New Fossil Evidence Reveals How Killer Whale And Other Hunting Whales’ Feeding Preferences Evolved

The origins of the killer whale ecomorph

Summarized by Lara Novalvos, a senior at the University of South Florida, majoring in Marine Biology and with a double minor in Geology and Environmental Science and Policy. After graduation, she expects to earn a Ph.D. in Oceanography. In her free time, she enjoys traveling, reading, and working out.

What was the hypothesis being tested: Scientists in this study are testing the hypothesis that hunting mammals is a trait that evolved more than once in whales and dolphins. The killer whale (Orcinus orca) and the false killer whale -a species of dolphin that resembles killer whales in feeding habits- (Pseudorca crassidens) are the only cetaceans (the group that contains whales and dolphins) that hunt marine mammals, a trait that was thought to have evolved once.

What data were used?:  A cetacean partial skeleton fossil found on Rhodes, Greece. The fossil skeleton preserved (among other bones) was its mandible (lower jaw), some teeth, and otoliths (“earstones”) belonging to fish that were eaten as the whale’s last meal.

Methods:  Morphological traits of the discovered species Rododelphis stamatiadisi were compared to those of other extinct and extant cetaceans (especially teeth count, size, and shape and upper jaw size) to build and infer an evolutionary tree that shows how the killer whale, the false killer whale, and the newly discovered Rododelphis stamatiadisi are related.

ResultsRododelphis stamatiadisi, the species found on the beach in Greece, is an extinct whale from the Pleistocene (2.59 million years ago – 11,700 years ago) that fed on fish. The results of the evolutionary (phylogenetic) analysis indicates that Rododelphis’ skull morphology is more closely related to that of false killer whales, placing them as sister taxa on the tree. However, another fish-feeding whale fossil previously discovered, Orcinus citoniensis, is considered killer whales’ closest relative. Researchers analyzed the teeth, jaws, and size of many cetaceans, extinct and extant, and concluded that delphinids (small whales with teeth such as dolphins, killer whales, pilot whales, and close relatives) evolved in six different lineages, half of them having many small teeth, and three of them having bigger, fewer teeth. From one of the lineages with few big teeth, killer whales evolved; false killer whales also appeared in a lineage of whales with big few teeth but not from the same one killer whales evolved.. These results indicate that the trait of feeding on other marine mammals appeared twice in the evolutionary history of whales, rather than having a single origin.

The figure above displays different delphinid species and its relationship based on morphological and molecular analysis. The phylogenetic tree shows a group of branches towards the middle of the tree colored in orange; these represent the lineage where the present killer whale evolved and its closest known ancestors. Towards the bottom of the tree, branches containing today’s false killer whale and its known lineage are colored in purple. The most closely related species to Orcinus orca is Orcinus citoniensis, while the most closely related species to Pseudorca crassidens is the recently discovered Rododelphis stamatiadisi; both extinct species fed on fish, whereas both killer whales species feed on other mammals, suggesting that the trait evolved in two separate instances.
The evolutionary tree containing extant killer whale and false killer whale. Orange branches show species in the killer whale’s lineage; purple branches show the false killer whale’s ancestors and lineage. Killer whales belong to a lineage that evolved before the false killer whale’s lineage did. Rododelphis stamatiadisi is more closely related to false killer whales than it is to killer whales; moreover, false killer whales are more closely related to Rododelphis stamatiadisi than to killer whales.

Why is this study important?: This study provided an opportunity to revise the evolutionary groups of the whale group. It focused on how different feeding habits evolved within the group, allowing for a deeper understanding on how the different species adapt to new environments, food abundance, or even climate change. Whaling has caused killer whales to switch to smaller prey, allowing us to observe how these whales adapt their feeding habits to different food abundance.

Broader Implications beyond this study: Research suggests that the coexistence of many large predators lead to the selection for even larger creatures; this study suggests that marine mammal predation can be correlated with whale’s gigantism. Both Orcinus orca and Pseudorca crassidens feed on marine mammals, but their most common ancestors did not.  Although the exact origin of why whales became so large is still unknown, it has been hypothesized that gigantism drove these two killer whales to develop their particular feeding preference.

Citation: Bianucci, Geisler, J. H., Citron, S., & Collareta, A. (2022). The origins of the killer whale ecomorph. Current Biology, 32(8), 1843–1851.e2.  

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