Calving Patterns Among American Bison

Oklahoma Cooperative Fish and Wildlife Research Unit
Department of Zoology
Oklahoma State University
Stillwater, OK 74078.

TRACY S. CARTER, Museum of Natural and Cultural History, Oklahoma State University, Stillwater, OK 74078

Reprinted from the Journal of Wildlife Management. 53(4):1989, 896-898


We monitored calving in American bison (Bison bison) for 8 consecutive years on the Wichita Mountains Wildlife Refuge, Oklahoma. Calving rates were significantly lower among 2- and > 13-year olds than among females 3-13 years old. Although mature females produced from 3 to 7 calves, none produced calves in alternate years. Nonlactating females produced equal numbers of male and female calves. Manipulation of age structure in bison should have little effect upon calf production unless a substantial proportion of adult females reach age 14.

Calving rates among mature female American bison range from <50% in the Slave River lowlands in the Northwest Territories (Van Camp and Calef 1987) and 52% in Yellowstone National Park, Wyoming (Meagher 1973), to 66.9% in the Wichita Mountains, Oklahoma (Halloran 1968), 78.4% for Fort Niobrara National Wildlife Refuge, Nebraska, and Wind Cave National Park, South Dakota (Haugen 1974), and 88.2% for the National Bison Range, Montana (Rutberg 1986). Meagher (1973) suggested that a preponderance of aged females could reduce calving rates. Halloran (1968) concluded that calving rates in the Wichita Mountains were influenced by a tendency of individual females to produce calves in alternate years, a pattern that he attributed to superior condition of nonlactating females. Rutberg (1986) reported that 25 of 29 nonlactating females at the National Bison Range carried male fetuses, compared with 26 of 53 male fetuses in lactating females. He argued that this difference supported the Trivers-Willard Hypothesis that predicts that females in good condition produce larger proportions of male offspring than do females in poorer condition (Trivers and Willard 1973).

We tested the following hypotheses: (1) calving rates show no change with age, (2) female bison often produce calves in alternate years, and (3) non lactating females are more likely to produce male than female offspring.

We thank F. Schitoskey, Jr. and 0. E. Maughan for their encouragement and technical assistance. We also thank A. A. Kocan, T. R. Thedford, and personnel of the Wichita Mountains Wildlife Refuge for their crucial assistance in marking bison. W. D. Warde and D. M. Leslie, Jr. advised us on specific statistical treatments, D. M. Leslie, Jr. provided constructive criticisms of an earlier draft. The New York Zoological Society's Wildlife Conservation International Program funded our investigation and the Oklahoma Cooperative Fish and Wildlife Research Unit, U.S. Fish and Wildlife Service, providedlogistic and administrative help.

Study Area and Methods

The 23,917-ha Wichita Mountains Wildlife Refuge in southwestern Oklahoma consisted of mixed-grass prairie (Buck 1964), post oak (Quercus stellata)-blackjack oak (Q. marilandica) woodlands (Crockett 1964), and granite mountains surrounded by a 2.6-m big game fence. Our investigation took place in the 13,500-ha Special Use Area (SUA) separated from the refuge by an internal big game fence. The SUA contained approximately 350 bison during our study.

As part of a study of long-term associations in bison (Shaw and Carter 1988) we captured all members of a female-calf group in May 1981 in 1 of the refuge's holding pastures. We worked each bison through a squeeze chute, where we applied numbered ear tags and determined ages. Ages of 10 adult females were determined from brands or brucellosis testing tags applied when they were calves; 5 females were tagged as calves; ages of the remaining 13 were estimated by tooth eruption and wear (Fuller 1959). Twelve more female calves born to tagged females were captured during summers 1982 and 1983 by using 2.5-mg etorphine hydrochloride in combination with 20-mg xylazine hydrochloride administered by dart fired from a powder charged projectile gun.

We determined calf production by observing tagged females in the field for prolonged periods each May and June and recording the sex of calves. Two-year-old females composed the first age category. Other age classes included young adults (3-7 yr), old adults (8-13 yr), and aged animals (> 13 yr). Differences in calf production by age class were tested with a 4 x 2 contingency table; 2-sample Z-tests were used to compare calving rates between adjacent age classes.


Three of 25 2-year olds produced calves. Although fecundity rates varied for females >2 years the mean was 71.8%. Overall, calf production rates varied significantly by age class (X2 = 44.49, 3 df, P 13 years produced significantly fewer calves than did 8-13-year olds (P < 0.005).

Calf production/female varied from 3 of 8 years to 7 of 8 years (0 = 0.66 " 0.17 [SD] for 10 mature F followed all 8 yr). None of these 10 females produced calves only in alternate years.

Lactating females (those producing a surviving calf the previous yr) produced 30 male and 39 female offspring. Nonlactating females gave birth to 26 male and 27 female calves (X2 = 0.38, 1 df, P > 0.50).


The low calving rate for 2-year olds was expected because bison rarely breed as yearlings (Reynolds et al. 1982, Meagher 1973). Our sample of 2-year olds calved at a rate (12%) close to that reported earlier for the same population (Halloran 1968), although our sample may contain bias. All 3 of the 2-year olds that gave birth were from a sample of 7 animals aged by tooth eruption, compared with zero births from 18 2-year olds tagged as calves. Tooth eruption rates can vary by age (J. H. Shaw, unpubl. data; Winchell 1963), so any of the 3 births reported here for 2-year olds could have actually been to 3-year olds.

