Thomas M. Campbell Iii

Population attributes for the black-footed ferret (Mustela nigripes) at Meeteetse, Wyoming, 1981-1985

Numbers of adults and juveniles in the single known free-ranging population of the endangered black-footed ferret ( Mustela nigripes ) at Meeteetse, Wyoming were estimated annually in July from spotlighting as 88 (1983), 129 (1984), and 58 (1985). Population sizes in September, determined from mark-recapture studies, were 128 ± 25 (1984) and 31 ± 8 (1985). Lower population estimates in 1985 reflected, at least in part, an ongoing epizootic of canine distemper in ferrets that decimated the population through November 1985, reducing it to ca. 6 individuals. From 1982 to 1985, adult sex ratio was 1 male: 2.2 females; juvenile sex ratio (1 male: 0.80 females) did not differ significantly from 1:1. The ratio of young to adults averaged 1.95:1 from 1982 to 1984 and 1.2:1 in 1985. At least 224 young were produced in 68 litters from 1982–1985, with a mean litter size at emergence of young of 3.3. Juvenile ferrets reached adult weight by September. Only one female tagged as a juvenile was recaught at 1 year of age, and she reproduced. Intercolony movements were primarily by juvenile males and occurred from September to October. Adults maintained geographic fidelity between years. Disappearance (mortality and emigration) rates ranged from 53 to 86% annually and were highest for juveniles. Observed ferret mortality in the absence of disease was primarily from predation. Reduction of the population during the epizootic suggests persistence of this population in the wild is unlikely.