Abstract
Nursing influences growth rate and overall health of mammals; however, the behavior is difficult to study in wild cetaceans because it occurs below the surface and can thus be misidentified from surface observations. Nursing has been observed in humpback whales on the breeding and calving grounds, but the behavior remains unstudied on the feeding grounds. We instrumented three dependent calves (four total deployments) with combined video and 3D-accelerometer data loggers (CATS) on two United States feeding grounds to document nursing events. Two associated mothers were also tagged to determine if behavior diagnostic of nursing was evident in the mother’s movement. Animal-borne video was manually analyzed and the average duration of successful nursing events was 23 s (±7 sd, n = 11). Nursing occurred at depths between 4.1–64.4 m (along the seafloor) and in close temporal proximity to foraging events by the mothers, but could not be predicted solely by relative positions of mother and calf. When combining all calf deployments, successful nursing was documented eleven times; totaling only 0.3% of 21.0 hours of video. During nursing events, calves had higher overall dynamic body acceleration (ODBA) and increased fluke-stroke rate (FSR) compared to non-nursing segments (Mixed effect models, ODBA: F1,107 = 13.57756, p = 0.0004, FSR: F1,107 = 32.31018, p < 0.0001). In contrast, mothers had lower ODBA and reduced FSR during nursing events compared to non-nursing segments. These data provide the first characterization of accelerometer data of humpback whale nursing confirmed by animal-borne video tags and the first analysis of nursing events on feeding grounds. This is an important step in understanding the energetic consequences of lactation while foraging.
Highlights
Humpback whale (Megaptera novaeangliae) calves exert different energetic demands on lactating females on the breeding and feeding grounds
We would expect nursing to comprise a smaller proportion of time on feeding grounds, but the average duration of successful nursing events on the feeding grounds (23 s ± 7 s, n = 11) was similar to the average duration observed by free divers on breeding grounds (30.6 s ± 16.9 s, n = 5) (Zoidis & Lomac-MacNair, 2017)
Based on the low activity level of mothers during nursing events, the data suggest that nursing did not occur while females were lunging, but our results show that females could modify their behavior to allow nursing for short durations while diving to depth to feed and during periods between lunges
Summary
Humpback whale (Megaptera novaeangliae) calves exert different energetic demands on lactating females on the breeding and feeding grounds. The duration and process of a calf transitioning from full dependence to complete independence of its mother (‘‘weaning’’) is not fully known It appears that the majority of mother-calf pairs separate after they leave the feeding grounds during the calf’s first year (Baker, Perry & Herman, 1987; Clapham & Mayo, 1987). A minority of pairs separate early while still on the feeding grounds (Baraff & Weinrich, 1993; Steiger & Calambokidis, 2000) and a few even remain together for a second feeding season as mother-yearling pairs (Baraff & Weinrich, 1993; Hammond, Mizroch & Donovan, 1990) Along this gradient, a humpback whale calf’s reliance on its mother for nutrients likely gradually decreases during the process of weaning (Oftedal, 1997), but neither the frequency nor the duration of visually verified humpback whale nursing events on the feeding grounds have been documented
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