Maternal care varies across taxa from brief, minimal care to long-term, intensive care. Mammalian mothers provide extensive and energetically expensive care by definition through pregnancy and lactation, which can extend for years, resulting in behavioural trade-offs between resource acquisition and direct care. In marine environments, mammalian mothers face unique challenges, such as the inability to cache or den their offspring while diving for prey. Dolphin newborns are precocious, accomplishing shallow dives in the first few weeks of life, however, fully mature diving and breath- holding capabilities take years to develop. Consequently, mothers are faced with a trade-off between diving and foraging or remaining close to and protecting their calves at the surface. Here we examined this trade-off, specifically by investigating whether mothers change their dive durations, especially during foraging, as a function of calf age. We used a longitudinal (1988-2014) data set on wild bottlenose dolphins, Tursiops aduncus, in Shark Bay, Western Australia, which included 27388 dive bouts from mothers (N=26) and calves (N=41). Our results show that maternal diving behaviour changes in response to calf age and sex. While both male and female calves increased their dive durations with age as expected, mothers were more likely to adjust their diving behaviour to accommodate female but not male calves, especially when daughters were in close proximity. This is consistent with findings that vertical social learning is more critical for daughters than for sons, and may reflect the sex-specific foraging and social tactics of the males and females more generally.
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