A chicken’s place on the family tree of life can be a little confusing. At first glance, it’s pretty simple. But the more you look, the more the answer becomes muddled.
It shares attributes to both the mammal and reptilian branches, so one could make an argument for both of these. Yet despite these commonalities, a chicken is neither a mammal or a reptile. A chicken is classified as a fowl: a family branch of its own distinction.
A chicken is a warm blooded animal, meaning body temperature is maintained internally. In the case of your favorite backyard egg-layer, normal body temperature is between 105 and 107 degrees Fahrenheit.
Ask anyone who has had a fever, and a temperature of 105 is just no fun! But this is not the real difference between the fowl family and mammals.
When it comes to staying warm, chickens use their combs and wattles (those fleshy pieces of red) and feathers to control body temperature. Mammals have sweat glands to keep them cool and hair to keep them warm.
Now you could make the argument that baby chicks are covered with fuzzy down. And the fuzz certainly looks and feels like it could pass for hair – at least to me. But chicks don’t nurse; another attribute of mammals.
Mammals generate milk to nourish their young. Chickens do not.
A baby chicks first meal will be pretty much the same food they’ll eat their entire lives. They do have to be shown by the mother hen – ‘this is food and this is water’ – otherwise they will die for lack of nourishment. But chicks do not nurse.
Gestation for the soon-to-be chick, happens outside of the body. The fertilized egg is not carried internally as is with mammals. It’s true that temperature plays a role in the development of the chick, as in the case of mammals, but this happens in a nest and not in a womb.
Reptiles lay eggs and some will make the argument that chickens should be part of the reptile family. But reptiles are unable to maintain their body temperature. If it’s cold, then the reptile will be sluggish. If it’s really cold, then some species of reptile will hibernate.
A chicken will never hibernate – another crucial difference defining species.
Interestingly enough, some mammals do hibernate, adding to the confusion of what’s what!
And if that’s not enough confusion, fish lay eggs. Sometimes these eggs are hidden in a safe place and then other times, the eggs are carried internally – meaning when the fry hatch, they do so internally and then exit their mother in a sort of delivery process.
In a mammal, when gestation is complete, the new offspring will travel through the birth canal to be delivered. When it comes to chickens, delivery has already happened. What is needed now is for the bird to break free of the egg shell. To do this, they utilize an egg tooth.
An egg tooth is a small boney ‘tooth’ that hangs on the end of chick’s nose. This tooth has one purpose and will fall off shortly after the chick has broken free of the egg shell.
Reptiles also use an egg tooth. In this regard, the fowl and the reptile share an attribute, just like the mammal and the fowl share a common ability to regulate body heat.
A Chicken Has Feathers – Not Hair, Not Scales
As mentioned above, chickens have feathers. Feathers are a unique identifier of the fowl species.
These incredible epidermal growths are as complex as they are unique. From their wide range of shape, consistency and coloring, feathers define the bird; whether it’s a hummingbird, an ostrich or a penguin.
To my knowledge, no current living species of mammal or reptile have feathers. Arguments are made regarding species of the past – but that’s a different article. The fowl of today have feathers.
A bird’s beak could be assumed as a defining feature of the fowl family. But incredibly, both mammals and reptiles share this attribute; not in shape but in composition.
The substance forming a bird’s beak is called Keratin. This substance is perfect for things that see a lot of wear as it never stops growing.
In the case of reptiles, the turtle’s beak is made of Keratin. But mammals see the biggest use of it as the substance is found in horns, hooves, nails and even human skin.
A parrot fish, named for its amazing resemblance to the parrot, also has a beak. And while its beak grows continuously just like in the case of the chicken, the beak on the parrot fish is actually made of bone.
The bones of a fowl are uniquely hollow. This maximizes strength for weight. And while it was commonly believed that some reptiles had hollow bones, their skeletal structure is not the same.
With the case of fowls, they actually have air sacs that sit within their lightweight frame. These air sacs will fill and deflate by input from their respiratory system.
To help visualize this, take a deep breath. The more air you bring in, the tighter and stiffer your chest cavity becomes.
With fowl, some of these air sacs are actually inside the bones. These pneumatized bones will fill with air – pressuring them and thereby adding strength and rigidity while not adding weight.
Also, specific to the fowl, is the wishbone. Ever pull apart a turkey wishbone at the Thanksgiving dinner? With any other species, you would not be able to do this. But in the case of fowl, their collar bone is uniquely fused.
Nature is full of examples where different species share characteristics. And while there are some incredible similarities, chickens are neither a mammal or a reptile. They are a part of the fowl family; a branch of species all its own.
Food for thought: If you really want a great example of animals sharing attributes, check out the Platypus. But be warned. If you thought fowls were confusing, then the platypus will probably leave scratching your head…for days!