Peter Reiser is intrigued by how muscles function. More specifically, how subtle differences in the proteins that comprise muscles influence their performance. Having frequently watched squirrels raiding the bird feeder in his garden and been struck by the speed at which they nibble sunflower seeds, Reiser was curious to find out more about the rodents' super speedy jaw muscles. So, when an undergraduate project student came to his office one day at a loose end, Reiser suggested that she took a look at the jaw muscles of a flying squirrel. Having separated the multiple forms of myosin (one of the main muscle proteins) from the flying squirrel's jaw on a gel, Reiser compared the protein gel with another of grey squirrel jaw myosin and quickly realised that something strange was going on. The mixture and sizes of myosin proteins from the two animals' jaws were completely different. The grey squirrel's jaw muscles were unlike those of other rodents. With his curiosity aroused, Reiser set about looking at the jaw muscles of more squirrels to see what else he might find (p. 2511).
Fortunately, Reiser had access to a wide range of squirrel species, courtesy of a local animal control company dealing with problem rodents. Teaming up with Sabahattin Bicer, Reiser began separating the myosin proteins from the jaw muscles of rodents ranging from woodchucks, eastern chipmunks and southern flying squirrels to red and grey squirrels to find out which forms of myosin were found in the animals' jaw muscles. Looking at the myosin distribution in the grey squirrel's jaw muscles, the duo were in for a shock. It looked as if a protein that shouldn't be found in rodent jaw muscles had turned up in the grey squirrel: masticatory myosin.
Reiser explains that masticatory myosin is usually found in the jaw muscles of carnivores such as lions crocodiles and sharks, allowing them to bite down hard on prey. What is more, no one had ever found a masticatory myosin in rodent jaw muscles, so it had been assumed that rodents simply didn't have the protein.
Needing more evidence that the unusual grey squirrel jaw muscle protein was a masticatory myosin, Reiser used an antibody that only sticks to masticatory myosin to pick it out of the gels, analysed the protein's sequence with mass spectrometry, and teamed up with Qun Chen, Ling Zhu and Ning Quan to look for the protein's mRNA in the grey squirrel's jaw muscles. Every test that he tried confirmed that the unusual protein was masticatory myosin. And when the team analysed the protein composition from five other species, the protein turned up in the jaws of eastern fox squirrels, woodchucks and eastern chipmunks. Only southern flying squirrels and red squirrels lacked the protein.
So why do omnivorous rodents use a jaw muscle protein that is usually associated with the life style of a hard-core carnivore? Reiser suspects that the answer lies in the ways that the animals approach their diet. He explains that southern flying squirrels and red squirrels, which lack the protein, make small incisions in shells to get to the nut within. However, grey and fox squirrels obliterate the shell. Reiser suspects that masticatory myosin may provide grey and fox squirrels with the additional contractile force necessary to shatter nutshells. He is also curious why some squirrels produce masticatory myosin while other squirrels do not, even though their teeth and skulls are almost indistinguishable.