as previously found in humpback whales (Cartwright & Sullivan 2009, Ejrnaes & Sprogis 2021 and observed in right whales (Cusano et al 2019, Dombroski et al 2021). As suggested in previous studies, neonate calves have limited lung capacity as they are still learning to manage their breathing cycle and buoyancy.…”

“…Longer bottom time and shorter surface interval of mothers with older calves were also found in Southern right whales (Dombroski et al 2021). In contrast, UAV monitoring of mother-calf pairs in Western Australia did not report any change of breathing rate in mothers with regards to calf length (i.e., age) (Ejrnaes & Sprogis 2021).…”

The Behavioural Impacts of Commercial Swimming With Whale Tours on Humpback Whales (Megaptera novaeangliae) in Hervey Bay, Australia

“…This is, however, not the case in other locations in Australia (e.g., Sprogis et al, 2020), and around the world (e.g., Fiori et al, 2019), where swimming with calves occurs, even in areas where it has been advised against. It is of great importance to limit disturbance on mother-calf pairs as they are the most sensitive to disturbance (Sprogis et al, 2020) and, over extended periods of time, increases in energy expenditure could become biologically significant for mothers with a dependent calf (Cartwright and Sullivan, 2009;Ejrnaes and Sprogis, 2021). Thus, in Hervey Bay it is of importance to retain best practice guidelines on not swimming with calves to reduce disturbance to the whales, but also to avoid injury to humans from high-risk situations with mother and calves (as documented in Sprogis et al, 2017Sprogis et al, , 2020Fiori et al, 2019;Barra et al, 2020;Hoarau et al, 2020).…”

Seasonal gain in body condition of foraging humpback whales along the Western Antarctic Peninsula

“…The opposite was observed on their lower latitude breeding grounds, where 32 humpback whale calves (TL range: 4.14-7.76 m, thus classified as n = 30 post-neonate, n = 2 neonate, see Irvine et al, 2017a) increased their TL, but not their body condition (Christiansen et al, 2016). Calves may first prioritize investing energy towards growth on the breeding grounds to reduce heat loss via reducing their surface area to volume ratio, to enhance breath holding ability (Ejrnaes and Sprogis, 2022), and to avoid predation by killer whales (Orcinus orca) and sharks (Pitman et al, 2015). Then, as they enter their high latitude foraging grounds, they begin to invest more energy toward fat and blubber storage.…”