What animals are mammals?

In this article, we discuss everything there is to know about mammals. We explore the taxonomical classification of mammals, learn who they are, and find out the specific traits that distinguishes them from other classes of the animals kingdom.

What animals are mammals?

Mammals are members of the animal kingdom characterized by having some of these distinct traits: mammary glands for suckling their offspring, hair or fur in any of their lifecycle stages, three middle ear bones, a neocortex region of the brain, and the ability to generate their own body heat and maintain a constant internal temperature (or are endothermic and homeothermic respectively)

Taxonomical classification of mammals

Mammals are the pinnacle of evolution. They are the most advanced and intelligent lifeforms of the animal kingdom, albeit, their total specie diversity is nothing compared to non-mammals such as birds and fishes. There are currently over 5000 species of mammals identified from which more than 1300 genera, 135 families and 25 orders have been drawn out.

All animals that belong to the class ‘Mammalia’ are called mammals and virtually all of them have a spinal cord surrounded by a cartilage or bone, or simply, they are vertebrates.

In short, here is the taxonomical classification of mammals. If you wish to understand more about animal grouping please read our article explaining Why insects are animals under the heading “Why are insects regarded as animals”.

Classification of mammals

  • Domain (Eukarya)
  • Kingdom (Animalia)
  • Phylum (chordate)
  • Sub-phylum (Vertebrate)
  • Class (mammalia, non-mammalia classes include birds, fish, reptiles, amphibians, arthropods and Invertebrates)

Splitting of mammals from non-mammals

A mammal is any vertebrate animal that is not a bird, reptile, amphibian or arthropoda; these are the remaining class of animals other than mammals classified under the phylum chordate.

The first mammals to have ever lived are believed to have evovled between 200 to 225 million years ago and the modern ancestors of mammals are believed to have existed around 65 to 55 million years ago.

The former period coincided with the period when dinosaurs walked the surfaced of earth and the latter period when they were underwent mass extinction to be wiped out completely. 

Mammals first diverged from birds and reptiles around 300 million years ago, and the archaic species as at then were very small, with long heterodont dentition and a triangular arrangement of molar cusps designed for shearing. They were most probably agile night creatures that resembled today’s insectivores.

The characteristics of mammals

Mammals have many autapomorphies that help set them apart for animals of other classes. These distinctive traits can only be pinpointed out from the mammalian class and nowhere else in the animal kingdom, although not all members of the mammalian class would still posses such trait.

We will start by listing out these distinctive traits, and then move onto general traits found on most species of mammals, but can also be found on other animal classes such as fish, amphibians or even birds. Like the autanophorphies, these traits too are common to the vast majority of mammals but may or may not be common to all species.

The autapomorphies of mammals

1) Body hair

Only mammals posses body hairs in at-least one stage of their life cycle be it embryo stage, early years after birth, after maturity or during full-time adulthood.

Humans and many other mammals have body hairs in the form of whiskers, quills, horns and coats which are only prominent in their young and adult stages while others such as the cetacean family develop most of their body hairs during the ambryo stages, although some species like whales still retain very little amount of body hairs on their faces or dorsal regions after birth.

Body hairs on mammals can serve as a means of sensing and perception as in the case of whiskers, for defense as in the case of horns and quills and for protection, camouflage, social signaling and thermoregulation as in the case of pelages and thick coats.

2) Neocortex region of the brain

Only mammals are known to posses a neocortex region of the brain who’s primary function is to aid higher order brain functions such as sensory perception, generation of motor command, spatial reasoning and cognition (among a few others). In humans, the neocortex is responsible for language.

The neocortex region is also called the neopallium, or isocortex.

3) Sweat gland

Only mammals have sweat glands which are tubular structures on the skin that produces sweat. Several sweat glands can be modified to form different features such as mammary gland (for breastfeeding), ciliary gland (for secreting sebum to keep eyelashes supple) and the ceruminous gland (for producing ear wax).

4) Mammary gland

Mammary gland is simply another sweat gland modified for breastfeeding young. Specifically, it is a modified apocrine sweat gland that produces milk for suckling of their offspring. Functional mammary glands are mostly found in female mammals, but on rare occasions, males, for example the the male dayak fruit bats, can have milk-producing mammary glands for nurturing of offspring after they are given birth to by the females. Talk about super father!

Mammary glands can be protected in and extended into externally visible organs such as breast (in primates), udder (in ruminant animals), and dug for animals such as dogs and cats.

These organs can have teats or nipples for allowing young to suckle out mill at will or have tiny perforations to forcefully squirt out milk for the young to lick or catch.

5) Auditory ossicles

Only mammals have three middle ear bones referred to as auditory ossicles. The bones are; malleus, incus, stapes, and the form one of the smallest bones in the mammalian body.

These bones are responsible for better and sharper auditory perception in mammals.

6) Single lower jaw bone

Only mammals have a single bone in their lower mandible (lower jaw) called the dentale, which articulates with the skull and provides strong bites of chewing and masticating of food.

7) Molars

Only mammals have molars as part of their dentition. The molars are set of individual tooth with flat surfaces to aid in proper chewing and grinding of food items.

