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December 2020 Natural Mystery Answered

posted Jan 12, 2021, 4:03 PM by Jonathan Poppele   [ updated Jan 13, 2021, 8:37 AM ]
Our December Natural Mystery proved to be a real challenge. We received a number of thoughtful guesses, but no correct answers. There is a modern proverb in American medical schools that has, appropriately, been adopted by many trackers. "When you hear hoofbeats in the night -- look for horses, not zebras."1 But I might add that if we examine carefully, and what we find is not a horse, we must consider...

This is the carcass of a Mongolian Jird (Meriones unguiculatus), better known as a gerbil. It was almost certainly someone's unfortunate escaped pet.


As we think through this mystery, it may help us understand that although we always want to "look for horses," we should not completely rule out zebras.

As several people noted in their answers, the animal has the large hind legs characteristic of a bipedal rodent, such as a kangaroo rat. But there no members of family Heteromyidae native to the Eastern U.S.--so we would need to come up with a plausible story for how one could have ended up at the edge of someone's yard in a small town in Pennsylvania. Even putting range aside, a study of Heteromyidae finds that their hind legs and their tails are even longer than what we see here.

The body form isn't a match for any of Pennsylvania's native rodents, either. Jumping mice, which are closely related to the bipedal Jerboa of the Mideast, have larger ears, a different shaped forehead, and different proportions in their hind legs. The same is true of woodrats, deer mice, and Old-World rats. Voles have shorter tails and less developed hind legs.

As we look for "horses," we come up empty. So we need to consider zebras. We need to consider animals that are not native to Pennsylvania, and not known to be invasive, but none the less might plausibly turn up there. That combination suggests a pet.

A quick search for "pet rodents" finds that hamsters, rats, mice, gerbils, and guinea pigs are the most common rodents kept as pets. A search for pet gerbils yielded this page. This and other information about pet gerbils suggest that is our match. Our "zebra" turned out to be a Mongolian Jirid.

Part of what supports this conclusion is that, despite the fact that we have identified a "zebra," there is still a plausible explanation for how it ended up in Oil City, Pennsylvania. From my research, it appears that there are other gerbils in the sub-family Gerbillinae have similar proportions. From the photo alone, I don't see any way to distinguish them. But the Mongolian Jirid is the only one that is known as a pet in the United States. So it's a zebra, but its the kind of zebra that are kept at the local zoo.

Mongolian Jirids are native to the grasslands, shrublands and deserts of China, Mongolia and Russia. The species was named by Henri Milne-Edwards in 1867 based on specimens sent to the French National Museum of Natural History from Northern China by the zoologist and Catholic missionary Armand David. The name roughly translates as "clawed femur."

Gerbils were brought to the United States from eastern Mongolia in 1954 for use as a lab animal, and soon found their way into the pet trade. All pet gerbils in the United States are decendents of the original lab stock of 20 males and 26 females.


Notes:
1) According to Quote Investigator, the first known record of this saying was in a column called "Our Town" by Charles Allbright, published in the "Arkansas Gazette" of Little Rock in October 1962. Here is an extened quote:

"The father of a young man who was there reports that at the University of Arkansas School of Medicine this week a doctor surrounded himself with about a dozen students and sought to go to the heart of proper diagnostic procedure.

In the end he summed up good diagnosis this way: “When you hear hoofbeats in the night, look for horses — not zebras.”
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