Many animals do not have a skull, but all mammals do. The shape of a mammal’s skull and head depends on the animal’s lifestyle – what it eats and which senses are the most important (the ears, the eyes, or the nose).
A mammal’s skull is a robust and bony box that supports the main sense organs – ears, nose, and eyes – and protects the brain. It also provides a firm anchorage for the lower jaw.
The sensitive brain, which processes all the messages from the sense organs, is housed in a special protective rigid box of bone – the cranium.
The rest of the skull comprises the face, which contains the sense organs, the movable lower jaw, and muscle attachments (consisting of a single bone).
Although the skull of each mammal species varies, the basic structure of a skull is the same in all mammals. More than 30 different bones are fitted together in a complex three-dimensional jigsaw. The lines where the bones interlock are called suture lines.
The sutures are not fused in a young mammal, whether a human baby or a badger cub. This allows the separate bones to grow, and so the skull can increase in size as the brain develops and grows within it. The head grows quicker than the rest of the body and reaches full size before a mammal is fully grown. When the skull is full-sized, its bones fuse along the suture lines.
The cranium is the name given to the prominent bones at the back half of the skull; they make a solid box that houses the delicate brain. The only openings in the box are small holes that allow the nerves and blood vessels to enter the skull and a large hole at the cranium base. This is where the spinal cord links with the brain.
The bones surrounding this hole are connected to the swivel mechanism, formed by the first two neck bones, that allows the head to move.
Biggest And Smallest Brains
The sperm whale has the biggest brain of any mammal, but this does not make it the most intelligent species. In general, the bigger a mammal is, the bigger is its brain – but the proportion of the bodyweight goes down as the mammal grows larger. Shrews have the smallest brains, but they are about as big as human brains compared to their size.
The cheekbones form an arch (the zygomatic arch) that comes from the side of the braincase and curves forwards to join the side of the skull above the back teeth. In most mammals, the widest part of the skull is across the cheekbones.
Weasels and shrews, on the other hand, are small mammals adapted for getting in and out of small holes. Their cheekbones start near the front of the braincase. The whole skull is relatively narrow and allows the weasel or shrew to investigate a narrow opening.
The stoat is similar to the much smaller weasel, but its greater size restricts it from entering small holes after prey – water vole holes and rabbit burrows are its size limit.
The cheekbones help protect the eyes and provide the hinges for the lower jaw. In the front area between the cheekbones and the skull, the orbit or socket is where the eye fits.
How badger skulls work
At the back of the cheek arch of the skull, there is a groove on the underside where the hinge joint of the lower jaw fits. In the case of the badger, this hinge is such an excellent fit that it will not dislocate unless fractured. This gives the badger a firm bite.
Once a badger has got its teeth into something, it is almost impossible to make it let go; the jaws clamp together with an unshakeable grip. The real power in a badger’s jaw comes from the strong muscles. The primary jaw muscles run from the angle of the jaw and are attached to the side of the braincase.
The bony ridge (the sagittal crest) along the top of the badger’s braincase provides a better attachment for these muscles. The jaw angle is pulled back when the muscles contract and the jaws snap shut. In mammals with powerful bites, the jaw hinge is in line with the teeth.
Herbivorous mammals, such as deer, are constantly chewing and do not need a powerful bite. However, they need a chewing action that does not tire the jaw muscles. This is why the jaw hinges are high up, a long way above the line of the teeth. This system ‘gears up’ the muscle action, giving less power but using less energy for chewing.
Deer have incisor teeth in the lower jaw only. These teeth are used to crop foliage by cutting against a leathery pad situated in the upper jaw.
Gnawing rodents shift their jaws backwards and forwards, switching from grinding to chewing and gnawing (with incisors) and biting up and down in the usual way. This is why voles and mice have a complex system of jaw muscles.
Aquatic Mammal Skulls
Seal skulls and jaws are similar to those of land-living carnivores. But the ‘full-time’ marine mammals, the whales and dolphins, have skulls with a most unusual shape – the upper and lower jaws being greatly elongated and containing many evenly shaped teeth.
The other peculiarity of whale and dolphin skulls is that the nose opening is on top of the head. This forms the blow-hole through which the mammal breathes when it surfaces. The ‘blow’ is produced by releasing compressed air from the bronchi and the other air passages after the whale has dived deep.
The palate bones are unique to mammals that make a roof to the mouth and separate it from the nasal cavity. Few other vertebrates have this adaptation. It allows the mammal to keep breathing while it is eating food. A mammal stops breathing when it swallows food.