Associations with multiple male groups increase the energy expenditure of humpback whale (Megaptera novaeangliae) female and calf pairs on the breeding grounds

In mammalian mating systems, where operational sex ratios are male skewed and males must compete for access to females, increased levels of male attention may amount to harassment of females and their offspring. To evaluate how male associations affect the behaviour of humpback whale (Megaptera novaeangliae) female and calf pairs in breeding regions, we compiled time budgets and monitored calf breathing regimes for females with calves in a range of associations, with (N = 71) and without (N = 19) males, in Hawaiian waters. We found that, while associations with a single male did not significantly change the behaviour of female-calf pairs, associations with multiple males led to increases in the time spent traveling (median increase 35%; p < 0.001) and decreases in time spent at rest (median decrease 29%; p < 0.001). Additionally, calves spent less time at the surface (median decrease from 10% to 0; p < 0.001) and the frequency of intermittent breaths between dives increased (median increase from 16 to 22%; p = 0.006). We show that these behavioural changes would require increased energy expenditure, which could impact calf fitness, and we speculate that the association between a female-calf pair and single male escort comprises a female counterstrategy that offsets male harassment, consistent with Mesnick's (1997) bodyguard hypothesis.