Living Tissue
Black Rhinoceros
Diceros bicornis
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Skulls are often associated with death, but like all bones, they are composed of living tissue.† Constantly changing, they grow larger, stronger or weaker depending on age, diet and exercise.† They can also heal themselves when broken.† Because of this, a skull is like a good book - it can record the major events and traumas in an animalís life.† Scientists read skulls to determine how long individuals lived, how healthy they were, and how they died.

Sea Lion
Sea Lion Skull


Q: How do bones grow?
All bones have construction zones called growth plates, where special bone-building cells work throughout an animal’s life. These cells, called osteoblasts, use blood-borne tools like calcium and vitamin D to construct and strengthen bone tissue.

Raymond Bandar
Field Associate
Department of Ornithology & Mammalogy

When they are born, male and female sea lions have very similar skulls. However, once they are old enough to reproduce, the males develop a large sagittal crest – a bony ridge where the jaw muscles attach to the top of the skull. In sea lion colonies, only the dominant male mates with the females. Males use strong jaw muscles to bite one another during competitions for that opportunity.

California sea lion skulls continue to grow in both length and width up through about age ten. By age fifteen, all of the sutures, or seams between the skull’s bony plates, have usually fused together completely. The males have an additional age marker – the sagittal crest along the top of their skulls begins to grow around age five and continues growing until about age ten.

Q: How do you determine a sea lion’s age from its teeth?
DL: Sea lions secrete a new layer of cement on the insides of their teeth each year. When one of these marine mammals dies, scientists can look at a cross section of a tooth, much like they do with the growth rings on trees, to determine its age.

Dr. Douglas J. Long
Collections Manager and Acting Department Chairman
Department of Ornithology & Mammalogy

Q: How did this sea lion die?
RB: A fisherman’s gill net left an incriminating line of evidence on this skull. The entangled youngster suffered a long cut along its brain case and lived long enough for the bone to begin regenerating, but ultimately it did not survive.

Raymond Bandar
Field Associate
Department of Ornithology & Mammalogy

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