The Estrous Cycle of Coyotes

Abstract

The estrous cycle in coyotes (Canis latrans) is characterized by a long proestrus (2-3 months). Vaginal smear patterns associated with proestrus, estrus, and metestrus are described. Data from 41 adults indicated estrus begins about 13 March and persists for 10 days. Neither onset date nor duration of estrus differed with body weight or age; however, the number of corpora lutea increased with age. Of 22 juveniles, 19 showed sexual activity but no more than 11 ovulated. In adults, ovulation apparently occurred as early as the 1st and as late as the 9th day of estrus. Left and right ovaries were similar in weight and number of corpora lutea. J. WILDL. MANAGE. 40(2):272-277 Reproduction has not been studied extensively in coyotes. Hamlett (1938) defined the breeding season, estimated the gestation period, and provided some data on the testicular and ovarian changes peculiar to this seasonal breeder. Whiteman (1940) reported on the breeding habits of several captive coyotes and was the first to call attention to the long proestrum period coyotes experience. Gier (1968) provided some information on the morphological and physiological changes associated with the reproductive cycle. Kennelly (1972) described the coyote spermatogenic cycle and estimated sperm transit time in the epididymis. Studies of dog-coyote hybrids have been a further but limited source of information on coyote reproduction (Kennelly and Roberts 1969, Silver and Silver 1969, Mengel 1971). The data reported here on the coyote estrous cycle and related events were gathered as part of a larger program to define more clearly those aspects of coyote reproduction that need to be understood if coyote populations are to be managed rationally. Our primary objective was to gather information to describe the estrous cycle as reflected by vaginal smears. In examining and breeding females coyotes, we accumulated observations on several related topics, and these data are included. MATERIALS AND METHODS Sixty-four female coyotes that were either born in captivity or captured near San Antonio, Texas, and Denver, Colorado, when 3-5 weeks of age were raised and maintained at Denver under routine dog colony procedures. Food (Wayne Dry Dog Food) and water were provided ad libitum. Each coyote was immunized against distemper, hepatitis, and leptospirosis and treated for parasites as needed. The animals were housed one, two, or three to a pen in outdoor concrete-lined runs containing weatherproofed but unheated shelters; illumination was by natural daylight only. Gross and microscopic changes associated with the estrous cycle were observed for both juveniles (less than a year old) and adults. Gier's (1960) technique for making vaginal smears, slightly modified, was used throughout the study. A plastic ca heter approximately 20 cm long with a rubber bulb on one end was inserted into the vagina as far as the cervix, and vaginal fluid was aspirated as the catheter was 1Present address: Massachusetts Cooperative Wildlife Research Unit, University of Massachusetts, Amherst 01002. 272 J. Wildl. Manage. 40 (2):1976 This content downloaded from 207.46.13.10 on Mon, 12 Sep 2016 04:35:11 UTC All use subject to http://about.jstor.org/terms EsTRous CYCLE OF COYOTES Kennelly and Johns 273 slowly withdrawn. When natural fluids were insufficient, a small volume of saline (1.0 cc) was introduced. A drop of the vaginal sample was mixed with an equal volume of an aqueous solution of 0.1 percent toluidine blue. After coverslips were placed on them, the smears were examined within 30 minutes at 400x. Routine aseptic surgical procedures were followed for all operations. Pentobarbitol sodium was given (I.V.) to effect and the reproductive organs exposed and examined through a midline incision. In some cases this was the extent of the operation and the incision was closed; in others, the unilateral ovary, oviduct, and uterus were removed before the incision was closed. The contra-lateral ovary, oviduct, and uterus were collected at different intervals following surgery. In this instance, the bitch was killed first with an overdose of pentobarbitol sodium (I.V.). Each ovary was dissected free of associated tissue, weighed, and sectioned transversely with a razor at about 1.5-mm intervals and examined grossly to determine the number of corpora lutea present. Each oviduct was cut just proximal to the tubo-uterine junction and flushed with physiological saline; the flushing medium was checked under a low power microscope (35x) for the presence of ova or zygotes. The basis for defining proestrus, estrus, and metestrus was primarily the cellular pattern observed in the vaginal smear. When appropriate, decisions relative to the onset of estrus were augmented by evidence of acceptance of the male; either copulation was observed or sperm were present in the vaginal smear.