Decline of Killer Whales Seen in Last Ice Age

Genome sequencing of killer whales has revealed a severe decline in population occurred during the last Ice Age, when climate change could have limited ocean productivity, according to researchers at Durham University.

With the help of collegues from the University of Lincoln, scientists studied DNA sequences from killer whale communities around the world. They discovered a drastic drop in whale numbers and loss in genetic diversity occurring 40,000 years ago, at a time when ice covered large parts of Earth.

The only stable killer whale population lived off the coast of South Africa. The population retained higher genetic diversity, which indicates high population size.

The research, published in Molecular Biology and Evolution, suggests that the Bengeula upwelling system - which delivers nutrient rich water to the coast of South Africa - was not disrupted by the last Ice Age.

Other major upwells, such as the California current off North America, Humboldt off South America, and Canary off North Africa, were either impacted or completely collapsed during the last glacial or Pleistocene periods, which occurred 40,000 to 2.5 million years ago.

While the researchers admit that factors affecting killer whale populations are "overlapping and complex," they hypothesis that lowered food supplies led to lower killer whale populations.

"Killer whales have a broad world-wide distribution, rivaling that of humans. At the same time, they have very low levels of genetic diversity," said corresponding author Rus Hoelzel, a professor at the School of Biological and Biomedical Sciences, in a statement. "Our data suggest that a severe reduction in population size during the coldest period of the last Ice Age could help explain this low diversity, and that it could have been an event affecting populations around the world."

"However, a global event is hard to explain, because regional modern-day killer whale populations seem quite isolated from each other. What could have affected multiple populations from around the world all at the same time?" he continued.

The researchers suggest surveying other top predators for further evidence of climate-caused disruptions, and feel their research could be relevant as we experience climate change today.

"If this is the case, then further research may suggest an impact on other ocean top predators during this time. It would also support concerns about the potential for climate disruptions to impact ocean ecosystems in future," said Hoelzel.