Comparative Anatomy

Dolphins and Humans

A few days ago we went out on a small boat and saw some spinner dolphins (hopefully they are visible in some of my pictures). It is hard to believe that humans and dolphins are related given their outward appearance, but dolphins and humans have surprisingly similar body structures, and are comparable in various ways.

Respiratory System

Being both of the class mammalia, both dolphins and humans breathe in air to get their oxygen. Unlike humans however, dolphins breathe through their blowholes instead of their mouths and noses. Being able to inhale and exhale from the tops of their heads makes I easier to breath because only the top of their head needs to exit the water.


The lungs of dolphins also need to be more efficient. They are not larger in proportion to their bodies than other land mammals, but they do have many differences to make them better suited for aquatic life. Dolphins have more alveoli than humans and a double layer of capillaries. Alveoli are small sacks at the end of the lungs, and are laces with capillaries where they exchange oxygen into the bloodstream and carbon dioxide out of it. Having more of these, laced with more capillaries, make the gas exchange much more efficient and helps dolphin gain more oxygen with every breath then humans do.

The respiratory system also has smaller differences that allow the dolphin to breathe more efficiently and keep water out of its system. The tissues making up the lungs are thick and flexible, making for tougher lungs. The end of the bronchioles that lead to the alveoli, where the gas exchange takes place, have muscles that can close off the area from the rest of the lung, making the gas exchange even more efficient. The dolphin also has muscles in the blowhole that prevent water from entering when the dolphin is under water.


The circulatory system of the dolphin is also well adapted to aquatic life. Dolphin blood contains more red blood cells than humans, and each cell also contains more hemoglobin, the substance that facilitates the transportation of oxygen in re blood cells. This also makes the dolphin’s breathing more efficient and allows it to carry more oxygen to its muscles. A substance like hemoglobin is also found within the muscles called myoglobin. This attracts oxygen more readily than hemoglobin to facilitate the transportation of oxygen from the blood to the muscles and also gives the muscles the capacity to store oxygen. Though the circulatory system of the dolphin is very similar to humans, it has also evolved many adaptations to aquatic life. The dolphin also has the adaptation that allows it to reduce or fully stop the flow of blood to on-vital organs to conserve oxygen for vital organs like to heart and the brain.

Skeletal System

The skeletal system of dolphins and humans are very similar despite the differing outward appearances of the two.



The general layout of the bones is very similar in both mammals. Both have vertebra for support and a rub cage to protect vital organs. Both have a pelvis and the structure of the fore limbs is remarkable similar considering the differences in the function and appearance.  Aside from the dolphin’s lack of hind limbs, the main differences in these two skeletal systems are in the structure of each individual bone.


The backbones of dolphins are very flexible to allow for the swimming motion they use to get around. This is due to less interlocking between the individual vertebras. The neck vertebras have evolved to become very short and have mostly fused together, making the neck shorter and more rigid than a humans.


Unlike human ribs, dolphin ribs are not strongly attached to the spine or breastbone, allowing them to safely collapse under pressure during deep dives

Pelvis and Hind Limbs

The hind limbs of dolphins have disappeared through evolution, but the remnants of a pelvis indicate that at one point in evolution the dolphins ancestors did have hind legs, and, assuming they resembled the general mammalian structure of leg bones found today, they would have had close the same bones as humans

Fore Limbs

The dolphin’s bones are shorter, have phalanges (the individual bones of each finger), and the number of fingers has been reduced to four instead of five, but other than that they are very close to each other. Having a skeletal structure this similar suggests evolutionary relatedness; that dolphins and humans share some common ancestor.



Unlike most mammals’ teeth, and unlike human teeth, dolphin’s teeth are all conical.



The skull of the dolphin has been elongated through evolution. The upper and lower jaws are both unusually long for a mammal, and the skull has also been tilted upward to match the dolphin’s horizontal position when swimming. This type of skull and jaw structure also helps the dolphins use sonar.

General Structure



The general positions of the organs of the dolphin are much like that in a human. The heart and lungs are both inside the ribcage, with the liver and stomach below. The intestines then trail down to the anus near the end of the dolphin. This structure is share with most mammals and suggests a common ancestor. Dolphins brains, like humans, are relatively large in proportion to body size, though this is likely not a sign of relatedness as humans brain size increased relatively recently, much colder to present day than when humans and dolphins had a common ancestor.

Taxa (Spinner) Dolphin  Human
Kingdom Animalia Animalia
Phylum Chordata Chordata
Class Mammalia Mammalia
Order Cetacea Primata
Family Delphinidae Homonidae
Genus Stenella Homo
Species Longirostris Sapiens

Humans and dolphins had a common ancestor many millennia ago, before or during the cretaceous period. After the extinction of the dinosaurs, larger mammals became feasible and became common. The ancient ancestors of the dolphin moved into swamp niches that used to be dominated by dinosaurs and began evolving to be better suited to aquatic life. The first primates appeared around this time, 60 to 70 million years ago, so by the extinction of the dinosaurs the ancestors of humans and dolphins were already dissimilar. Any of the most recent common ancestors would have looked neither aquatic nor primate like, and though the dolphin and human have a remarkably comparable anatomy, they are likely only related by a common ancestor many millions of years ago, possibly only by one of the first mammals.



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