Mouse Howls Like a Wolf, Bites Like a Tiger

By Brian Switek


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.


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