In South Texas, there truly are field mice that are carnivorous and they stand on their hind legs, put their heads back and howl like a wolf.
They are loud, nocturnal predators, heard up to 100 yards away. This is not from a script for a Sci-Fi horror movie. We’ve heard their calls in the grassy utility right-of-way behind our house. We did not know what the critters were until Karen Benson, fellow Master Naturalist, encountered them and researched their background. Here’s her story.
The find
Carter Crouch, wildlife biologist at Cesar Kleberg Research Institute in Kingsville, claimed he heard a mouse caught in his research traps “howl” when he was removing it from the trap. I was hard-pressed to believe something 6 1/2 inches long, weighing less than .08 pounds could produce a piercing, high-pitched howl that hurt Crouch’s ears. But I’ve seen strange things as a naturalist; I checked my “Mammals of Texas” to learn more. Texas is home to 60 species of rodents: squirrels, prairie dogs, pocket gophers, voles, even beavers, as well as 40 kinds of mice and rats. One of the mice had a shape and behaviors that matched Dr. Crouch’s description, and it howled like a wolf – the Northern Grasshopper Mouse.
Start with research
Fascinated by this, I checked other sources. Native to this part of Texas, the northern grasshopper mouse (Onychomys leucogaster) is rather squat and blocky, has a white tummy and a white tip to its short tail. They’re not fast runners but are very agile – able to twist and turn to subdue prey in their longer-and-stronger-than-average jaws.
Why a grasshopper mouse?
Rodents, as a rule, are herbivorous. They feed on grain, seeds and gnaw on nuts. However, northern grasshopper mice are carnivores named for their favorite summer food: grasshoppers and other insects. Analysis of their stomach contents showed 89 percent of their diet is meat – insects, other invertebrates, small mammals (for example, other mice and voles), reptiles and small snakes. They’re “veritable tigers among mice,” hunting prey (especially their larger prey) much like a cat, or a weasel. They stalk, then rush in, seize the prey and kill it with a bite to its head. Their longer jaw and adapted jaw muscles allow them to achieve crushing bite strength at a larger gape – jaw opening greater than 40 degrees. The bite force of seed-eating mice declines after 40 degrees.
A solitary lifestyle
As with all predators, grasshopper mice have lower population densities than small herbivores. Their home ranges go up to 28 acres. Wanderers, almost always on the go, they do not stay anywhere long. Rather than construct their own burrows, they usually parasitize the burrows of other small mammals. They are solitary animals except during breeding season, May to October. If food gets scarce, they will cannibalize their own species.
Why do they howl?
Whether they howl to establish and maintain territories, call or “sing” to attract a mate, or for some other reason, is unclear. But howling may be a part of their complex courtship and mating behaviors. An alternative explanation of howling: it is exhibited just before a kill is made, and the prey is paralyzed by the sound? Or, do they howl spontaneously, enjoying standing up on their hind legs and “singing”? Who knows? If you want to hear their song, go to http://bit.ly/1sHLrgt and listen.
If someone suggests captured grasshopper mice as pets, be aware of several things. They don’t live long – less than a year (three to seven months in the wild). They are a host for 57 species of fleas, some of which are hosts for plague bacterium. They are serious diggers. And you better have lots of live insects around, although they show a preference for strips of raw liver. Plus, they do howl at night, loud enough to wake a family.
Thanks, Karen, for helping us understand a rarely-seen critter. For us, we’ll leave “ours” in the right-of-way and appreciate their songs.
Sources: The Mammals of Texas – Online Edition, nsrl.ttu.edu/tmot1/onycleuc.htm; “Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: Does jaw-muscle anatomy predict performance?”, Williams, Peiffer and Ford, Journal of Morphology; Volume 270, Issue 11, November 2009; wired.com; en.wikipedia.org; ckwri.tamuk.edu; The Bee-Picayune, on Nov. 15
Paul and Mary Meredith