Difference Between Fish and Mammals

Fish vs Mammals

These are two large and distinct groups of animals and the differences are prevalent than the similarities. It is important to explore some marked differences between mammals and fish. Usually, the fish live totally in water and the mammals are mostly terrestrial. However, the living habitats of some glamorous mammalian species, viz. dolphins and whales, live in water while there are fish species with capabilities of ‘walking’ on ground and surviving without water for months. In addition to these adaptive radiations of both mammals and fish, the scientific base for classification is important.


Mammals evolved with birds, from the reptiles. And the most evolved group of animals are the mammals with more than 5650 extant species. They are warm-blooded, that means body temperature of mammals is maintained at a constant level without changing it according to the environmental temperature. Therefore, the physiology of the mammals does not change and which has been extremely important for them to survive long ice ages without being extinct. It is a bony skeleton in all mammals, and sometimes there are cartilaginous structures present within bones. The gills are present during the foetal stage and after birth the lungs become functional until the death. Some of the most interesting features of mammals are the presence of hairs, sweat glands, and mammary glands. The name, mammals, is because of the presence of mammary glands which are functioning to feed the newborns. With the presence of mammary glands along with some other behaviours; mammals do care on their young with great courage and love.


Fish were the first evolved vertebrates (animals with a back bone) and consist of many groups with about 32,000 extant species. They are cold-blooded, i.e. change the body temperature with the environmental temperature. There is a group of fish without jaws, and another group has cartilaginous skeletons. Usually, the oxygen intake occurs through gills which are present throughout the lifespan. The skin of fish is covered with scales and there is a lateral line present on a series of scales. The lateral line is a sensitive for the water movements which are useful for their lifestyle (foraging, mating…etc). The fins are the structures developed in fish for their locomotion. With the dorsal, anal, caudal, pelvic, and pectoral are the types of fins named according to the location on body and they function in moving through different directions and balancing in the three-dimensional aquatic habitat. Some fish species demonstrate parental care and which is a higher vertebrate feature. Scientists have discovered that, some fish species communicate via sounds which can’t be heard for humans. Fish come in all the sizes up to 15 metres long whale shark.

Fish vs Mammals

- Being vertebrates, both fish and mammals are highly evolved taxonomic groups.

- The number of fish species is comparatively higher to mammals. But the mammals are more evolved than the fish.

- The adaptive radiations of the mammals to different environments are predominant and successful with dolphins and whales are adapted for aquatic habitat, bats have developed wings to fly, primates have a developed and large brain to conquer the terrestrial habitat.

- Fish are cold-blooded, scaly bodied, laterally flattened bodied, and finned whereas the mammals are warm-blooded, hairy bodied, not flattened, and mostly four-legged in the body forms.

Both fish and mammals are equally important for the humans in umpteen ways.