According to evolutionary morphologist Matthew Ravosa, you can learn a lot about an animal by looking at how it chews. Fascinated by the evolution of the feeding apparatus in primates and other mammals, Ravosa wanted to know more about how diet affects jaw joints, because this might help him understand the structure of ancient fossil jaws. To probe further, Ravosa teamed up with colleague Sharon Stack at the University of Missouri School of Medicine to investigate how rabbit jaw joints adapt to soft and hard diets().
The team already knew that rabbits fed the equivalent of jaw-breaking sticky toffee for a few weeks developed stronger and denser jaw bones to support the harder-working muscles. But no-one had looked at longer term changes before, and these might not necessarily be the same as short-term changes because animals' responses to their environment changes as they age. To examine longer term changes in bones and cartilage, the team fed one month old weanling rabbits on mushy powdered rabbit pellets, or on tough hay and whole pellets.
After the rabbits had munched their way through their specialised diets for four months, the team measured the bone dimensions in two joints vital for chewing: the temporomandibular joint (TMJ), which is a hinge joint connecting the lower jaw to the main portion of the skull; and the symphysis, which is the joint between the two lower jaw bones, situated at the `chin'. This joint is fused in humans and higher primates, but unfused in lower primates.
Taking into account each animal's size, they measured the bones' external dimensions and found that the rabbits on the harder diet had 10% thicker bones than their peers on the soft diet. Soft-diet rabbits don't use their muscles much; `it's the equivalent of sitting on the couch all day', says Ravosa, so their bones aren't as thick. Aided by engineer Stuart Stock, the team used micro CT scans to peer inside each bone, finding that the bones from hard-diet rabbits were thicker than they were expecting on the inside: by up to 20%. Micro CT also told the team how biomineralised, and therefore how dense, each bone was: hard-diet rabbits had stronger and denser bones because when the TMJ and symphysis joints are overworked, the bone bulks up to compensate.
Next, the team compared how the cartilage fared on the two diets. Cell biotechnician Ravinder Kunwar measured the amount of type 2 collagen and molecules called proteoglycans in the cartilage. These give cartilage its rubbery quality and strength, helping it resist compression. Expecting to find over-expression of these two proteins in the cartilage of hard-diet rabbits,which would help the joints cope with their extra strain, the team actually found that there was less of these proteins in the cartilage, and more cell death at the end of the bone. Wear and tear had taken its toll; `they looked like joints in older animals' says Ravosa. However, `some aspects of the bony morphology could compensate for the cartilage' he adds. In the older animals,the symphysis was beginning to fuse, which hadn't been seen before. This would stiffen the joint and overcome some of the effects of damaged cartilage.
Next Ravosa wants to understand how the jaw joints adapt in even older rabbits, to see if the symphysis fuses further. The results `could help explain the evolution of increased symphysis fusion in primates' he explains,`it could have been due to diet'.
