Sharks share waters with other large marine animals such as dolphins and whales. At a quick glance, they appear to have many similarities.
Many sharks give live birth, which can cause confusion about whether or not they can produce milk for their young.
Sharks do not produce milk because they are not mammals. They lack mammary glands and do not nurse live young outside of the womb. However, some species of sharks produce a milk-like substance within the uterus for intrauterine nutrition. Different species of sharks have other mammalian characteristics as well, but ultimately they are classified as fish.
Read on to learn more about how sharks are different from mammals, and how they are sometimes confused as being one.
Topics
Sharks Vs. Mammals
This table shows general information about the differences between sharks and mammals.
| Information | Sharks | Mammals |
|---|---|---|
| Biological classification | Kingdom: Animalia (multicellular animal) Phylum: Chordata (vertebrate) Class: Chondrichthyes (cartilaginous fish) | Kingdom: Animalia (multicellular animal) Phylum: Chordata (vertebrate) Class: Mammalia (mammal) |
| Skin | Scales | Hair, hair follicles, or fur |
| Internal temperature | Cold-blooded (ectothermic) | Warm-blooded (endothermic) |
| Mammary glands? | No | Yes |
| Care for their young | Left alone once born | Nursed and cared for until mature enough to survive on own |
| Reproduction | Viviparous (live birth) or oviparous (laid eggs) | Live birth (except for egg-laying Monotremes) |
| Respiration | Gills process oxygen from passing water; sharks stay indefinitely in water | Lungs breathe in oxygen from the air (via nostrils or blowhole); marine mammals must come to the surface of the water to access air |
| Neocortex in the brain? | No | Yes |
| Ears with three middle bones? | No, but they have small ear holes | Yes |
| Tail | Vertical fin (caudal) | Horizontal tail (fluke) in marine mammals; short or long tails in land mammals; (no tail: humans, great apes, and a few other mammals) |
| Swimming motion | Spine and tail move side to side | Marine mammals use flukes in an up and down motion |
| Appendages | Rigid fins with cartilage | Marine mammals’ flippers have bones, much like the ones found in a human’s hand |
Interestingly, some shark species do not strictly present all of the characteristics in this table.
Some sharks show mammalian or a resemblance to mammalian characteristics, especially to marine mammals such as whales. This can cause confusion as to their true identities.
Regardless, they are still fish.
Read on to learn more about these conflicting characteristics.
Mammalian-Like Characteristics In Sharks
Warm-Blooded Sharks
Five species in the Lamnidae (mackerel) shark family are warm-blooded or referred to as endothermic.
For example, the great white shark can maintain a stomach temperature of 57ºF warmer than ambient water temperature. The salmon shark maintains a body temperature of approximately 77ºF which is considerably warmer than the sub-arctic waters of its habitat.
These sharks use a network of blood vessels to elevate their internal body temperatures, retaining muscular heat.
This is different from the traditional warm-blooded mammals, which use metabolic processes (through energy and consumption of food) to generate heat.
Endothermic sharks consist of the following species:
- Great white (Carcharodon carcharias)
- Shortfin mako (Isurus oxyrinchus)
- Longfin mako (Isurus paucus)
- Porbeagle (Lamna nasus)
- Salmon (Lamna ditropis)
Whale Shark
Whales are mammals, causing confusion over the name given to the whale shark (Rhincodon typus). This shark resembles the whale only by its name and similarities in body shape.
A closer look will reveal that the whale shark has a vertical tail (caudal fin), whereas a whale has a horizontal tail (fluke).
This creature is the largest shark in marine waters, capable of growing over 65 feet long. Despite its size, it is still classified as a fish.
This video shows the majestic size of the whale shark as it swims:
Reproduction: Live Births
Sharks can reproduce in 3 ways:
- Viviparous (live birth) – most common, about 70% of all sharks
- Oviparous (lay eggs) – about 30% of all sharks
- Parthenogenesis (reproduction without mating) – rare; seen in captivity
Viviparous sharks “incubate” developing eggs in the final section of their oviduct for approximately 9 to 12 months. In many viviparous sharks, the fetus consumes yolk from within its egg casing.
Even though the baby shark (pup) is born live, it developed internally inside an egg, in a way that is much different than that of mammals.
Some sharks even hold several live pups in their womb, where one eats the siblings in order to gain nutrition and is the strongest, most skilled shark upon birth.
Once a baby shark (pup) is born, the mother does not offer additional nutrition or care, unlike mammals.
Placental Milk
Some species of sharks exhibit placental viviparity, with a yolk-placenta, called matrotrophic nutrition.
The egg case and yolk interlock in the shark’s uterus, creating a nutritional transport surface area much like how an umbilical cord functions. This lipid-rich uterine milk-like fluid is created sending additional nutrients to the developing fetus.
This is not the same as milk that is produced from mammary glands after birth as seen in mammals.
Milk Shark
The milk shark (Rhizoprionodon acutus) is a fish that does not produce milk.
It is named after some regional beliefs that eating the shark’s flesh will induce or promote lactation in human mothers.
This particular species uses placental viviparity in supplying unborn pups with nutrition, thus also promoting the idea that it offers nutritional milk.
However, the milk shark is a cold-blooded fish, and not a mammal.
In Conclusion
While sharks can bear some resemblance to mammals with live births, milk-like substances, confusing names, and some are warm-blooded, they do not produce milk.
Sharks do not have mammary glands, nor do they nurse their born pups.
The majority of sharks are cold-blooded (ectothermic) fish. They are also covered in scales, swim in a side-to-side pattern, and breathe using gills.
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