Fossil Friday – grasshopper mouse

Abstract deadlines are coming up for conferences taking place in the first 6 months of 2019, so we’re trying to collect as much data as possible for our submissions. One of these is a follow-up to our abstracts from last year on the Harveston locality in Temecula, but this time looking at the small animals (“microvertebrates”). One of our high school volunteers (or “Max’s Minions”, as we call them), Charlotte Hohman, is taking the lead on this project, sorting vials of tiny rodent, lizard, and bird bones and trying to identify them. This week we may have found a bone from a relatively uncommon mouse, the most metal mouse in Southern California.

The bone in question is a left calcaneus (heel bone). The image isn’t the greatest because it was taken by holding an iPhone up to a microscope eyepiece; we just wanted a quick reference image. The total length is about 5 mm.

After comparing this to references, we’re pretty sure this calcaneus comes from a southern grasshopper mouse, Onychomys torridus. If correct, this is our first grasshopper mouse from Harveston. Onychomys is known from the Diamond Valley Lake fauna, but is quite rare compared to the other rodents (in fact, I think we have more mastodons than grasshopper mice!).

So this is just a mouse; what makes it metal? Unlike their close relatives such as deer mice, which are omnivores leaning toward herbivory, grasshopper mice are voracious predators. They primarily eat insects and other arthropods, but will eat other mice on occasion.

They are known especially for eating bark scorpions. Apparently, when stung by a scorpion, the mouse’s pain receptors shut down; even though they may get stung multiple times, they don’t feel the pain. Northern grasshopper mice will attack centipedes, which are also venomous; the mouse is just fast and sting (I haven’t been able to confirm if southern grasshopper mice also eat centipedes).

And, if all that isn’t enough, grasshopper mice are territorial. But their territories can be over 20 acres (!), and they’ll warn away interlopers by howling with a high-pitched scream. Presumably if that doesn’t work they just eat the invader.

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