In all of human history, only two blue whale births have been recorded, with both having taken place decades ago.
It is unclear why blue whale births and even calves themselves are so rarely seen, especially since there used to be hundreds of thousands of blue whales before whaling began.
Today, around 10,000 to 25,000 blue whales exist, and they give birth every two to three years. Still, we have not managed to detect any births or calves swimming around.
A new study may explain when and where the births are happening and where the calves spend their first few months of life, alleviating some concerns over the health of the species.
Generally, researchers prefer to study the feeding areas of blue whales in the summer. But blue whale calves are born in the fall and winter, which is why they are hardly spotted.
Then, they are weaned before returning to feeding areas. In the summer, blue whales migrate to colder regions, like off the coast of California, where krill is abundant.
In winter, they return to warmer regions, such as the Gulf of California and the eastern tropical Pacific, to give birth.
By the time blue whale calves are seven months old, they are already 52 feet long and have stopped associating with their mothers.
However, the high pregnancy rates of 33 to 50 percent a year across blue whale populations seem to contradict the average 3.1 percent rate of sightings of mother-calf pairs.
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After accounting for low fetal and calf survival, low birth rates, and calf separation from the mother, the timing hypothesis appeared to be the best explanation for the observed patterns.
“My conceptual model can explain the mystery of the missing calves: 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,” said Trevor Branch, the lead author of the study and a professor of aquatic and fishery sciences from the University of Washington.
Data from long-term field studies and historical whaling records were combined and analyzed to come up with this hypothesis. The research revealed higher numbers of calves in winter regions and lower numbers in summer regions.
The low calf sightings raised concerns about low birth rates or low calf survival, but instead, the fresh findings offer hope that more calves can be spotted in regions that blue whales travel to in winter and spring.
“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,” Branch concluded.
The details of the full study were published in the journal Endangered Species Research.
