Camera trap pioneers: O.P.Pearson

Game trails have always interested naturalists, because they tell us where wild animals move and how they use their habitat. Trails come in all sizes, are usually "engineered" for energetic economy, and are most pronounced where activity is concentrated--near feeding areas, water, or mineral licks. Climb a tree or hide near a game trail, and sooner or later you will see who uses it.

In the 1950s, Dr. Oliver Pearson of the University of California embarked on a traffic survey of mouse runways on his property in Orinda, California. Professor Pearson was a distinguished and particularly inventive scientist who studied mammals and birds. His interests ranged from taxonomy to physiological ecology and predation. He was also a pioneer in the use of camera traps.

Pearson wanted to know what goes on in the microcosm of mouse runways. To a mouse, a patch of weeds or a grassy field is like a jungle, and species like meadow mice bushwhack and maintain their own runways with their teeth. In a clear runway they can zip along at a brisk clip and tend to their daily business. Harvest mice enjoy the labors of their larger neighbors.

Pearson's property harbored populations of meadow voles and harvest mice only 50 feet from the house, and the epicenter of his study was a 20 x 20' weed patch with a brush pile in the middle. Could a professor of mammalogy ask for anything more?

He used two 16mm movie cameras in tandem with weather stations to survey the traffic. The camera shutter was electrically activated whenever a mouse or even a large insect passed the camera lens. He used two trigger mechanisms with equal success, a treadle and a photoelectric cell.

The cameras were synchronized with electronic flash units. Nowadays we can snoop on sensitive species using infra-red cameras. To avoid frightening the mice, Pearson masked the flash with 18 layers of red cellophane, and noted that "a muffled clunk made by the mechanism seemed not to alarm the mice unduly."

Powered with alternating current, the cameras ran day and night, but there was a problem with differential exposure. Daytime pictures were overexposed, because the shutter was slower than 1/30th of a second and the f-stop was adjusted for night. Again the solution was red cellophane, this time over the camera's lens. The camera units were sheltered in glass-fronted housings. Overhanging eaves and a blackened light bulb counteracted the effects of rain and dew.

Pearson positioned a camera unit on one side of a mouse runway. On the other side, and framed within the camera's view, were the instruments of a miniature weather station: a dial thermometer, a hygrometer, a ruler, and an electric clock with a sweep second hand. Thus, whenever a creature took its own portrait it also recorded its body length, the time, and a weather report. Since he live-trapped the mice and gave them distinctive "haircuts" he could identify many of them individually.

Not long after he deployed the equipment, the neighbor's Siamese cat discovered that the camera housings were superb perches for mousing. He solved the problem by fencing the weed patch. Then a slender salamander electrocuted itself while short-circuiting the treadle. Delusional song sparrows were another problem. They wasted a lot of film shadow boxing their own reflections on the weather station window. Curiously, one bird side-stepped an oncoming meadow mouse, a gesture of road etiquette rarely seen on California highways.

In nineteen months of operation (111 recorder weeks) the camera traps generated 8,495 photographs. In other words, weed-patch wildlife made an average of 11 passages per day. The professor noted that, "A patient, non-selective predator waiting for a single catch at runways such as these could expect, theoretically, a reward each 2.2 hours." The cameras mined enough information for three scholarly articles.

Pearson's first paper on the traffic survey contains the information of most interest to camera trappers. I'll go out on a limb here, and state my belief that the results also apply to larger mammals, especially other herbivores like deer.

His "pleasant surprise" was that 26 species of animals, from weasels to snakes and mole crickets--made use of the runways. He never saw three of these species within at least a mile of the study area, proving that camera traps often disclose nature's little secrets. Ironically, he never photographed three other species he commonly saw within 100 feet of the recorders! The take home message here is that placement of camera traps is critical.

Though two species of mice used the runways, Pearson's camera traps revealed that meadow mice were the "public works custodians". When meadow mice stopped using a runway, weeds and seedlings quickly filled them in. Another curious finding was that mice apparently used runways selectively even though nearby runways seemed equally suitable.

Pearson's camera trap studies of rodent ecology are still among the most elegant and detailed studies of their kind. The work also seemed to stimulate Pearson's interest in predator-prey relations, for he went on to study predation of mice in Tilden Park not far from the UC campus. During the next twenty years a smattering of biologists, including your's truly used homemade camera traps for wildlife studies and surveys. Then in the 1980s camera traps became commercially available, and wildlife biologists developed camera-trap-fever.

Often we don't discover common interests till it's too late. This camera trap codger wishes he had mustered the courage earlier in his career to ask Pearson about his camera trapping days. Many times I saw the professor at meetings of the American Society of Mammalogists, but stood speechless and in awe. I guess we are a lot like meadow mice, politely passing each other through the runway of life. Mice and men are alike that way.

Acknowledgement: My appreciation to the Allen Press for permission to reproduce the plate from Pearson's 1959 paper, and to Kathleen Berge of the California Academy of Sciences for scanning it.

References
Osterberg, D.M. 1962. Activity of small mammals as recorded by a photographic device. Journal of Mammalogy, 43:219-229.

Pearson, O.P. 1959. A traffic survey of Microtus-Reithrodontomys runways. Journal of Mammalogy., 40:169-180.

Pearson, O.P. 1960. Habits of Microtus californicus revealed by automatic photograph records. Ecological Monographs, 30:231-249.

Pearson, O.P. 1960. Habits of harvest mice revealed by automatic photographic recorders. Journal of Mammalogy, 41(1):58-74.