Why do orcas harass porpoises?

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Orcas, also known as killer whales, have been observed harassing and killing porpoises without consuming them, which has puzzled scientists for decades. A recent study analyzing 78 recorded interactions between orcas and porpoises from 1962 to 2020 has proposed three plausible explanations for this behavior: social play, hunting practice, and mismothering behavior1.
  1. Social play: Orcas are cognitively complex animals that engage in playful activities to bond with one another. Harassing porpoises may be a form of entertainment for the orcas, as they continue engaging with porpoises even after they've killed them1.
  2. Hunting practice: Killer whales often catch and manipulate their food before eating it, which may allow young members to learn and practice hunting skills. Harassing porpoises might help the orcas hone their hunting techniques, even if they don't intend to consume them1.
  3. Mismothering behavior: This theory suggests that orcas may be attempting to provide care for porpoises they perceive as weak or ill. Such empathetic behavior has been documented in killer whales before, such as when a Southern Resident mother carried her dead calf for 17 days1.
While these explanations provide some insight, researchers acknowledge that the exact reason behind orcas harassing porpoises may never be fully understood1.
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