A Rat’s Niche : Look Who’s Moved Into the Neighborhood
The wedding guests stood around in groups, sipping California Coolers. Suspended over the roof of the house across the alley was the moon--a great, silver medallion with a thick power line dividing it into two halves. Suddenly, the intimate backyard ambiance was disrupted by a loud gasp.
“Oh, gross!”
“Revolting!” came a second opinion.
Looking up, I saw a very large roof rat--about the size of a potato--running along the power line above us. It traversed the cable with the almost mechanical dexterity of a remote-controlled toy. And then, just as the creature reached the midpoint of the yard, it stopped and remained absolutely motionless, right in the middle of the moon. It sat there, a silhouette in a round frame, and swayed gently on the power Edison line.
“What a wonderful thing!” I blurted out.
The words just escaped. Thirteen years as a biologist had bent my values, and what I saw suspended above the wedding party was a masterpiece of evolution, a creature that had been designed and built to live in the camp of its foe, the most formidable adversary any animal could have drawn in the entire history of nature. This rat had evolved a form, a size, habits--the overall life strategy--to survive in the attics, the sheds, the garages and especially the shrubbery that we humans are so fond of. And no poison, no gun, no extermination program has eliminated him.
I also appreciated a theoretical aspect of the roof rat’s accomplishment. Unlike us humans--who put clothes on and take them off, who modify the environment to suit our needs and desires--animals are so precisely crafted to deal with an exact set of conditions that they cannot survive anywhere else. For instance, their legs are designed for particular kinds of running or digging or climbing. Their guts can digest only certain foods. Their brains produce a narrow range of habits and reactions, and they can learn only specific things.
But mainly I appreciated the rat because of a conversation I had had recently with a biologist doing postdoctoral work at UCLA. Michael Recht is actually what you’d call a modern naturalist; he’s interested in the whole animal and its natural habitat, and he uses modern technology for his studies. Several years ago he took on a project funded by the Orange County Vector Control District. He decided to capture a group of roof rats in the City of Orange, collar them with tiny radio transmitters and follow them around, at all hours of the day and night, to see where they went, when they went, what they did, what they ate--in short, “how they made a living,” as ecologists like to say. He persuaded an entire block of homeowners (except one, who refused to cooperate) to let him appear in their backyards at any time. An odd sight it was to see him clambering over fences, running along walls and sneaking under telephone lines, all the while carrying a Panasonic radio receiver to pick up the signals. But that’s what a naturalist has to do sometimes.
To urban ecologists like Recht, a neighborhood is actually much more than a place for human beings; it is an ecosystem--a place where animals with the right stuff can survive and multiply, where they are able to take advantage of a good, protective habitat. For roof rats, ivy and bougainvillea, for example, are ideal. These plants grow next to walls, drape over fences and, left unchecked, can produce masses up to two or three yards thick. The foliage takes up the outer six to eight inches and acts as a kind of shell; but inside, a relatively open space interlaced with bare branches and twigs provides a kind of grand shelter that is easily traveled and shielded from rain, wind, people, dogs and cats. It is happily inhabited by roof rats; they often build their nests there. They also revel in run-down sheds, unused attics and loosely stacked woodpiles, which become rat mansions, full of halls and chambers. Such places make up a metropolis interconnected by telephone wires, fences and trees.
In designing the ideal roof rat, evolution has used several key attributes. Size is one, because an uninvited guest such as a rat must be small enough to hide in cracks and crannies. Color also is important--or, actually, the lack of color. It’s no accident that rats are a neutral gray; they literally disappear in the shadows.
Then there’s the matter of diet. A successful rat must enjoy, and at the same time be able to digest, the same food that its human hosts eat. Roof rats dearly love treats such as peanut butter, but they feed mostly on backyard produce--walnuts, avocados, apricots and so on. They will also do just fine on a diet of 60% ivy shoots, with a few snails thrown in for protein.
Probably the most important factor for living among hostile hosts is the proper behavior. The proper habits and the proper reactions must more or less be programmed in the rodents’ genetic constitution, because there is not enough time in a rat’s life to learn, say, stealth in a world crawling with cats. Stealth must be instinctive. In Recht’s backyard observations, he discovered just how appropriate roof rat behavior is.
First and foremost, roof rats are totally arboreal; they stay off the ground, almost at all costs. (On the other hand, the Norway rat, sometimes called the sewer rat, can live in the same neighborhoods, but it stays almost exclusively on the ground. In that way, the two species have divided up a very desirable environment to minimize competition.) Roof rats use power lines, telephone wires and the tops of fences for high-speed travel, scurrying quickly across the open spaces. They are nocturnal, make nightly rounds and follow the same pathways to their favorite haunts.
They are also extremely wary, and if you see one at all, it is because of overcrowding, which forces them to forage longer hours and to take more risks. If you try trapping one and fail, it will not re-enter the trap. And if you disturb a nest area--if you rummage through the attic, for instance--the rats will clear out for some time; only if you stay out of the attic for about a week will they return.
All this knowledge can be used against the rats, however. Recht found that one of the most effective rat-population controls was to block off their aerial pathways. By cutting back trees so they don’t bridge the gaps between neighboring houses, by eliminating loosely stacked woodpiles, by screening off entry points to houses, by getting rid of thick masses of ivy, you could drive them from an area. The problem was that to achieve real success, everybody on the block would have to cooperate.
So all this was scrolling across my mind while the rat sat there frozen against the moon, its tail hanging down like a cord plugged into the darkness. Finally, it scurried down the cable toward the loquat tree in the corner of the yard. A few rustlings, and it was gone.
The reception never recovered. A subdued, somber mood pervaded the conversation as if we all knew, instinctively if not consciously, that this niche that we took for granted was not our exclusive domain. The roof rat was at least as suited to it as we. I looked up at the cable that innervated the house with electrical power, and one last thought occurred: The rat, in adapting to this niche, has in many respects become similar to the human, eating the same diet and thriving in man-made settings. It is not surprising that rats are used universally in medical research.
