Turns out the mite is mightier than we thought: A tiny arachnid that lives in southern California is the world’s fastest land animal relative to size, according to a new study.
At its quickest, the sesame seed-size Paratarsotomus macropalpis zips along at about 322 body lengths per second (a measure of speed that shows how quickly an animal moves relative to its size). For a human, that’d be like running 1,300 miles (2,000 kilometers) an hour.
The previous record-holder, the Australian tiger beetle, travels at 171 body lengths per second. By comparison, the cheetah—the fastest land animal overall—can move at only about 16 body lengths per second. (Related: “Cheetah Breaks Speed Record—Beats Usain Bolt by Seconds.”) The fastest known human, Usain Bolt, covers a little more than 6 body lengths per second.
Scientists at Pomona College, Pitzer College, and Harvey Mudd College witnessed the new record while observing P. macropalpis with high-speed cameras outside in Claremont, California, where the mite can be seen scuttling along sidewalks in the heat of the day.
A southern California native, the tiny creature—whose legs are invisible to the naked eye—had mostly been ignored by scientists since it was identified in 1916.
“First I thought, ‘Wow, that can’t be true’—[its body lengths per second] seemed ridiculously high,” said researcher Samuel Rubin, an undergraduate in physics at Pitzer who presented the new research April 28 at the 2014 Experimental Biology meeting in San Diego. ”I better redo the measurements.”
But repeated experiments showed the same result: He had found one speedy mite.
Some Like It Hot
The team filmed the mites running on concrete up to 140 degrees Fahrenheit (60 degrees Celsius)—a temperature scorching enough to kill most animals. (See “Urban Heat May Warm Faraway Places.”)
A closer look at the mites’ bodies revealed a well-sealed outer layer that acts like a barrier against extreme heat. The mite’s desert home is full of stone slabs and baked clay surfaces that would be as hot as a city sidewalk, said Rubin, who’s in the process of submitting his research paper to the Journal of Experimental Biology.
That outer layer, and the fact that the mite doesn’t take refuge in the shadows, suggests that it didn’t evolve its speediness to escape the heat. Rather, the team suspects the mite moves fast to hunt its prey, which are so small and fast that scientists haven’t seen them yet, even with high-speed cameras.
“We never observed them being fed on or feeding because it’s so infrequent, and they’re so fast,” said Rubin, who observed the mites for an entire summer and didn’t witness a single act of hunting. “It’s hard enough to get any footage on them.” The camera’s field of view is less than four inches (ten centimeters), and these mites will move that distance in less than a second.
Small and Speedy
Rubin said the team’s findings also reinforce a scientific theory called scaling, which says that relative speed increases as an animal’s body mass gets smaller. The theory holds that the smaller an animal gets, the less force it needs to move fast. And less force means not much need for muscle.
The space where a big muscle would go can instead be populated by other things, such as molecules like mitochondria—cellular powerhouses that can make an animal’s strides even faster. The mite’s stride frequency is 135 Hertz (Hz), a measurement of how many strides occur a second. By comparison, an average human runner’s stride frequency is less than 3 Hz.
The new research “sounds exciting,” said Walter Federle, an expert in insect biomechanics at the University of Cambridge in the U.K. who wasn’t involved in the study. “I have often wondered about the stride frequencies of rapidly running mites, and I am amazed how incredibly high they are.”
Federle agreed that smaller body sizes allow for faster speeds and acceleration, but noted that there may be a practical limit to the phenomenon: the time needed to activate muscles.
“There is clearly very little time [for the mite] to contract the leg muscles at 135 Hz,” said Federle.
As for Rubin, he’s now curious about the mite’s meals. According to the scaling theory, the mite’s prey—since it’s presumably smaller than its predator—is likely even faster.
But until such a creature is found, the mighty mite is leader of the pack.
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