While not being particularly knowledgeable in such things, I would presume the shrews. A whales muscle needs to optimise for efficiency and oxygen storage to a

While not being particularly knowledgeable in such things, I would presume the shrews. A whales muscle needs to optimise for efficiency and oxygen storage to allow for extended deep dives and continuous use. Compared to a shrew whose muscles would favour compact size, low mass, and fast reactions to permit quick getaways.