What Animals are not Mammals?

Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands, which in turn produce milk for the nourishment of the young. The defining characteristic of mammals is the endothermic metabolism that allows for sustained life with a defined generation time.

A closer examination into the genetic makeup reveals that all mammals have unique corresponding DNA alterations compared to other vertebrates.

These alterations allow for variations among species but also generate identical proteins across mammalian species.

Class Mammalia contains about 4500 species including humans, whales, dolphins, and bats. However, there are 5 classes within this classification, each containing various subgroups made up from orders which contain many families.

The five mammal classes include monocytes (egg-laying), marsupials (pouched), Eutheria (placental, live birth), Chiroptera (bat-like) and cetacea or whales, dolphins, and porpoises.

The following animals are not mammals because they do not fall into any of the subgroups within the class Mammalia:



Fish – fish are members of a group of animals that consist primarily of cartilaginous fishes (sharks, rays and chimeras).

They are aquatic vertebrates lacking limbs with digits; most use their gills to take in oxygen from water, although some species can breathe air. Fish do not depend on lungs to breathe oxygen like other vertebrate species.


Amphibians – Amphibians are cold-blooded ectothermic vertebrates that undergo metamorphosis, and along with fish and reptiles, are on their way to evolving into amniotes.

Amphibians differ from other tetrapods in several ways: the first pair of ribs is usually absent, skeletons and internal organs are generally covered by a non-protective amniotic membrane; they produce either eggs or live young (usually both).


Reptiles – these creatures lay terrestrial hard-shelled eggs rather than laying water-borne soft-shelled ones as do fish and amphibians. They also have scales as an external covering as opposed to naked skin as do mammals, birds, and amphibians.

Scientists believe there were at one point aquatic reptile species living on land but over time they morphed into modern-day reptiles, such as lizards and snakes.

They also believe that snakes were once limbless creatures but over time developed them to be able to move quickly through the air and on land.


Saurians – this group of warm-blooded tetrapods evolved during the Jurassic period and became extinct at the end of the Cretaceous.

The last mesosaurs died off around 65 million years ago, just before the mass extinction event took place which destroyed all terrestrial dinosaurs including Tyrannosaurus Rex and Triceratops.

However many saurian species survived until the K-T Extinction Event occurred 65 million years ago wiping out almost all forms of life on earth except for certain plants, insects, amphibians and fish.

Some scientists believe if the K-T Extinction Event had not occurred these creatures may have evolved into intelligent life and humans as we know them would never exist.

Placental mammals

Placental mammals – the placenta is a specialized organ that provides for nutrient uptake, gas exchange, and waste removal from a developing embryo as an alternative means to a maternal source of nutrients and oxygen when internal gestation is physiologically impracticable. The placenta allows for embryonic diapause (resting).

All non-marsupial eutherian mammal species develop a placenta, which serves as the only source of energy and nutrition to growing fetuses during pregnancy.


Marsupials – this diverse group of animals has two distinctive features: pouches and teats/nipples on their stomach for their young to feed on.

Marsupials are born at a very early stage of development and most species carry only one baby at a time in order to maximize the amount of care it receives.

This is achieved by having an immune system that gives the newborn protection against infection, adequate space to crawl around so they develop proper muscle tone and sufficient nutrition before birth.

Common misconception: bats are not mammals

Many people believe that bats are not mammals due to their highly developed sense of hearing which comes from having special bones in their ears or membrane-like skin wings made of stretched skin between finger bones, allowing them dive. Also the answer here tells us that birds evolved from dinosaurs who were reptiles which mean bats cannot be mammals.

Scientists believe birds are dinosaurs that evolved over time into what they are today.

Due to the fact that bats have wings, specialized ears, and their own unique species classification they too must be considered a mammal due to having mammary glands for breastfeeding their young much like mammals. Many bats live in colonies with pups nursing off of their mothers until they are old enough to fly on their own.

Some scientists disagree with the above findings because there is no fossil evidence proving bats were ever warm-blooded, however by looking at bats’ internal organs one would notice that there is a structure similar to those found in placental mammals.

It should also be noted that even though reptiles lay eggs so do some amphibians and fish but scientists consider them all tetrapods as well as mammals because of their amphibious habits which makes them tetrapods.

Common misconception: octopuses are not mollusks

Many people believe that octopuses are not mollusks due to the fact they don’t have a hard shell like other mollusks such as a clam, snail or oyster do.

Also, many people fail to realize that it’s been scientifically proven over time that octopus evolved from a snail-like ancestor with shells and morphed into what we know today as an octopus due to their ability to adapt quickly changing environmental conditions through evolving by adapting specific traits which make them unique from all other species on earth today.

It should also be noted that being called an octopus, octopi, or octopus is all the same.

Common misconception: mosquitoes are not insects

Many people believe that mosquitoes are not insects due to their ability to fly and suck blood from other animals which in return causes them pain and suffering.

Also, many people fail to realize that it’s been scientifically proven over time that mosquito evolved from a wasp-like ancestor having wings and morphed into what we know today as a mosquito due to their ability to adapt quickly changing environmental conditions through evolving by adapting specific traits which make them unique from all other species on earth today. It should also be noted that being called mosquitos, mosquitoes, or skeeter is all the same.

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