Onychomys: Tiny Terror of the Western Deserts
Here’s a riddle for you: What stalks its prey like a cat, howls like a wolf, and weighs about one ounce?
Give up? The grasshopper mouse! Onychomys is a genus of small mice native to western North America. The three species, O. leucogaster, O. torridus, and O. arenicola, have overlapping ranges with O. arenicola (Chihuahuan grasshopper mouse) stretching down into Mexico, while O. leucogaster‘s range reaches up to southewestern Canada. They are desert and arid adapted rodents, able to survive harsh conditions very well and are mostly found in desert, scrub, and dry plains in their ranges. Their characteristics and behavior are all very similar so I’m going to talk about the genus as a whole, although there are minor differences between species in terms of diet, habitat, etc.
Onychomys mice are very small, usually just a little over an ounce. They are actually quite adorable, with darkish brown to gray fur on their backs and light to white fur on their undersides, and a short, thick tail less than half the length of their bodies. They have relatively small ears and, although not bipedal, rise up on their hind legs and use their front limbs for various tasks rather regularly. If you were to see one in the wild, you would just think, “hey, a mouse with a short tail!”.
What makes Onychomys special – and fiercesome -among the hordes of North America rodents is its carnivory. Actually, carnivory itself is not that unusual among rodents, particularly desert ones. While many species are adapted to specialize in seeds or plant material, most are more omnivorous, incorporating small arthropods and sometimes even carrion into their diets. It’s the level of carnivory of these little guys that is impressive. They eat almost exclusively meat – mostly insects but also scorpions, small snakes and lizards, and even other rodents!
Now, these are not animals they have found dead and consumed. Onychomys are hunters. Quite aggressive ones. They will pretty much go after anything about their size or smaller that moves (with a very few exceptions in species which have toxins that make them inedible which the mice can smell from a distance), and will attack unrelentingly until it is dead. When they are near prey they sniff to identify it and then track its scent trail, silently stalking it until they are close. Then they attack, lunging at the prey with open mouths and front paws, trying to subdue it with a killing bite. In the case of vertebrates, they often go for the back of the neck, severing the spinal cord with their teeth. With arthropods they often first try to disable whatever defense the animal has, removing the stinging tails of scorpions, for instance.
They almost never give up on prey once they’ve decided to attack, although sometimes after it is dead they may decide it’s not edible and leave it. Onychomys are of the “kill first, ask questions later” school of predation it would seem. Scientists who have studied them are often astounded at their level of aggression and their heedlessness of danger, as they attack everything from harmless crickets to other mice to the most poisonous species of scorpions in North American with the same vigor. They’ve even been known to kill kangaroo rats and cotton rats much, much larger than them in the lab (although it’s unlikely they hunt those species in the wild). In the eastern part of their range, they hunt small prairie birds.
It takes them a bit longer to subdue something heavily armed like a scorpion, but they usually manage it. Part of the reason they are able to do this is that Onychomys species seem to be at least partially immune to neurotoxins. The scorpions will sting them during an attack, and while they may retreat briefly and seem to experience pain from the wound, the normal effects of paralysis didn’t set in and the mice will return to their attacks. In the lab, trials have been done giving the mice various drugs, including sedatives and anti-psychotics, to see if that will decrease their aggression towards prey or each other.
They don’t. Drugs that would lay you or I out flat seem to roll off the backs of Onychomys and they just keep on going. They are born fighters – fairly young mice raised exclusively in a lab setting are still efficient killers when presented with prey for the first time, even when compared with wild-caught mice.
So why are these mice so aggressive and carnivorous? It seems to be a desert adaptation. Water is a problem for everyone in the desert, particularly mammals. Many rodents have developed similar adaptations to conserve/obtain water. These are often a combination of be behavioral (only coming out at night, eating only the moist parts of plants) and physiological (kidneys that concentrate urine to prevent water loss, the ability to extract metabolic water even from fairly dry food sources). While Onychomys can concentrate their urine somewhat, they are not as efficient at it as many other desert species and they can’t get metabolic water from seeds like kangaroo rats (many of which never need to drink or even eat particularly succulent foods).
They get their water from meat. Animals have a lot of water in them, more than many desert plants. Onychomys essentially usurp the water carefully hoarded by other species in their own bodies for their survival. Trials have been done feeding these rodents exclusively on meat (other mice, beef, and liver) and they were able to live quite happily without drinking water for months. If the meat was cooked, reducing the water content, they lost weight and their health immediately went downhill. It takes a lot meat to supply an active mouse with all the water and calories it need to survive in the desert, and it takes a lot of aggression to produce the high number of kills necessary to maintain that supply.
Although Onychomys are rodents, they eat like carnivores and thus sort of act like them too. They live alone or in pairs with young, not unusual for mice, but they maintain particularly huge home ranges, up to nearly 8 acres for some males. Which is simply gigantic if you’re talking about a mouse the size of a salt shaker! They are nocturnal and maintain extensive burrows in their territory which they use to live in, to escape from other predators, and to store food in the winter. They patrol and defend their territories viciously, eagerly fighting, killing, and eating intruders of the same species and other rodent species. Females have even been known to kill and eat their mates.
And they are vocal. They make various calls of alarm and identification, including the infamous howl. When patrolling their territory, they will often rise up on their hind limbs, throw back their heads, and make a loud piercing, cry. They do this, of course, particularly on spring spring and summer nights when the moon is full, although it is unknown if this behavior is a mating call or primarily a territorial display. This has led to another common name for Onychomys – wolf mouse.
Rowe, A., & Rowe, M. (2006). Risk assessment by grasshopper mice (Onychomys spp.) feeding on neurotoxic prey (Centruroides spp.). Animal Behaviour, 71(3), 725-734. doi: 10.1016/j.anbehav.2005.08.003.
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