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Why Does the Walrus Look Like a Seal but Share DNA with Sea Lions?

Category: Living Things

Sea lions have small external ear flaps visible on the sides of their heads. The walrus, an animal that also swims with flippers, does not have these flaps [1]. However, studies using DNA suggest that walruses are more closely related to sea lions, which have ear flaps, than to seals, which do not [3]. This creates a conflict between physical appearance and genetic lineage. By looking at this discrepancy, we can explore the possible paths that led to the loss of external ears.

1. How Ear Flaps Divide Pinnipeds into Sea Lions, Seals, and Walruses

Marine animals that swim using flippers are classified as pinnipeds. This group is divided into three families: Otariidae (sea lions and fur seals), Phocidae (true seals), and Odobenidae (walruses). Today, only one species in the Odobenidae family, the walrus, still exists [1]. Sea lions and fur seals have small external ear flaps visible from the outside. True seals and walruses do not have these flaps [1]. In Japanese explanations, it is often stated that sea lions and seals can be distinguished by the presence or absence of these ear flaps [7]. If we classify walruses solely by the presence of ear flaps, they fall into the same category as true seals. However, when we look at other parts of the body and genetic relationships, the story changes.

2. The Walrus Body Mixes Traits of Sea Lions and Seals

The walrus can move on land using four limbs, turning its hind flippers forward to walk, much like a sea lion [2]. This is a sea lion-like trait. On the other hand, when swimming, the walrus is said to use a method similar to true seals, wriggling its entire body. The lack of external ear flaps is also a trait shared with true seals [2]. Thus, a single walrus body contains a mix of sea lion-like and seal-like characteristics. This raises the question: which group is the walrus more closely related to in terms of lineage?

3. Why Molecular Data and Fossils Place Walruses Closer to Sea Lions

In the past, classification relied heavily on comparing body shapes. Some of these analyses placed walruses closer to the seal side [4]. Currently, the mainstream view is based on molecular data, such as DNA. A study that examined 129 taxonomic groups by combining molecular data and fossils suggests that the walrus family and the sea lion family form a group called "Otarioidea" (eared seals). According to this study, they split from the true seal family about 20.1 million years ago [3]. However, this date is an estimate. The debate has not disappeared. There is still disagreement about whether the true seal family diverged before or after the walrus [4].

4. Did Walruses and True Seals Lose Their Ear Flaps Separately?

If walruses are indeed close relatives of sea lions, it can be interpreted that having ear flaps was the ancestral state. In this view, the true seal family and the walrus family each lost their external ears independently. This is an interpretation based on molecular phylogeny [1]. When different lineages develop similar traits, it is called "convergence." It is possible that similar changes occurred to adapt to life in the water. A commonly cited reason for losing ear flaps is water resistance. Protruding ear flaps increase drag while swimming, so losing them may be better suited for marine life [1]. This is one hypothesis and has not been directly verified in walruses.

5. How Walruses Hear Underwater Without External Ears

Even without external ear flaps, walruses can hear underwater. A study measuring underwater hearing in one Pacific walrus found that the most sensitive frequency was around 12 kHz, with a good hearing range of about 1 to 12 kHz [5]. Since the measurement was taken from a single adult, it cannot definitively represent the traits of all walruses. In 2026, a peer-reviewed paper was published investigating how "amphibious hearing" evolved in pinnipeds that hear in both air and water [6]. As this is a new study, details should be checked in the original paper. However, how sound from water reaches the inner ear in a head without external ears is still considered an area of ongoing research.

6. How to Compare Sea Lions, Seals, and Walruses in Photos

Try comparing the heads and hind flippers of sea lions, true seals, and walruses in encyclopedias or photos. If you make a table for the three species comparing "ear flaps," "direction of hind flippers," and "swimming style," you can see where they are the same and where they differ. If you visit an aquarium or zoo, observe the animals quietly from behind glass or fences so as not to startle them. After comparing them, you can form your own prediction about which group walruses are closer to and verify it using the sources listed in this article.

Sources

  1. Wikipedia, "Pinniped" https://en.wikipedia.org/wiki/Pinniped (Describes the three pinniped families, the presence of external ears, and the link between ear loss and streamlined bodies.)
  2. Wikipedia, "Walrus" https://en.wikipedia.org/wiki/Walrus (Details walrus hind flipper orientation, swimming style, and lack of external ears.)
  3. Evolution journal, "Charting the course of pinniped evolution" https://academic.oup.com/evolut/article/78/7/1212/7655703 (Discusses the Otarioidea grouping and estimated divergence dates from true seals.)
  4. Wikipedia, "Eared seal" https://en.wikipedia.org/wiki/Eared_seal (Notes that debates regarding pinniped classification persist.)
  5. Kastelein et al., J. Acoust. Soc. Am. https://pubmed.ncbi.nlm.nih.gov/12430829/ (Reports underwater hearing measurements for a single Pacific walrus.)
  6. Proceedings of the Royal Society B, "The origin and evolution of amphibious hearing in pinnipeds" https://doi.org/10.1098/rspb.2026.0178 (A peer-reviewed study on the evolution of amphibious hearing in pinnipeds.)
  7. Marine Diving web, "How to tell sea lions and seals apart" https://marinediving.com/marine_life/serial_marine_life/sealion_and_seal/ (Explains the distinction between sea lions and seals based on ear flaps in Japanese context.)