CALVING
Preparing to Give Birth in Hawaii
Each year, pregnant females migrate from higher latitude feeding areas to lower latitude regions associated with breeding and calving, such as Hawaii. Why this is necessary is still debated in the literature and has been complicated by recent observations of newborn humpback calves in more traditional feeding areas like California. However, the vast majority of humpback births occur in warm waters.
Female Reproductive Cycle
Female humpbacks typically give birth every two to three years once sexual maturity is reached. The gestation period is about 11 months. This suggests that the majority of females become pregnant during the winter months and return the following winter to give birth. A calf will stay with its mother for a period ranging from 6-12 months.3 Since lactating female humpbacks lose up to 50% of their body mass while fasting on the breeding grounds, most mothers will take a year off between births.4 However, observations of individual females with newborn calves in successive years suggests that some females – perhaps those in their reproductive prime – can withstand annual births.
Births
Female humpbacks are especially vulnerable while giving birth and only a few cases of the birthing process have been documented worldwide. In 2020, in the waters off Maui, a partial birth was video-recorded by a local whale watching company. This is the first documented birth in Hawaii and provided valuable insight into the process. In this particular case, the female was accompanied by a primary and secondary escort, demonstrating that even females in labor can still attract male attention. Several social sounds, similar to those in surface active groups, were recorded during the encounter. Although the full delivery was not observed, the mom was sighted 40 days later with a healthy calf.
You can find the video of the partial birth on Captain Steve’s Facebook page and the full scientific publication in Aquatic Mammals.
Social Bond between Mother and Calf
During their time on the breeding grounds, which averages about 6 weeks, moms and calves form and strengthen their social bond through communication, teaching, and play.5 Vocal communication between mom and calf is relatively quiet and grunt-like, which may help them avoid detection by males or predators, but this remains unknown. In addition to vocalizations, physical touch between moms and calves also appears important for maintaining contact. Calves also spend significant amounts of time learning and mimicking the behaviors of their mothers and male escorts, such as breaching, pec-slapping, tail lobbing, rolling, head lunging, and spy-hopping. These bouts of play are thrilling to observe, but also essential for building muscle and increasing their breath-holding capacity, critical for their migration north to the feeding grounds.
Do Mothers Avoid Males on the Breeding Grounds?
Seventy-five percent of mom-calf pairs in Maui are accompanied by a male escort. Although males are interested in mothers with calves, Whale Trust research has shown that moms are far more likely to avoid males than are solo females. Some males behave aggressively by chasing moms with calves. Females, however, have developed a few tactics to avoid unwanted male escorts. One technique is to use acoustic cues to avoid interactions with multiple whales. When we played back the sounds of a multiple male active group to mothers with calves, most of them moved away from the source of the sound.1 But what if a female already has an escort that she doesn’t want? In this case, we have observed females rub off unwanted escorts by approaching nearby female groups and then swimming away, leaving him behind.
Mom (unsuccessfully) rubs off unwanted escort on another nearby Mom-calf pair.
KEY FINDINGS
- Results of playback experiments and behavioral observations both showed that mothers with calves avoided males on breeding grounds more than females without calves.1
- Females may also use different tactics to avoid males (e.g., “rubbing off an unwanted escort” on another nearby female group).1
- Singers accompanying females is relatively rare [need to find out if its 1 in 10]. However, females with calves are more likely to be accompanied by a male singer than females without calves, further suggesting that singing does not play a role in mating or courtship.1,2
- Like other female mammals with offspring, humpback whale mothers may conflict with breeding males that stand to maximize reproductive success by increasing access to receptive females. 25% of females and calves on the Hawaiian breeding grounds were actively chased by lone males.1
References
- Jones, Meagan E. 2010. Female Humpback Whale (Megaptera novaeangliae) Reproductive Class and Male-Female Interactions during the Breeding Season. Antioch University, New England. 184 pp. (Download PDF)
- Darling, J.D., Jones, M.E., and Nicklin, C.F. 2006. Humpback Whale Songs: Do they organize males on the breeding grounds? Behaviour. 143: 1051-1101. (Download PDF)
- Chittleborough, R. G. (1958). The breeding cycle of the female humpback whale, Megaptera nodosa (Bonnaterre). Marine and Freshwater Research, 9(1), 1-18.
- Lockyer, C., et al. (1981). “Growth and energy budgets of large baleen whales from the southern hemisphere.” XF2006134403 FAO Fisheries Series 5: 379-487.
- Darling, J.D. and C.M. Jurasz. 1983. Migratory destinations of North Pacific humpback whales (Megaptera novaeangliae). In: Communication and Behavior of Whales. Ed. by R. Payne. AAAS Selected Symposia Series. Westview Press, Boulder, CO. pp. 359-368.
