Reproduction in the Plains Bison

Arnold 0. Haugen

Former Leader
Iowa Cooperative Wildlife Research Unit
Iowa State University
Ames, Iowa 50010. 
Present address: RR 1, Box 7, Dorchester, Iowa 52140.

Reprinted from the Iowa State Journal of Research. Vol 49, No. 1, August 1974

Journal Paper No. J- 7552 of the Iowa Agriculture and Home Economics Experiment Station Ames, Iowa. Project 1664. The study was financed by a contract between the Bureau of Sport Fisheries and Wildlife and the Iowa Cooperative Wildlife Research Unit.


Observations on female reproduction in Bison bison bison were made under range conditions in South Dakota and Nebraska in 1965, 1967, 1968, and 1971. Counts were obtained for both corpora lutea and embryos.

Only one of the 17 1½-year-old cows observed had ovulated, but she was not visibly pregnant when sacrificed Oct. 10. The ovaries of all 39 2½-year-old cows had corpora lutea and 34 were pregnant as indicated by the presence of an embryo or fetus. Of 23 cows, 3 ½ to 4 ½ years old examined, 15 had embryos, and an additional four had only corpora lutea. Seventy-one (88%) of 81 cows 5 years old and older were pregnant.

Approximate ages of bison embryos were estimated through use of a rump-crown curve for domestic cattle. The sex ratio for 101 fetuses was 119.4 males: l00 females. Of 154 corpoa lutea observed, 51.9% were in the right ovary.

Breeding dates were calculated from data on age of the embryo and the date the cow was sacrificed. Results indicate that most bison cows conceived between July 21 and August 19, with the peak about August 1. The earliest conception was July 4, and the latest October 2, which indicates that the peak of calving occurred between May 7 and 21 after a gestation period of about 285 days.


Management of bison (Bison bison bison) herds at Fort Niobrara National Wildlife Refuge Nebraska, and at Wind Cave National Park and Custer State Park, both in the Black Hills of South Dakota, provided opportunity for a study of reproductive performance or this species. Reproductive tracts were obtained from cows culled during the months of October and November of 1965, 1967, 1968, and 1971. Opportunity to salvage the reproductive tracts was timely in that the study or corpora lutea and the early stages of prenatal development could be correlated with other studies in progress on bison behavior at Wind Cave and Fort Niobrara. Contemporary behavior studies (Fischer, 1968; Herrig and Haugen, 1970; Shult, 1972; Petersburg, 1973) included bison activity during the rutting and calving seasons.


Bison uteri collected at the time of slaughtering were refrigerated at about 30°F until they could be examined for evidence of breeding. Age of most of the animals concerned was determined from ear-tag records. Where individual females had not previously been tagged as calves (when vaccinated against brucellosis), age was estimated on the basis of tooth replacement and wear (Fuller, 1959). In a few instances a cow's age could only be approximated as adult.

The ovaries, when available, were placed in 70% alcohol for temporary holding, and later were measured for size and cross-sectioned with a razor blade to expose corpora lutea. In some cases, one or both ovaries were not available for examination. Membranes, indicative of pregnancy, if present, were noted even if occurring only as a blastocyst. Grossly visible embryos (for convenience, the terms embryo and fetus are here used as synonymous) were measured for rump-crown length by the technique of Cheatum and Morton (1946). Because a crown-rump length growth curve for bison is not available, the curve reported by Winters et al. (1942) for domestic cows was used to estimate ages of embryos. Sex of the fetuses was noted.

Numbers of sets of ovaries, numbers of uteri, and numbers for which fetal sex was determined vary because, in some instances, only one, and in other instances no ovaries were available. For some animals the uterus was not available. Sex of embryos less than about 40 days old could not be determined accurately.

Results and Discussion

Corpora lutea

Of 154 corpora lutea found in bison ovaries, 80 (51.9%.) were in the right and 74 in the left ovary. In cattle, ovulation has been reported as occurring about 60% of the time in the right ovary (Ulberg, 1962). Of the 132 embryos examined, 64 (48.5%) were from the right, and 68 from the left uterine horn. Eight of 132 embryos were in the opposite horn of the uterus from the ovary containing the corpus luteum of pregnancy.

Three bison were found with two corpora lutea each. Because twinning is a very rare occurrence in bison, the second corpus luteum may represent a second estrus. In one 6 ½ year-old cow killed in December, both corpora lutea were in the right ovary, whereas in another (a 9-year or older cow killed October 1) both corpora lutea were in the left ovary. In a third cow 2 ½ years old, a corpus luteum was in each ovary when the cow was killed on October 26. No sign of chorionic-membrane development could be found in any of these three cows. McHugh (1958) reported one set of twins for the Lamar herd in Yellowstone Park. Fuller (1962) and Halloran (1968) agree on the rarity of twinning. Fuller (1961) found only a single fetus in each of 481 gravid uteri from bison at Wood Buffalo National Park in Canada.

Age-Breeding Relationship of Cows

Bison cows ordinarily conceive for the first time at 2 years of age. This is borne out by the fact that 34 of 39 2-year-old cows studied were carrying embryos. The remaining five, although they had ovulated, were not yet showing signs of pregnancy, lacking visible fetal membranes. They either had not conceived or had been in estrus so recently that embryo tissues were not yet visible. Only one of 17 yearling cows showed a corpus luteum, but no sign of embryo development could be found.

Nineteen of 23 3- and 4-year-old cows had been in estrus, as indicated by the presence of corpora luteum when removed in the October culling. Fifteen of these showed developing embryos.

