AN ABSTRACT OF THE THESIS OF Roger Alan Powers for the Master of Science (Name) (Degree) in Wildlife presented on (Major) (Date) Title:THE REPRODUCTIVE BIOLOGY OF THE MOUNTAIN COTTONTAIL RABBIT IN OREGON Redacted for privacy Abstract approved: B. J. Verts The reproductive characteristics of themountain cottontail rabbit, Sylvilagus nuttalli (Bachman),were investigated from 269 specimens collected in central Oregon between28 May 1968 and 28 August 1969. Male rabbits were in breeding condition betweenmid-February and 3 July 1969.Nearly 77% of the females bore four litters between 7 March and 28 June; 23% bore five littersbetween 22 February and 31 July 1969. Evidence of immediate postpartum estrus and synchronous breeding were observed.The mean size of litters was 4.3young per litter in 1969.There were no significant differences (P > 0.05)among the sizes of synchronous litters. Juvenile males became sexually mature in February when they were at least seven months old.Pregnancy was observed in one juvenile female two to three months old. Maximum annual productivity was 18.2 young per adult female, and 0.3 young per juvenile female in 1969. The 1969 breeding season may have been delayed due to severe weather in January and February 1969. The Reproductive Biology of the Mountain Cottontail Rabbit in Oregon by Roger Alan Powers A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1970 APPROVED: Redacted for privacy Assistant Professor of Wildlife Ecology in charge of major Redacted for privacy He /d of Department of Fisheries and Wildlife Redacted for privacy Dean of Graduate School Date thesis is presented Typed by Cheryl E. Curb for Roger Alan Powers ACKNOWLEDGEMENTS I would like to express my appreciation tothe Theodore Roose- velt Memorial Fund Committee,American Museum of Natural History, for their generous grant whichfinanced the major portion of this in- vestigation. I am indebted to many persons whovolunteered their aid when it was most needed, among them wereMr. John A. Gill and Mr. Donald E. Trethewey of the Department ofFisheries and Wildlife Oregon State University, and Mr. Richard L.Lantz of the Oregon State Game Commission. My special thanks must go to Mr. MurlG. Bright of Corvallis, whose aid, regardless of personal inconvenience ordiscomfort, greatly contributed to the success of thisinvestigation, and whose experience, knowledge of the outdoors,and good humor were invalu- able. I feel fortunate to have worked withDr. B. J. Verts, Assistant Professor of Wildlife Ecology, OregonState University, whose efforts on my behalf wereappreciated. TABLE OF CONTENTS Page INTRODUCTION 1 MATERIALS AND METHODS 5 Study Area 5 Collection 7 Necropsy 9 RESULTS AND DISCUSSION 14 Sex Ratios 14 Breeding Season 15 Breeding Synchrony 17 Number of Litters in 1969 20 Size of Litters 21 Male Reproductive Cycle 22 Juvenile Sexual Development 27 Males 27 Females 28 Annual Productivity 29 BIBLIOGRAPHY 30 LIST OF FIGURES Figure Page 1 Locations from which mountain cottontail rabbits 6 were collected, Deschutes and Jefferson counties, Oregon, June 1968-August 1969. 2 Mean numbers of man-hours required to collect a 8 mountain cottontail, by month and by collecting trip, Deschutes and Jefferson counties, Oregon, 8 June 1968-28 August 1969. 3 Frequency distribution of dates of breeding for 18 mountain cottontail rabbits collected, Deschutes and Jefferson counties, Oregon, 1969. 4 Average weights by month of testes of adult male 25 mountain cottontail rabbits.Deschutes and Jeffer- son counties, Oregon, June 1968-August 1969. LIST OF TABLES Table Page 1 Size of litters and duration of breeding season 4' for the mountain cottontail rabbit as reported by various authors. 2 Numbers of female mountain cottontail rabbits 16 collected in Deschutes and Jefferson counties, Oregon, each collecting trip,1 January-28 August 1969, and numbers of females consid- ered to be pregnant through use of each of sev- eral criteria of pregnancy. 3 Mean sizes of litters and mean rates of ovulation, 23 implantation, and resorption for each litter among female mountain cottontail rabbits, Deschutes and Jefferson counties, Oregon, 1969. THE REPRODUCTIVE BIOLOGY OF THE MOUNTAIN COTTONTAIL RABBIT IN OREGON INTRODUCTION The mountain cottontail rabbit, Sylvilagus nuttalli (Bachman), is a species native to much of the western portion of the contiguous United States (Hall and Kelson1959:261, Map 185).In Oregon, these rabbits occur in varying abundance in the Upper Sonoran life zone east of the Cascade mountains(Bailey1936:107, Dice1926:17). They appear to be most common in the vicinity of lava buttes, low lava hummocks, and other rocky outcrops (Bailey1936:107-108, Orr 1940:105-106). The mountain cottontail is designated a game species inonly 5 of the 14 western states constituting its principal range(Kozicky and Madson 1969).The species is not considered to be a game animal in Oregon, thus, its populations do not receive the benefitof restric- ted hunting seasons or bag limits.Nevertheless, mountain cotton- tails appear to possess many of the characteristicsof an excellent game animal. Some of these rabbits areknown to be killed for sport and food in Oregon, but most are probably killed by sportsmenhunting upland game birds.The number harvested annually in Oregon is unknown. 