Factors influencing reproductive success and litter size in captive island foxes

A severe decline of island foxes (Urocyon littoralis) on the northern Channel Islands in the 1990s prompted the National Park Service to begin a captive breedingprogram to increase their numbers. Using detailed records of all the fox pairs(N = 267) that were part of the program on San Miguel, Santa Rosa, and Santa Cruz Islands from its inception in 2000 through 2007, we identified factors influencing the breeding success of pairsin captivityin the interest of formulating strategies that could increase captiveproductivity. We compiled a database of variables including litter size, reproductive success, distance to nearest occupied pen during the breeding season, subspecies, exposure, female age, male age, age difference, female and male origin (wild vs. captiveborn), same versus different origin, years paired, previous reproductive successby the pair, previous reproductive successby the female, mate aggression-related injuries, male previous involvement in a pairwith mate aggression, and female previous involvement in a pairwith mate aggression. We used multiple linear regression to identify factors predictive of litter size, and logistic regression to predict the probability of reproductive success. A larger inter-pen distance, older male age, less exposure, and a smaller intra- pairage difference positively affected litter size. The probabilities of reproductive successincreased with fewer years pairedand less exposure. Comparatively, pairswith wild born females (vs. captiveborn females), and previously successful pairs(vs. previously unsuccessful and new pairs) were most likely to be successful. These results indicate that the optimal situation was to pairwild-caught females with older males in sheltered pens that were as far from other pens as possible, to maintain successful pairsand repair unsuccessful ones. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.