8) Nostrils and a nose

Only mammals have nostril opening at a common structure identified as a nose. Note that some animals don’t even have nose for example members of the whale cetacean family who primarily breath out using structures called blowholes.

10) Four chambered heart with left aorta being persistent

Only mammals have four chambered heart with the left aorta being persistent. Bird too have four chambered heart, but only their right aorta is persistent. Other animal classes such as the reptiles and fish don’t even have a four chambered heart.

11) Muscular diaphgram

Only mammals have been observed to posses a muscular diaphgram.

12) Monophyodont dentition

Only mammals have been observed to switch between two types of dentition at different lifecycle stages. Humans and other members of example, switch from milk or deciduous teeth present during their early days to stronger permanent teeth as they grow much older.

13) Voice organ in larynx with several pairs of membranous muscles

Only mammals have their voice organ in the larynx having several pairs of membranous muscles.

The traits common to majority of mammals but can also be found elsewhere in the animal kingdom.

14) Endothermic and homeothermic

Most mammals are endothermic and are homeothermic as well. This means that they are able to generate their own internal heat and be able to maintain a stable internal body temperature (using this heat) and regardless of external influence.

The famous mammalian exception to this rule is of course the sloth which is arguably ectothermic and as a consequence behaves like a poikilothermic animal; depending on behavioral and postural adjustments (or rather, relaying on external influence) to regulate body heat.

15) Nurturing of young

Virtually all mammalian species nurture their young ones to independence before abandoning them to fend for and defend themselves. During he nurturing days, the nurturers, which could be either or both parents; either which is mostly the female parent, feed, train, care for and offer protection to their vulnerable and care-free young.

They suckle them milk from their mammary glands and teach them basic survival and foraging skills.

At independence, the offspring are abandoned and expected to put to practice whatever feeding and survival skills they’ve garnered from their mentor in order to survive on their own and find food.

Mammals aren’t the only ones that nurture their offspring for extended periods after birth, animals like birds too care for their young ones until they are able to forage and defend themselves.

16) Giving birth to live young

Most mammals give birth to their young ones alive. The young can either be anatomically complete or underdeveloped which would require another system called a pouch to complete development.

The mammalian exception to this rule are monotremes, which lay eggs in order to give birth. Only two living species exist in this as you would find in the next section below.

The other animals classes that also give birth to live young ones aside mammals include some species reptiles, fish and insects.

17) High metabolic rate

Mammals have a high metabolic rate which is the consequence of being endothermic and homeothermic. The energy demand of mammals is massive because of how complex their bodies are and so need to generate and use huge amount of heat to fuel and maintain their body processes. As a result, their metabolism is high and thus must eat food consistently to stay alive and active.

Most mammals have high metabolic rates but bird which is another animal class entirely also has a high metabolic rate.

Types of mammals

The mammalia class is broadly split into three major clades based on mode of reproduction and pattern of embryonic development. Of course, other features and traits exists and can be used to distinguish members of one group from the other.

The three major clades of mammals are:

  • Monotreme mammals (prototheria first beast).
  • Marsupial mammals (methateria changed beasts)
  • Placental mammals (Eutheria, true beasts)

The later two clades are denoted as Theria: while monotremes are denoted as Prototheria. Prototherian mammals evolved first before placentals and marsupials.

1) Monotreme mammals

Monotremes are the most primitive of the three clades of mammals. They evolved from the mammalian linage right before marsupials and placentals, about 150 million years ago. 

Monotreme mammals are notable for their weird mode of reproduction which involves laying eggs from their cloaca that are rich in yolk and incubating them for around 10 to 11 days.

They inherited this mode of reproduction from the ansestral aminote (reptile) group they evolved from, and only made little changes to it.

Even the structure of their reproductive system resembles that of this archaic ancestor; the ovary is large and short with oviducts coming via paired uteri into a broad vagina, which opens with the urinary bladder and rectum into a common cloaca. 

Like all other mammals though, monotremes suckle their young after giving birth to them in a nest. Because the mother lacks any true breast, the milk is sprinkled on the face of the young and it’s left for the young to lick or catch the droplets.

2) Marsupial Mammals

Marsupials are the second primitive of the mammalian clade. They, instead of laying eggs like their ancestral amniote and monotremes, under go intrauterine embryonic development similar to placental mammals and give birth their young ones alive.

The main difference between marsupial reproduction and that of placentals is the stage at which their babies are born.

Marsupials give birth to an altricial young, around two week of age, which resembles a fetus. The young then crawls up to through the mothers abdominal hairs and reach for her extendable pouch and begin to suckle milk form the teats located in them.

They complete their embryonic development there and remain there until they are able to survive the outside world on their own.

3) Placental mammals

Placental mammals are basically mammals that give birth to an anatomically complete young. The young ones upon birth, are given teats to suckle milk, and are well taken care of until they reach independence.

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Cite this Article (APA Format)

Bunu. M. (2020, May 26). What animals are mammals?. Retrieved from http://emborawild.com/what-animals-are-mammals/

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