2 Knowledge of the biology of the mountain cottontailis extremely limited compared with that of other membersof the genus. A search of the literature failed to reveal a singlepublished report devoted solely to the ecology and life history of thespecies.Other than brief notes on the life history of mountaincottontails published as part of regional works on mammals (Bailey1936:107-109, Ingles 1965:148- 149, Dalquest 1948:387-389, Hall1946:607-612, Davis 1939:360-363, among others), in unpublishedtheses (Janson 1946:51-52 63-65), and in monographs devoted principally tothe taxonomy of the Order Lago- morpha (Hall 1951:119-202, Orr1940:98-109, Nelson 1909:199-211), information about the species is restrictedprimarily to that concerned with distribution (Dalquest 1941:408-411)and parasites (Jellison 1945:958-961, Honess 1939:281-284). In order to more completely understandthe role of mountain cottontails in the ecology of arid and semiaridregions, it is impera- tive that knowledge of the basicbiology of the species be augmented. Knowledge of the reproductive characteristicsand potential produc- tivity of mountain cottontailsshould contribute to that understanding, and hopefully should provide a basisfor certain decisions regarding the species when initiation of managementpractices are deemed necessary, Information concerning the reproductivebiology of mountain cottontails tends to be sketchy, and, in most cases,based on small 3 numbers of specimens.Size of litters was reported to range from one to eight (Table 1),The onset of the breeding season was reported to range from February to April, andthe reproductive season was re- ported to terminate from June to August(Table 1).The gestation period was listed as 28-30 days (Cowan andGuiguet 1956:107), but the basis for this estimate was not reported.Orr (1940:108) believed that two litters annually were probably maximumfor the species in California, Cowan and Guiguet (1956:107) reportedtwo to three litters annually in British Columbia, and Bailey(1936:108) was unable to de- termine the occurrence of more than a single annuallitter in Oregon. The primary objective of this research wasto describe the reproductive biology of the mountain cottontailin central Oregon. Specific objectives were: 1. To determine the ages at which male andfemale mountain cottontails attained sexual maturity. 2. To determine the chronology of thebreeding season. 3. To determine the size of litters andnumber of litters produced annually. 4. To determine if breeding amongfemales was synchronous. 4 Table 1.Size of litters and duration of breeding season for the mountain cottontail rabbit as reported by various authors. State No. Litter Size or Females Aver- Authority Province Examined age Range Season Hall r) 8 6.1 4-8 ? (1951) Hall Nevada 6 5 4-6 March-July (1946) Dalquest Wash. ? ? 1-4 Apr. -June (1948) Davis Idaho 1 6 Apr. -June (1939) Bailey Oregon Apr. -July (1936) Orr Calif. & 8 6.1 4-8 Apr. -July (1940) Nevada Dice Wash. & 3 4.7 4-5 ? (1926) Oregon Janson Utah 13 6, 6 5-8 Late Feb. - (1946) March; Late July or Aug. Cowan and British ? 2 Late April- Guiguet Columbia Late June (1956) 5 MATERIALS AND METHODS Study Area Eight areas located in Deschutes and Jefferson counties of central Oregon were used in this study (Figure 1).All areas used were within the Deschutes river drainage.Broad valleys and prominant buttes were characteristic of the main river valley, but deep incised canyons were associated with the smaller tributary streams.Elevations ranged from 2100-5100 feet among the eight study areas. Eroded Columbian lava flows, dipping gently eastward from the Cascade mountains, formed extensive continuous escarpments. Locally, lava occurred in small tortuous hummocks 10-30 ft high partially buried by eolian sands.These hummocky areas were identified on local maps as "lava lakes or beds. " The climate of the region was semiarid; annual precipitation averaged approximately ten inches.The greatest precipitation occurred as rain or snow between October and May.Monthly pre- cipitation during the remainder of the year averaged one inch or less. Absence of cloud cover and low relative humidity contributed to large diel fluctuations in temperature (U.S. Weather Bureau 1968). Agriculture, except for cattle grazing, was restricted to irri- gated valley and canyon floors due to the steep slopes and rocky soil DESCHUTES CO, N A Collection Site Map Location Figure 1.Locations from which mountaincottontail rabbits were collected, Deschutes and Jefferson counties,Oregon, June 1968-August 1969. 7 of the uplands.Extensive upland areas were publicly owned and under the management of the U. S. Forest Service orBureau of Land Manage- ment.