Calving rates for young and old adults did not differ significantly from one another and were close to those of Halloran (1968). After age 13, calving rates declined. This pattern is consistent with Reynolds et al.'s (1982) suggestion that reproductive success was highest from 3 years old until the onset of old age.

Halloran's (1968) data were collected from 1959 to 1966 when the bison population averaged 3.46/km2, compared with 2.30/km2 during this investigation (Wichita Mt. Wildl. Ref., unpubl. data). Perhaps nutritional stresses imposed by the higher population density may explain the alternate year pattern in calving that Halloran (1968) observed.

Trivers and Willard (1973) predicted that a species with small litter size, strong male-male competition, and a correlation between maternal condition and condition of offspring at maturity should produce a greater proportion of male offspring from females in superior condition. Rutberg (1986) assumed that nonlactating females would be in better condition than would lactating ones. He reported that 86% of a sample of nonlactating females from the National Bison Range carried male fetuses, thus supporting the Trivers- Willard hypothesis.

Although lactating females in our sample produced more female than male offspring (39 vs. 30), the sex ratio of calves produced by non-lactating females was balanced (27 and 26). Our sample did not support the Trivers-Willard hypothesis.

Management Implications

Managers of intensively managed bison populations routinely corral them, examine them in squeeze chutes, and estimate individual ages. In some populations, individuals are branded as calves, permitting precise age determination (R. M. Ellis, Fort Niobrara Natl. Wildl. Ref., pers.commun. ). Bison numbers are usually controlled by sale at live auction. Managers of these populations thus have greater opportunities to manipulate age structures than do most other wildlife managers. While consistent demand for calves and yearlings requires managers to dispose of large numbers of pre-reproductive aged individuals, managers have some discretion in the sale of older bison. They could, for example, favor a young age structure through sale of more old adults and aged animals, or they could favor old age structures by auctioning larger proportions of young adults.

Our study implies that there is no advantage to maintaining a younger age structure (e.g., having virtually all F in the 3-7 yr age group). Theoretically, there is an advantage in maintaining an older age structure (e.g., breeding F throughout 3-13 yr) that results in a greater mean generation interval, thereby reducing effects of inbreeding in closed populations (Soulé 1980). This theoretical advantage may be of limited practical importance in managed bison populations because generation interval may have less effect on inbreeding than reproductive rate, variance in lifetime production, or both

(Shull and Tipton 1987).

Literature Cited

BUCK, P 1964 Relationships of the woody vegetation of the Wichita Mountains National Wildlife Refuge to geological formations and soil types.

Ecology 45:336-344

CROCKETT, J. J. 1964 Influence of soils and parent material on the grasslands of Wichita Mountains Wildlife Refuge, Okla. Ecology 45:326-335.

FULLER, W. A. 1959. The horns and teeth as indicators of age in bison J. Wildl. Manage 23:342-344.

HALLORAN, A. F 1968. Bison (Bovidae) productivity on the Wichita Mountains Wildlife Refuge, Okla. Southwest. Nat. 13:23-26.

HAUGEN, A 0. 1974. Reproduction in plains bison. Iowa State J Res. 49:1-8.

MEAGHER, M 1973 The bison of Yellowstone National Park. Nat. Park Serv. Sci Monogr. 1. 161pp.

REYNOLDS, H. W, R. D. GLAHOLT, AND A. W. L. HAWLEY 1982. Bison(Bison bison).Pages 972-1107 in J. A Chapman and G. A. Feldhamer, eds. Wild mammals of North America biology, management, and economics. The Johns Hopkins

Univ Press, Baltimore, Md.

RUTBERG, A T. 1986. Lactations and fetal sex ratios in American bison. Am. Nat. 127.89-94.

SHAW, J H., AND T S. CARTER. 1988 Long-term associations between bison cows and their off-spring. implications for the management of closed gene pools. Pages 50-55 in J. Malcomb, ed. Proc. North Am. Bison Workshop, Missoula, Mont.

SHULL, A M., AND A. R. TIPTON. 1987. Effective population size of bison on the Wichita Mountains Wildlife Refuge. Conserv. Biol 1:35-41.

SOULE, M. E. 1980. Thresholds for survival. maintaining fitness and evolutionary potential Pages 151-169 in M. E. Soule., and B. A Wilcox, eds. Conservation biology: an evolutionary-ecological perspective. Sinauer Assoc. Inc., Sunderland, Mass.

TRIVERS, R L., AND D. E. WILLARD. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90-92.

VAN CAMP, J, AND G. W. CALEF. 1987. Population dynamics of bison. Pages 21-24 in H. W. Reynolds and A. W. L. Hawley, eds. Bison ecology in relation to agricultural development in the Slave River lowlands, NWT. Can. Wildl Serv. Occas. Pap. 63.

WINCHELL, J. R 1963 Age criteria for American bison. MS. Thesis, Univ. Montana., Missoula. 70pp.

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