Reproduction in Cottontails Fed Diets Containing a PCB

Abstract

Fifty-five female cottontail rabbits (Sylvilagus floridanus) were placed on an ad libitum or a 75 percent ad libitum diet for 12 weeks while confined in laboratory cages. One-half of the animals of each feeding regime received 10 ppm (based on ad libitum intake) of a polychlorinated biphenyl (Aroclor 1254) in their diet during this time. All animals then were released into 0.1-ha outdoor enclosures with males for 3 to 4 weeks and received untreated feed ad libitum. Animals fed the restricted diet gained weight rapidly when placed outdoors on ad libitum feed and had higher ovulation rates and fetal rates than animals fed ad libitum throughout the study. PCB treatment had no effect on body weight change or any reproductive characteristics studied. J. WILDL. MANAGE. 40(3):491-495 The cottontail rabbit is one of the most important game mammals in the United States. It is also one of the few mammalian species often encountered in urban areas. Much ecological research has been conducted on this species, but relatively little is known of specific effects of nutrition and exposure to environmental contaminants on its reproductive functions or its abundance. Previous work (Stevens 1962, Kirkpatrick and Kibbe 1971, Hill 1972) indicated that reproductive rates of cottontails may be influenced by nutritional intake. Work by Ringer et al. (1972) demonstrated that low level ingestion of a widespread industrial contaminant, a polychlorinated biphenyl (PCB), at relatively low concentrations (1 ppm) can impair reproduction in ranch mink (Mustela vison). Further, Conney and Burns (1972) suggested that PCB ingestion with the accompanying induction of hepatic microsomal enzyme activity may result in increased metabolism of steroid hormones with subsequent endocrine imbalance and impaired reproduction in many mammalian and avian species. Many environmental contaminants (especially the chlorinated hydrocarbon insecticides) are potent hepatic microsomal enzyme inducers, so the study of PCB effects on reproductive hormone systems is applicable to similar compounds as well. PCBs are particularly well suited for the study of hepatic microsomal enzyme induction because they are strong microsomal enzyme inducers but, in contrast to chlorinated hydrocarbon insecticides, are relatively nontoxic. Thus, they allow the study of enzyme induction without the complicating effects of toxicity. The present study was conducted to determine the singular and combined effects of nutritive restriction and PCB ingestion on reproductive and associated characteristics of female cottontail rabbits under controlled conditions. This research was supported in part by U.S. Public Health Service Grant 1-R01-ES00863. We gratefully acknowledge Monsanto Chemical Co., St. Louis, Mo., for supplying the Aroclor 1254. MATERIALS AND METHODS Fifty-five female cottontail rabbits, livetrapped in Montgomery County, Virginia, were used in a 2 x 2 factorially designed experiment. The four groups were designated as follows: ad libitum-no PCB, re1 Present address: Equitable Environmental Health, Inc., Box 247, Batavia, Illinois 60510. J. Wildl. Manage. 40(3):1976 491 This content downloaded from 157.55.39.207 on Thu, 20 Oct 2016 04:13:06 UTC All use subject to http://about.jstor.org/terms 492 REPRODUCTION IN COTTONTAILS FED PCB Zepp and Kirkpatrick stricted-no PCB, ad libitum-PCB, and restricted-PCB. The animals were housed individually in steel laboratory rabbit cages (Hoeltge, Inc., Cincinnati, Ohio), each 55 x 25 x 33 cm in dimension. Water was treated with sulfaquinoxaline (25 percent S. Q. soluble, Merck & Co., Inc., Rahway, N.J.) at a concentration of 0.0125 percent to control coccidiosis. The laboratory was maintained on a 14:10 hour light:dark schedule and the temperature was kept at approximately 25 C. Feed (Purina Rabbit Checkers, Ralston Purina Co., St. Louis, Mo.) was provided in excess to the ad libitum rabbits, whereas the restricted rabbits received a 75 percent ad libitum diet, based on the mean consumption of the ad libitum groups during the previous week. The ad libitum-PCB rabbits were fed the same ration treated with 10 ppm PCB (Aroclor 1254). The PCB was dissolved in acetone and sprayed onto 2 kg of feed while it was being mixed in a food mixer. The restricted-PCB rabbits received a ration containing 13.3 ppm PCB to equalize PCB intake with that of the ad libitumPCB group. Feeding of the diets began, after 1 to 2 months of pre-experimental confinement, on 22 January 1973, and continued for 12 weeks. Four or 5 animals from each treatment were bled from a marginal ear vein during weeks 2, 5, 8, and 16 to obtain plasma. Plasma corticoids were determined by use of a competitive protein binding assay (Murphy 1964, 1967). A primary objective of the study was to determine ovulation and conception rates of rabbits on the various treatments. Cottontails will not breed under laboratory confinement, however, so the rabbits were released during the 12th week of treatment into four 0.1-ha rabbit enclosures located in a woodlot on the VPI & SU campus. All females were assigned randomly to the pens regardless of treatment group. Four to five male rabbits, on a similar experiment, were assigned randomly to each pen for breeding purpo es. During outdoor confinement, untreated food and water were supplied in