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Scientists may finally have an explanation for the longstanding mystery of why blue whale calves are rarely sighted, an advance that could help better conserve the critically endangered species.
There have been only two recorded instances of the world’s largest animal giving birth in all of human history, both from decades ago.
Thousands of blue whales roam the oceans and the giant mammals give birth every two to three years. But the births are very stealthy, with calves rarely sighted.
Given pregnancy rates as high as 33-50 per cent for the species, it is a mystery that the rate of sightings of blue whale mother-calf pairs is on average an abysmal 3.1 per cent.
Researchers generally suspect low foetal survival, low calf survival, low birth rates and separation from mother as likely reasons for why so few calves are spotted, sparking concern about the survival of the species.
A new study, published in the journal Endangered Species Research, hints that the key reason why blue whale calves are hardly ever spotted may be that researchers prefer summertime to study congregations of the animal.
The calves are born in fall and winter and weaned before they return to feeding areas, study co-author Trevor Branch from the University of Washington explains.
Blue whales migrate during summer to feed in colder regions where krill is plentiful, like in the waters off California. They return to warmer regions such as the Gulf of California and the eastern tropical Pacific in winter when ready to give birth.
Seven months after being born and already over 16 meters long, the calves are weaned and stop associating with their mothers. “Blue whales produce calves or give birth shortly after departing their summer feeding grounds and wean their calves seven months later, just before they return,” Dr Branch explains.
This explains why most researchers conducting field studies in summer rarely see blue whale mothers with calves.
“This new idea provides an alternative explanation for why some blue whale populations appear to produce very few calves: it’s not a failure of calf production, it’s because fieldwork in those populations is understandably concentrated in easily accessible summer feeding areas,” Dr Branch says.
For the latest study, Dr Branch combined and assessed data from field studies and biological information from historical whaling records. The analysis revealed a higher proportion of calves spotted in winter regions.
“These results suggest that the mystery of the missing blue whale calves can largely be explained by mothers calving immediately after leaving summer feeding grounds and weaning on their return,” Dr Branch notes.
The study also suggests that a higher number of calves could be sighted during field studies concentrated in regions that blue whales travel to in winter and spring.
“For summer feeding areas, the conceptual model predicts rapidly declining proportions as the season progresses from November to February in the Southern Hemisphere and from May to August in the Northern Hemisphere,” Dr Branch says.
The marine biologist is now preparing to test this idea with field data by month in each region, combined with estimates of the size of calves by month.
