Jeff Mitton
Natural Selections (appeared June 26, 2004 in the Daily Camera)
I have lived in Boulder for thirty years, and in the first twenty I did not see a fox. Now I see foxes every week. What has happened?
The red fox, Vulpes vulpes, is distributed throughout the northern hemisphere from the Arctic Circle to North Africa, Central America, and the Asiatic steppes. In the seventeenth century, red foxes were introduced from Europe to the eastern United States for hunting, but they escaped, and are believed to have mixed with the resident red foxes. In the nineteenth century, red foxes were introduced to Australia to control introduced rabbits.
Foxes are in the same family as coyotes and wolves, but are much smaller. The canids differ by factors of three. The largest fox weighs 15 pounds, while the largest coyotes and wolves are 45 and 135 pounds, respectively.
The most common color pattern of our local foxes is reddish brown hair on the body and tail, with black ears, feet, and forelegs. Their large, bushy tails can be tipped in white or black.
Foxes mate between December and April, and after a gestation period of fifty one days, vixens give birth to up to ten, but usually three or four kits. Kits are kept in a den dug by the vixen, and are weaned in about two months. In the fall, young males disperse to establish their own territories. The vixen and male may separate at this time, but they usually pair again at the beginning of the mating season. Both males and females are reproductively mature at 11 months. Large litter size and early sexual maturation enable foxes to increase their population size quickly.
My fellow biologists suggested several changes that might explain more frequent sightings of foxes. Vegetation in town and along creeks and ditches has increased, providing more cover. Fox squirrels have become more abundant in town, providing more prey. Hunting and trapping of foxes has declined to nil. But I do not think that these changes, singly or in concert, are sufficient to explain the flush of foxes.
The key to this puzzle is in the learned behavior of foxes, which I assert has changed. An analogous precedent can be taken from a behavioral change in a bird in the British Isles.
The great tit, Parus major, is common in the British Isles and Western Europe. In the 1930s, milk was delivered to doorsteps in bottles. This was before milk was homogenized, so in each bottle a layer of cream rose to the top, with milk below. The bottles had cardboard caps, each with a tab so they could be easily removed. A resourceful tit grasped a tab with its beak, yanked the top off, and drank the rich cream. This behavior was mimicked by other tits, and in just a few years the behavioral innovation spread throughout the British Isles.
In places where mule deer are hunted, they are elusive and wary. But in the Boulder Mountain Parks, mule deer have learned that people are no threat, and can be ignored with impunity (see photo). The foxes have learned the same trick.
In the past, our foxes were crepuscular and nocturnal, but now they forage in town during the day. They brazenly steal food from dog dishes left on porches. They are harvesting our abundance of fox squirrels, and even taking cats.
I have seen kits romping on the lawn near Sewall Hall on the CU campus, and they are commonly seen on the sidewalks and in the alleys on University Hill. Foxes may have increased in abundance, but they are increasingly conspicuous because they have changed their behavior.
A young red fox, Vulpes vulpes, ignores nearby hikers and bikers to focus its attention on a fly.
Photo by Jeff Mitton
Most red foxes are red or cream, as in the first two photos
Kits are cute.
