Ecology of pocket gophers with emphasis on Thomomys bottae mewa

Abstract

From the fall of 1947 to the summer of 1954, the life history of the Digger pine pocket gopher (Thomomys bottae mewa) was investigated at the San Joaquin Experimental Range, O'Neals, California, More than 1,000 gophers were handled. Most of the information about the population dynamics of gophers was obtained by live-trapping 330 marked individuals 1,798 times on a 3.7 acre study plot. This repeated live-trapping lasted for five years. Others were maintained in laboratory cages. Gophers live a solitary existence except during the breeding season when multiple captures occurred in the same burrow. Few of the females were breeding before they were one year old. A few females produced two litters a season, but generally they had onlyone litter, usually in February or March. The best external characteristic for indicating breeding activity in females, according to 222 that were autopsied, is a swollen or open vagina. Body weight is not a reliable indication of age. Males continue to grow throughout their life. "Weights of live-trapped gophers changed from + 0.75 to - 1.14 grams per hour while confined in a trap. One female lived for another year after losing 22.5 per cent of her weight when held in a trap with insufficient food for 20 hours. This is easy to understand, however, for the weight of the alimentary tract of seven gophers (T.b. navus) averaged about one fifth (18.1 to 25.7 per cent) of their gross body weight. It is not uncommon for females to live as long as three or four years. More than half of the female gophers reaching the age of one year also live to be two years old, except when the population density is high and the turn-over, consequently, more rapid. Males do not live as long as females. Our data indicate that gophers are polygamous. The adult sex ratio of males to females varied from about 1:1 to 1:4. Females predominated (1:4) when the population density was high. The home range of a pocket gopher is also its "territory," for adults vigorously defend their entire burrow system from individuals of both sexes, except for brief periods during the breeding season. Male territories occupied an average surface area of 2,700 square feet, whereas females only one half that, or 1,300 square feet. During March, April, and May juveniles left home, to become sub-adults, by often traveling aboveground in their dispersal movement. More than 200 sub-adults were captured in funnel traps placed on the ground surface along hardware-cloth drift fences. Gophers often seem instinctively capable of returning home when released in other burrows 200 feet or more away. They apparently return by traveling through existing burrow systems, passing through the territories of other gophers. Individuals released on the opposite side of a drift fence returned without getting caught in traps along the fence and without digging under the wire fence. Attempts to correlate soil and forage conditions with gopher distribution and weights in a cause-and-effect relation are discussed. No ticks, fleas, or warbles were found on gophers at the Experimental Range, which seems unusual However, one species of louse, three kinds of mites, and tapeworms were found. The ecological factors responsible for creating fluctuations in the density of gophers are discussed. Also the significance of pocket gophers with respect to animal associates, forage relationships, effect of burrows, and importance to man is discussed.