Before crunching into its prey, the grasshopper mouse howls. The sound is a high, sustained whistle which pierces the desert night. It is as if the rodent is imitating a wolf at miniature scale – the grasshopper mouse even stands on its hind legs and throws its head back during the shrill call. And while the rodent may cry spontaneously or emit the sound as a warning when it spots another of its kind, the grasshopper mouse regularly howls just before a kill.
The three species of grasshopper mice – all members of the genus Onychomys – are among the most carnivorous of all rodents. These are not adorable grain-eaters. Grasshopper mice are agile little predators which regularly take on prey as large, if not larger, than themselves. Insects, scorpions, lizards, and other mice make up about ninety percent of a grasshopper mouse’s diet. And, like other carnivores, they roam relatively large territories but have low population densities – a swath of habitat can only support so many hunters.
Since the diets of grasshopper mice are so different than those of their herbivorous relatives, we would expect their jaws to reflect this different lifestyle. But exactly how grasshopper mice diverge from their plant-eating cousins has been a matter of debate. In a 2006 paper on the jaw anatomy of the northern grasshopper mouse Onychomys leucogaster, anatomists Kazuhiko Satoh and Fumihiko Iwaku proposed that the carnivorous rodent had a relatively weak bite force but a wider gape compared to plant-eating mice. This seemed to be consistent with the predator’s attack strategy. Chewing grains all day requires a good deal of crushing power, but cutting into flesh and insect carapaces might not require as much force. Evolution had caused the grasshopper mouse to sacrifice bite strength for a wider gape to enfold its unfortunate prey.
But zoologists Susan Williams, Erika Peiffer, and Sonya Ford came to a different conclusion in a 2009 paper on the gape and bite force of the same grasshopper mouse species. Both features are important for determining how animals feed. Bite force involves how animals cope with the mechanical properties of their food, and gape relates to the size of the food they can fit into their mouths. The trouble is that the kind of adaptations that allow for higher bite forces – such as increased muscle mass in the jaws – constrain the gape an animal is capable of. Remember the sabercats. These hypercarnivores could open their mouths to a ridiculous extent, but they had relatively weak bite forces compared to shorter-fanged bit cats such as lions and tigers.
In their analysis, Williams and colleagues found that grasshopper mice did not have especially wide gapes. The relatively long jaws of the mice compared to the deer mouse Peromyscus maniculatus created the impression of a larger gape, but, when scaled to the same size, the two mice species had relatively similar gapes. Contrary to what Satoh and Iwaku reported, the slightly different muscle anatomy of the grasshopper mouse limits its gape to a range comparable to herbivorous mice. Where the grasshopper mouse differs is in bite force. Williams and collaborators found that bites delivered by grasshopper mice were significantly more powerful than those recorded for deer mice. Rather than opening wider, the jaws of the grasshopper mice are adapted to deliver powerful, piercing bites to prey. Grasshopper mice are not the sabercats of the rodent world, but tiny tigers of the North American deserts and prairies.
Epilogue: There are a few clips of grasshopper mice hunting on YouTube, but I did not include them in this post because of the atrocious narration which accompanies them. Two National Geographic clips (here and here), in particular, use narration which sounds better suited to a trailer for the latest moronic Rob Schneider film than a nature program. Another frustrating confirmation that basic cable nature channels are the bottomless chum bucket of science communication.
Christiansen, P. (2011). A dynamic model for the evolution of sabrecat predatory bite mechanics Zoological Journal of the Linnean Society, 162 (1), 220-242 DOI: 10.1111/j.1096-3642.2010.00675.x
McHenry, C., Wroe, S., Clausen, P., Moreno, K., & Cunningham, E. (2007). Supermodeled sabercat, predatory behavior in Smilodon fatalis revealed by high-resolution 3D computer simulation Proceedings of the National Academy of Sciences, 104 (41), 16010-16015 DOI: 10.1073/pnas.0706086104
Nowak, R. 1999. Walker’s Mammals of the World, 6th ed., Volume II. Baltimore: Johns Hopkins University Press. pp. 1357-1358
Satoh, K., & Iwaku, F. (2006). Jaw muscle functional anatomy in northern grasshopper mouse, Onychomys leucogaster, a carnivorous murid Journal of Morphology, 267 (8), 987-999 DOI: 10.1002/jmor.10443
Williams, S., Peiffer, E., & Ford, S. (2009). Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: Does jaw-muscle anatomy predict performance? Journal of Morphology, 270 (11), 1338-1347 DOI: 10.1002/jmor.10761