Call number: LD2668 .T4 1957 B76 / Master of Science

Call number: LD2668 .R4 1957 E47 / Master of Science / Entomology

The antelope jackrabbit (Lepus alleni) inhabits the seasonal landscape of the subtropical Sonoran savanna grassland in southern Arizona. Basic ecological information on this understudied lagomorph is lacking beyond historical responses to rangeland conditions. This is the first study to utilize radio collars to assess space use of antelope jackrabbits. In the semidesert grassland of Buenos Aires National Wildlife Refuge, Arizona, we estimated antelope jackrabbit home range size, seasonal ranges, and movement patterns. Home range estimates were comparable to other Lepus species, however, seasonal range sizes did not differ. We analyzed antelope jackrabbit habitat structure, measured vegetation characteristics, and determined whether there was a spatial association between antelope jackrabbits and the endangered Pima pineapple cactus (Coryphantha scheeri var. robustispina). Antelope jackrabbits selected vegetation structure and characteristics similarly to available areas on the refuge. We did not detect a spatial association between antelope jackrabbits and Pima pineapple cacti, however given the importance of understanding endangered species relationships, further investigation is warranted. Our results add to the limited ecological information known about antelope jackrabbits and provide baseline data for future studies. Knowledge about spatial ecology and habitat selection helps managers and biologists make informed recommendations for land and wildlife management.

The purpose of this report is to describe (1) the demographic pattern of a black-tailed jackrabbit (Lopus californicus) population in 450-square-mile Curlow Valley, northern Utah and southern Idaho, during the period April, 1962 through March, 1965; and (2) the methodology developed to obtain the data on which the description is based. The research discussed here occupied the first 3 years of a projected, long-range study of the population ecology of the species in this area with special attention to the mechanics and causes of short-term fluctuations.

Population biology of the black-tailed jackrabbit population on a 250-square-mile area in Curlew Valley, northern Utah, was studied from 1962-70. During this period the fall population density index increased from 40.0 in 1962, to 60.6 in 1963, decreased progressively to a low of 21.2 in 1967, then increased the following 3 years to a high of 185.0 in 1970. Breeding was synchronous with four conception periods each year; in some years a fifth conception period was evident. The first conception period occurred about the last half of January; other periods followed at 40-day intervals indicating a 40-day gestation period and postpartum estrus with subsequent conception. Over the 9 years of study, the mean percentages of females breeding during the five conception periods were 88, 99, 100, 70, and 11 percent, respectively. The mean number of ova ovulated per breeding female for the five periods was 1.9, 5.1, 6.4, 4.9, and 3.6, respectively. During the period of decreasing density, 1963-67, the yearly mean number of ova ovulated per female surviving the breeding season ranged from 13.2-19.3, but varied independently of density. During the 3 consecutive years of density increase, 1968-70, however, the number of ova ovulated per female decreased progressively from 19.8 in 1968 to 14.2 in 1970. Mortality rates of the total population from October-March remained relatively constant (mean: 63 percent) during the years of population decline, but dropped to 33 percent during the first year of population increase (1968). March-October mortality of adults decreased to 9 percent during the first year of population increase from a previous mean of 73 percent, and juvenile mortality from parturition to fall census, decreased from a mean of 68 percent to 38 percent. The effects of variations in mortality rates on population density have overshadowed the effects of the relatively less extreme variations in natality rates. As a result the pattern of density change was almost entirely a result of changes in mortality rates. Changes in mortality rates of adults and juveniles were well correlated with the coyote/rabbit ratio on the study area. Exceptions occurred with juvenile mortality rates at the relatively high rabbit densities observed in 1969-70. During these two years, juvenile mortality rates from parturition to fall census (61 and 68 percent, respectively) were greater than could be accounted for by coyote predation. The factor or factors responsible for the increased juvenile mortality are not known. Observed annual density changes were described with the mathematical model: Nt+4 = Nt (1 - 37.8 - 988x1) (1 + 11.2 - 1130x2 - 0.0581x3 + 42000x22 + 0.00115x32) where Nt is the number of animals at the end of October, Nt+1 is the number of animals at the end of the following October, x1 is the coyote/rabbit ratio from October-March, x2 is the coyote/rabbit ratio from March-October, and x3 is the mean number of rabbits per square mile from March-October. The model accounts for 99 percent of the observed change in rabbit density from 1968-70.

Field and laboratory studies were conducted to develop a model of energy expenditure in a population of white-tailed jackrabbits (Lepus townsendii). Field work in southwestern Wyoming during 1985-87 showed that the breeding season commenced at snowmelt and ceased during late- July drought. Adult females reproduced relatively synchronously and produced a mean of three litters annually. Greatest fetal production occurred in the second litter period. Collections indicated a 1:1 sex ratio, few jackrabbits >2 years-old, and a density of 7 animals / km2 in the population. Postnatal growth was sigmoidal, culminating in heavier adult females than males. Using radio -telemetry, a circadian rhythm was detected in jackrabbit activity, with movement beginning after sunset and ending by sunrise. Observations showed that season, snow cover, weather, lunar phase, and predators influenced activity. Energetics studies established the pattern of seasonal acclimatization in the jackrabbit. Basal metabolic rate (BMR), pelage thickness, and body temperature increased but overall thermal conductance (C) and the lower critical temperature (LCT) declined from summer to winter. High winds and low air temperatures elevated metabolism interactively and their effects were most pronounced during summer. Metabolic rate dee lined with incident radiation at Ta< LCT during winter but not during summer. In newborn jackrabbits, body temperature dropped despite increased metabolism at Ta< 25°C. Cold tolerance and homeothermy developed with age. Based on these and published data, a FORTRAN model was written that simulated the energy expenditure of a population of jackrabbits. Metabolizable energy requirements for maintenance , thermoregulation, reproduction, growth, and activity were estimated. The model indicated that most energy (kJ·kg - l.day- 1) was required by adult females during lactation, adult males at the onset of breeding, and newborn juveniles. Energy expenditures for adult females, adult males, and juveniles were 191, 130, and 224 MJ·individual-1·km-2·yr-1, respectively. Total energy expenditure increased with wind and lower air temperature and decreased if juvenile huddling was simulated. The model indicated that the jackrabbit population is not limited by food. Estimated percent consumption of forage energy was 4%, assuming 50% of phytomass was edible, the population density = 100 jackrabbits/km2, metabolizable energy efficiency= 0.4, and the air was calm.