Louise Richardson

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.

Black-footed ferret Mustela nigripes energy expenditure and prey requirements

Abstract Black-footed ferrets Mustela nigripes are exceedingly rare and only a single extant population in Wyoming is known. Tracks in the snow and direct observations have provided data on activity and show that a ferret can travel from 0 to 7 km and may move up to 50 litres of soil from prairie dog Cynomys spp. burrows in a night. We constructed an additive model to estimate ferret energy expenditure, including energy for running, digging, investigating burrows, and thermoregulation. From field data, we estimate that ferrets expended an average of 130 kcal day−1 during winter. We used the Siberian polecat M. eversmanni as a biological model for the endangered ferret to estimate energy and nutrient aquisition from two ferret prey species. Gross energy content, proximate analyses and utilisation by the polecats of the two prey did not differ and were comparable to results for other carnivores. The polecats consumed an average of 125 kcal day−1 during trials, which is equivalent to 104 metabolisable kcal day−1. At least 20 prairie dogs must be eaten by a ferret during the four winter months (December–March) to meet these requirements. During summer months lactating female ferrets might need to eat prairie dogs at up six times this rate. Our results have conservation implications, including expected ferret densities in prairie dog towns.