Humpback whale suckling behavior: an insight into the mother-offspring strategy in mysticetes

Abstract: Despite its biological importance, the nursing behavior in baleen whales has been poorly documented mainly because of the challenges in tracking whales in their natural environment. We studied the suckling behavior (nursing from the calf's perspective) in < 3-month-old humpback whale calves off Sainte Marie Island, Madagascar, South Western Indian Ocean (breeding/calving area). We investigated the temporal pattern of the suckling behavior, its spatial and behavioral context, and the ontogeny of these characteristics. We exploited data from both camera-equipped and non-camera-equipped animal-borne multi-sensor tags (Acousonde and CATS cam) collected from 2013 to 2022 (37 calves). Our dataset constitutes the largest sample assembled so far to investigate the suckling behavior in free-ranging whales. We found that the suckling event duration does not vary much with the calves’ relative age. The estimated proportion of time spent suckling ranged between 0.8 and 1.6%. The suckling events mainly occurred during the descent and bottom phases of dives and rarely during the ascent phase of dives or surface activities. Strong evidence of humpback whale suckling at night is presented for the first time. We found that suckling events were often performed in bouts of 2–6 events. Suckling bouts mostly occurred less than 10 min apart, and suckling sessions were separated by about 2 h of non-suckling periods. In other words, humpback whale calves suckled several times throughout the day in short sessions similar to terrestrial “followers.” Our results add to the evidence that humpback whales, and potentially most baleen whales, are behaviorally and functionally similar to terrestrial “followers” regarding maternal strategy. Significance statement: Young mammals rely on the milk provided by their mother to survive during their early life stage. The suckling behavior, i.e., the procurement of milk from the mother, has been widely studied for terrestrial mammals but not for fully aquatic mammals like the baleen whales due to the challenges in tracking them. By placing sensors and cameras on calves, we described the suckling pattern in humpback whales, the most common baleen whale species. We found that the temporal pattern of suckling in humpback whales is similar to those of terrestrial precocial mammals experiencing comparable environments (open habitat with predators): the young suckle frequently but in short sessions. Our results offer a new insight supporting the hypothesis that the rearing strategy used by baleen whales is an aquatic version of the “following” behavior found in these precocial terrestrial species.