The misconception that sharks could be mammals likely stems from a combination of factors, including certain shared behaviors, similarities to specific marine mammals, and misconceptions about what defines a mammal.

Here are a few reasons why this confusion might arise:

Shared traits and behaviors

Firstly, let’s explore the shared traits and behaviors. Take, for example, the process of reproduction in certain shark species. Unlike most fish that lay eggs, most of the 500 shark species, including the bull shark, lemon shark, and tiger shark, give birth to live young, known as viviparity.

Even those sharks like the Thresher shark that birth via ovoviviparity — where fertilization and development of the eggs occur inside the mother’s body — results in the birth of live young. This process can superficially resemble the reproductive process of many mammals, which also give birth to live young.

Unlike most fish that lay eggs, some shark species, including the bull shark, lemon shark, and tiger shark, give birth to live young, known as viviparity. pic.twitter.com/A6auDyAZWg

— Discvr.blog (@DiscvrB) June 30, 2023

This mirrors the reproductive process seen in mammals, leading to some confusion. Moreover, a few species of shark, like the spiny dogfish, also exhibit parental care, a trait largely associated with mammals, adding to the ambiguity.

Similarities to marine mammals

The similarities between certain sharks and marine mammals can also lead to misidentification. The classic case is the whale shark. Despite its whale-like size and behavior of filter-feeding – sucking in large quantities of water and filtering out tiny plankton, similar to what whales do, it’s not a mammal.

Another example is the basking shark, which has a similar feeding style. These visual similarities to marine mammals can sometimes lead to wrong classifications.

Moreover, the iconic image of a great white shark breaching the water surface in pursuit of prey, much like how dolphins and whales occasionally leap above water, could also trigger erroneous associations.

Here are a few additional examples of similarities sharks share with mammals:

1. Warm-bloodedness: Most fish are cold-blooded, meaning their body temperature changes with the environment. However, certain shark species, such as the great white shark and the mako shark, are partially warm-blooded. They can maintain a body temperature higher than the surrounding water, which is a characteristic most commonly associated with mammals. It’s important to note, however, that these sharks are not fully warm-blooded like mammals.
2. Complex Social Behaviors: While fish are generally thought of as less social or interactive than mammals, some shark species exhibit complex social behaviors that can resemble those of certain mammals. For example, certain types of hammerhead sharks have been observed schooling in large numbers, while others, like the grey reef shark, demonstrate a social hierarchy when feeding.
3. Long Lifespan: Some shark species can live for a very long time, much like many mammals. For example, the Greenland shark has one of the longest known lifespans of any animal, with some living up to 400 years or more.

Defining characteristics of mammals

A lack of understanding about what truly defines a mammal may contribute to the confusion. Mammals are a group of vertebrates characterized by the presence of mammary glands, which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones.

These features differentiate mammals from fish like sharks, which lack mammary glands, have a different brain structure, are covered in scales, and have a single bone in their middle ear.