Study on facilitating fertility for breeding of red foxes in captivity

Abstract

　There are few reports about reproduction of captive red foxes（Vulpes vulpes）in Japan. The present investigation was carried out in red foxes maintained at the Zao Fox Village to clarify the backgrounds and factors involved in their reproduction, to maintain their genetic diversity, and to facilitate their fertility with improved conception rates. First, the experiment was designed to delineate the reference ranges for vital signs, hematology, coagulation and serum biochemistry in the non-breeding season during November using clinically healthy adult foxes. No sex difference was noted in any of the items tested. Both mean values for male and female foxes were compared with the reference ranges for healthy dogs. In foxes, elevated rectal temperature, high values in erythrocyte and eosinophil counts with lowered erythrocyte indices, lowered butyrylcholinesterase and elevations in glucose, albumin, and urea nitrogen in sera were observed. Reference ranges for various biological items assist veterinary clinicians and researchers in their interpretations of observations of red foxes. Next, the study was conducted to elucidate a practical tool for predicting conception in red fox vixens subjected to natural mating. Basal vaginal electrical resistance（VER）during post-breeding seasons consistently ranged from 90 to 220 units（mean: ca. 140 units）. The VER displayed a single peak after the vulva became well-swollen during breeding seasons. Median VER magnitude was 350 units among all vixens that became pregnant, 230 units among vixens that did not become pregnant, and 190 units among vixens that did not mate. Conception rates in vixens with VER above 350 units were dramatically higher when coital-lock time was more than 40 min. These results suggest that a peak magnitude（over 280 units）of VER with coital-lock time may be an expedient predictor of conception in red foxes. Then, the appropriate voltage（V）in electroejaculation for semen collection from stud male red foxes was investigated, as well as whether frozen semen with bovine semen extender can be used for artificial insemination. The proper load voltage for electroejaculation was 3-4 V in 2-4 cycles（1 cycle: 1-sec duration each, with a 1-sec rest）based on semen collection rates with concentrations of spermatozoa. The semen collection rate was 66.2% under the conditions of this study. In stud donors, the ejaculate volume of semen was 0.47 ± 0.3μℓ, the concentrations of spermatozoa were 592 ± 85 × 10⁶ cells/mℓ, and the score for the viability index of spermatozoa was 65 ± 5. Frozen semen was prepared according to the known procedure for cows. In frozen-thawed semen, a relatively high conception rate（81.3%）was obtained in vixens. These findings demonstrate that the defined condition for semen collection by electroejaculation with cryopreservation of semen using bovine semen extender can be applied to artificial insemination of red foxes. Finally, breeding profiles at the periparturient stage in red foxes subjected to natural mating or artificial insemination were retrospectively surveyed using 130 vixens during their reproductive seasons of 2012 to 2017.Coupling of vixens with natural mating was attempted a maximum of three times with the same male, while artificial insemination was conducted using frozen-thawed semen with the bovine semen extender. With natural mating, conception rates in once, twice, and thrice copulations were 55.8%, 68.0%, and 85.7%, respectively, showing a significant difference between once and thrice copulation. Conception rates with artificial insemination were 82.4%. Mean gestation periods were between 52.1 and 53.3 days in all groups. Mean litter sizes were 3.7 to 4.3 cubs with natural mating, and 4.4 cubs with artificial insemination. No difference in breeding profiles was noted between primiparous and multiparous vixens. These results are available for understanding basic fox theriogenology in Japan.