Age Determination of Foxes by X-Ray of Forefeet

Abstract

An investigation of breeding, denning and range of the red fox (Vulpes fulva) and the gray fox (Urocyon cinereoargenteus) in Alabama provided an opportunity to record certain age characteristics. Emphasis was placed on the possibility of identifying young of the year among older foxes by the condition of the epiphyses of the bones in the forefeet as determined by X-ray. Similar X-ray studies of the upper tibial epiphysis of dogs (Schlotthauer and Janes, 1952, Amer. Jour. Vet. Res. 13(46): 90) have indicated that the femoral and upper tibial epiphyses close at about the age of 10 months. Two foxes, a red and a gray, were available for these studies during the first fall (1952) of the investigation. The approximate age of each fox was known since they were captured while small. X-rays of the lower radius and ulna of these foxes were made at intervals (Table 1, Animals 2134 and B). The first two X-rays (October and November) showed that cartilage still separated the epiphyses on the radius and ulna from the diaphysis or shaft. The December X-rays showed the epiphyseal plate beginning to disappear but not completely ossified. Unfortunately the red fox escaped in December. An X-ray of the gray fox in early January showed the epiphyses of both the radius and ulna completely ossified. The following fall two young red foxes and two young gray foxes were used in similar X-ray studies. One of these red foxes also escaped before the investigation was completed. X-rays late in December of the remaining red fox and the grays showed the epiphyses almost completely ossified. X-rays of the fore-feet of seven foxes of unknown ages in January and February showed the epiphyses of both the radius .and ulna completely ossified. It appears that foxes can be classified dependably as adults or as young of the year through November by means of X-rays of the distal epiphyses of the radius and ulna (Figure 1). Some young foxes were distinguishable through most of December. This indicates that ossification is completed in foxes between eight and nine months of age. A faint and incomplete line along the epiphyseal plate is still visible on some foxes in January at which time they are approximately 10 months old. Whelping of foxes in Alabama occurs primarily during late March and April (Sullivan, 1953, M.S. Thesis, Alabama Polytechnic Institute). To test this technique, 60 fox feet were secured from trappers who were trapping for bounty in connection with a fox control campaign in Lee County, Alabama. Most of these foxes were trapped at or near dens in late summer and fall. The feet were first preserved in alcohol-formalin solution, then dried and pinned on cardboard squares of the same size as the X-ray negatives to be used. As many as 25 feet were X-rayed at one time. By studying the X-rays, it was a simple matter to separate the adult foxes from young of the year. Forty-four (73 per cent) of the 60 foxes were juveniles. Subsequent experiments showed that fox feet can be dried fresh without preservative and kept for several months before they are X-rayed. The clearest egatives were secured when the feet were layed directly on the film holder and X-rayed without being first pinned to cardboard. The foreleg when removed from the fox should be cut off above the joint at the pad, preferably halfway up on the first long bones (radius and ulna). Successful X-rays were made on Kodak (no screen) safety film No. 246 with exposures of 60 KVP at 20 MA at one-half-second. All exposures were made at 36. Personnel of the Small Animal Clinic of Alabama Polytechnic Institute did the X-raying. By X-raying fox feet collected from trappers through November, biologists can determine the ratio of young to old foxes harvested. The final date for collecting feet in different regions should be not later than eight months following the earliest whelping of fox pups in that area. Information on percentage of young should provide a better understanding of productivity of the fox. This technique has two principal advantages: (1) it permits determination of age without having the entire carcass in hand, and (2) it permits accurate determination of the ratio of young of the year to older foxes for about three months longer 1 A contribution from the Alabama Cooperative Wildlife Research Unit; jointly sponsored by the Alabama Department of Conservation, Alabama Polytechnic Institute Agricultural Experiment Station, the U. S. Fish and Wildlife Service, and the Wildlife Management Institute.