The researchers believe whale mothers with calves employ a strategy in habitat selection that may help them avoid male harassment.
It is well established that humpback whale mothers with newborn calves favor shallow waters in their breeding grounds. Recent research indicates that this appears to be a tactic employed by mothers to avoid energetically costly associations with males. Adam Pack, a professor at the University of Hawai‘i at Hilo who is an expert in North Pacific humpback whales in the Hawaiian Islands, points out in an article in Science Trends, that it is not known how this shallow water preference varies as a calf ages and grows and whether mothers, who are less likely to ovulate than females without a calf, employ other tactics to avoid males when in deeper waters.
Pack, a professor and researcher at UH Hilo with joint appointments in psychology and biology whose research focuses on marine mammal behavioral ecology and cognition, has been studying humpback whales in Hawai‘i, their major breeding grounds, for over 20 years.
In the late 1990s, Pack, with a team of colleagues, developed a new technique called underwater videogrammetry to measure the body sizes of humpback whales at sea while simultaneously recording their behavior and social interactions. Using this method, the research team showed 1) that body size confers an advantage to male humpback whales when competing for females, 2) that larger females produce larger calves and consequently attract greater numbers of males than do smaller females, 3) that mature females prefer to associate with the largest males, and 4) that both mature and immature males produce long complex vocalizations called “song.”
In their 2017 study, Pack’s team measured the lengths of 96 humpback whale calves in waters off Maui, Hawai‘i ranging in size from 3.8 to 7.6 meters. They found that the depth of water that a mother and her calf occupy is positively related to the size of the calf, so individual females tend to be found in deeper waters as their calves mature. To confirm this, the team also examined the habitat preferences of 72 mother-calf pairs who were resighted over various intervals within a breeding season.
In addition to Pack, who is also affiliated with The Dolphin Institute, research team members included Louis Herman (UH Mānoa and Dolphin Institute), Alison Craig (Edinburgh Napier University), Scott Spitz (Dolphin Institute), James Waterman (Liverpool John Moores University), Elia Herman (Dolphin Institute), Mark Deakos (Hawai‘i Association for Marine Education and Research), Siri Hakala (Pacific Islands Fisheries Science Center, NOAA), and Carley Lowe (UH Hilo).
In the 2017 study, the researchers found that the magnitude of water depth change between initial and final sightings of a mother-calf pair increased significantly with their resighting interval—as a calf aged, it tended to be found in deeper waters with its mother.
“Interestingly, the team also found that when mother-calf pairs were in deeper waters they had a preference for rugged seabed terrain,” writes Pack in the article.
The researchers suspect that this kind of terrain provides more “acoustic camouflage” from males than does flat terrain as it tends to be noisier from snapping shrimp. The team posits that this might help mothers avoid harassment from males seeking mating opportunities by making it less likely that they are overheard communicating with their calves.
“This final bit is very intriguing as it may indicate a strategy evolved in mothers for mitigating male harassment while preparing (in deeper waters) for emigration back to the feeding grounds,” writes Pack.
These findings are reported in the journal Animal Behaviour in the article “Habitat preferences by individual humpback whale mothers in the Hawaiian breeding grounds vary with the age and size of their calves.”