Hybridization of S outhern H emisphere blue whale subspecies and a sympatric area off A ntarctica: impacts of whaling or climate change?

Understanding the degree of genetic exchange between subspecies and populations is vital for the appropriate management of endangered species. Blue whales ( B alaenoptera musculus ) have two recognized S outhern H emisphere subspecies that show differences in geographic distribution, morphology, vocalizations and genetics. During the austral summer feeding season, the Antarctic blue whale ( B . m. intermedia ) is found in polar waters and the pygmy blue whale ( B. m. brevicauda ) in temperate waters. Here, we genetically analyzed samples collected during the feeding season to report on several cases of hybridization between the two recognized blue whale S outhern H emisphere subspecies in a previously unconfirmed sympatric area off A ntarctica. This means the pygmy blue whales using waters off A ntarctica may migrate and then breed during the austral winter with the A ntarctic subspecies. Alternatively, the subspecies may interbreed off Antarctica outside the expected austral winter breeding season. The genetically estimated recent migration rates from the pygmy to A ntarctic subspecies were greater than estimates of evolutionary migration rates and previous estimates based on morphology of whaling catches. This discrepancy may be due to differences in the methods or an increase in the proportion of pygmy blue whales off Antarctica within the last four decades. Potential causes for the latter are whaling, anthropogenic climate change or a combination of these and may have led to hybridization between the subspecies. Our findings challenge the current knowledge about the breeding behaviour of the world's largest animal and provide key information that can be incorporated into management and conservation practices for this endangered species.