The round, crater-like marks on whales are most likely made by cookie-cutter sharks in the genus Isistius. The shark attaches to its much larger prey with suction, twists, and uses its lower teeth to remove a plug of flesh. A completed bite leaves a circular crater; an interrupted bite can leave a crescent-shaped mark.
Why do cookie-cutter shark bites look circular?
The shape comes from the shark’s unusual way of feeding. Rather than taking a conventional bite and tearing off a strip, a cookie-cutter shark grips an animal’s skin and cuts out a small, rounded piece of tissue. The resulting wound can look strikingly neat, but “perfect circle” is a visual description, not evidence that every mark has identical dimensions or edges.
Smithsonian Ocean describes the shark attaching by suction, twisting its body, and biting with its lower teeth. The circular wound is the mark left by that specialized action. If the shark does not complete the bite, the wound may instead appear crescent-shaped. Smithsonian Ocean’s cookie-cutter shark overview describes the feeding mechanism and the animals known to be bitten.
Which shark is responsible for the marks?
The leading explanation is a cookie-cutter shark, a small shark in the genus Isistius. “Tiny” is relative to its prey: these sharks can feed on much larger animals, including whales, dolphins, tuna, and seals. The wound shape and the shark’s feeding anatomy support the attribution.
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In a 2016 analysis of historical whale observations at Donkergat, South Africa, Best and Photopoulou reviewed the crater-like wounds and considered possible causes. They concluded that Isistius was the most likely source. This is a well-supported attribution, not a claim that the researchers witnessed each bite as it happened. Earlier proposed explanations included barnacle displacement, parasites, lampreys, remoras, and microorganisms; the cookie-cutter shark better fits the wound characteristics and its specialized bite. The 2016 PLOS ONE analysis explains that assessment.
What do observations of whales tell scientists?
Wound records show that cookie-cutter shark bites are not limited to one whale population or species, though the evidence often comes from marks on animals rather than observed attacks.
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- Across Gulf of Mexico cetaceans: NOAA Fisheries’ 2021 summary of a study published in 2018 reports evidence of cookie-cutter shark predation in 12 of the 19 cetacean species evaluated. NOAA Fisheries’ summary discusses the findings.
- On short-finned pilot whales in Hawai‘i: A 2025 study recorded 9,293 fresh, healed, and scarred bite marks on 396 of 403 whales in its study sample. That is a result for the whales examined, not an estimate of how common bites are among all pilot whales. Fresh marks were described as pink or red crater-shaped depressions. The 2025 study in Marine Biology reports the observations.
These records help establish where bites occur and what they look like, but a scar alone does not reveal the moment of attack. Even fresh marks provide clues rather than a direct view of the shark feeding.
When do the bites happen?
The 2025 Hawai‘i pilot-whale study found that the probability of observing bites varied with season, lunar phase, and sea-surface temperature. The authors also estimated bite age and the timing of wound colonization; those estimates are not direct observations of when a shark made a bite.
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A separate 2025 study of damage to fish caught on Hawai‘i longlines found associations with fishing operations that continued later into nightfall, low lunar illumination, and fishing near the edges of the North Pacific subtropical gyre. Those patterns offer ecological context about cookie-cutter shark activity, but they are not direct evidence of when sharks bite whales. The NOAA Institutional Repository record for the longline-fishery study describes that work.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the scars do—and do not—establish
Cookie-cutter sharks remove tissue from large animals, and their bites leave recognizable wounds. The evidence does not establish that these bites ordinarily kill whales, nor does it show that every circular whale wound has been traced to a witnessed shark attack. Whale scars, observations of fresh marks, and other ecological clues help researchers infer how and where this elusive shark feeds.
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