Eighty-one cows, 5 years old and older, when slaughtered, had a corpus luteum in one or the other of their ovaries, indicating they had undergone estrus in the current breeding season. The remaining 10 (12%) did not show any chorionic membranes. Some of these 10 cows, however, might have bred just before or could have bred later if they had not been sacrificed. Such late breeding is known to occur on rare occasions. In the Yellowstone Park herd winter breeding has been noted in rare instances (McHugh, 1958). Other authors of earlier studies also have noted rare instances of out-of-regular season calving, even winter calving, which would indicate late winter or spring breeding.

Sex Ratios

Embryos (ranging in rump-crown lengths from 7 to 243 mm) were recovered from bison, cows slaughtered between the first week in October and November 12. Sex was determined for specimens roughly 45 mm (rump-crown) in length or longer.

Reproduction in the Plains Bison

The sex ratio of 101 embryos from the years, 1965,1967,1968, and 1971 (63 from Wind Cave, 4 from Ft. Niobrara, and 34 from Custer State Park) averaged 119.4 male (54.5%): 100 female (45.5%). Ten embryos checked in 1969 and 1970 were not included in the sample because of some doubt as to accuracy in determining sex of the small-sizedembryos involved and the limited experience of the assistant at the time.

The 54.5% male component in the embryo count compares favorably with findings for Yellowstone National Park bison as reported by Meagher (1970) quoting records of Rush (1932), and from four later herd reductions where the percentages of male embryos in uteri ranged from 52% to 62% and averaged 56% for 348 fetuses. Fuller (1960) found a sex ratio of 53% males for 472 embryos from wild bison in Wood Buffalo National Park in Canada.

Rutting Season

After assigning ages to the various embryos studied, it was possible to back-date the embryos to an earlier date believed to represent date of conception. The distribution of the calculated breeding dates checks closely with bull-cow, tending-bond activity (the rut) observed in the field each July through early September by Shult (1972) and Petersburg (1973). Therefore, the growth curve for embryo fetuses of bison and mixed-breed cattle, both of which have roughly a 9½-month (285 days) gestation period, seems to differ little for the first 3 months after breeding.

Resulting data indicate that the peak of breeding occurred between July 21 and mid-August. A secondary and smaller peak of breeding occurred between August 25 and September 3. Cows breeding at 2½ years of age showed a bimodal distribution, with 10 or the 34 breeding in the latter period. Possibly the second and smaller peak in breeding may represent a second estrus. If bison are similar to cattle (Ulberg, 1962), a bison cow can be expected to experience another estrus 17 to 24 days later, if she does not become gravid as a result of the first estrus. Chances for heifers being bred successfully on the first estrus seem smaller than for the older animals because heifers have a shorter estrus than do older domestic cows. The normal duration of estrus in domestic cows ranges from 9 to 28 hours (Hafez and Schein, 1962).

The sample of calculated breeding dates, however, probably is biased toward early breeding because the animals culled were not taken at random but often were selected because they showed no signs of suckling a calf. Animals in the 3 ½ -year class and older can be expected to have calved; therefore, if still suckling a calf, may breed a little later in the season, as has been indicated by Petersburg (1973).

In cows nursing calves, the first heat occurred 72 days after calving (Clapp, 1937; Casida and Wisnicky, 1950). These authors also indicated that cows nursing calves had a longer period between calving and the onset of estrus than cows milked twice a day. If bison have a similar delay before the first estrus after calving, and with the peak of calving occurring in mid-May, the cows suckling calves born during that peak may still come into estrus during the peak of the breeding season in early or mid-August.

Age-Breeding Condition of Bulls

Breeding condition in bulls was indicated by the presence of sperm in the epididymis of testes of 16 of 22 bulls sacrificed in the fall. Sperm occurred in three of six yearlings, five of SIX 2-year-old bulls, four of five 3-year-old bulls, three of three 4-year-old bulls, and one 5-year-old bull (Shult, 1972). From observations on dominance among bulls on the range however, it is unlikely that a 1-year-old bull would have opportunity to breed. A bull at the age lacks dominance over cows, a dominance that seems necessary for successful copulation in cattle. An even more important condition precluding breeding by a yearling bull is that a dominant older bull establishes a bull-cow tending bond with any cow coming into estrus. The dominant bull drives away intruders of lesser dominance. Younger bulls and others of lower rank, which commonly trail after the tending bond pair, may be realistically referred to as "satellite" bulls.


Approximate ages of bison embryos were obtained by comparing rump-crown lengths of the bison embryos with the growth curve for domestic cattle embryo lengths. The back-dating of those ages to determine approximate dates of conception shows that breeding occurred well within the period of bull-cow, tending-bond activity as noted in field studies by Shult (1972) and Petersburg (1973).

Indications are that 72 of the 121 (60%) of the cows studied would have calved between May 7 and 21. This peak in calving checks closely with postpartum calf counts on the same area in 1969 (Shult, 1972) (Fig. 2). His study showed that, of 61 calves produced that year, one-third had been dropped by May 7, two-thirds by May 13, and the remaining third thereafter. Evidence therefore indicates that, with use of the embryo growth curve for cattle (Winters et al., 1942), both breeding and calving peaks can be calculated with adequate accuracy and that the gestation period for bison is about 285 days, as reported by Seton (1929). The 9-month period reported by Brown (1936) is too short.


Acknowledgment is extended to Milo Shult and Allan Egbert, graduate students, and to undergraduate part time employees, who assisted in the collection and handling of some of the female reproductive organ specimens. The cooperation of Custer State Park personnel in the salvaging of uteri of bison slaughtered also is appreciated. The cooperation of personnel of the National Parks Service and the Bureau of Indian Affairs is acknowledged.


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