A new study from the University of Washington highlighted why blue whale calves are rarely sighted. Only two blue whale births have ever been recorded, even though there are around 10,000 to 25,000 blue whales today. Blue whales give birth every two to three years, so where are all the calves?
The research suggests that the mystery is due to the timing and location of blue whale births.
Lead researcher Trevor Branch explains that most field studies on blue whales are conducted in summer when whales feed in colder regions like California. However, blue whales give birth in fall and winter in warmer areas such as the Gulf of California.
When blue whale calves are about seven months old and 52 feet (16 meters) long, they are weaned and no longer stay with their mothers. Despite high pregnancy rates of 33-50% annually among blue whale populations, mother-calf pairs are only seen 3.1% of the time.
The study found that calves are weaned when they return to feeding areas, explaining why researchers rarely see mother-calf pairs. This discovery offers hope for the health of the blue whale population, suggesting there are more calves than previously thought.
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Branch’s team used data from long-term studies and historical whaling records to support their hypothesis. They found higher calf proportions in winter regions and lower in summer. The team plans further research to test this idea across different areas and times.
The study indicates that the low sighting rates of calves may not be due to low birth rates or survival but rather the timing and location of field studies. The findings could lead to new strategies for observing and protecting blue whale 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,” Branch said.
Journal Reference:
- Trevor A. Branch. The timing hypothesis explains the mystery of the missing blue whale calves. Endangered Species Research. DOI: 10.3354/esr01383
