IBC2000-1  Archeology

Preferential Selection and Killing of Bison by Plains Indians

Jack W. Brink
Archaeological Survey
Provincial Museum of Alberta
12845 - 102 Avenue
Edmonton  AB  Canada  T5N 0M6

 

The following article was originally presented at the International Bison Conference in Edmonton, Alberta in August 2000.  The conference covered a wide array of bison topics including production, marketing, genetics, history and much more.  This article has been reprinted with the permission of the IBC2000 Chairman.

Summary

This paper looks at two broad topics; a review of bioenergetic processes in large mammals, and an examination of historic bison hunting patterns on the northern Plains. I argue that these diverse areas are related in that hunting preferences are regulated primarily by differing animal condition, which in turn is dictated by bioenergetic processes. The bioenergetic trends that I discuss are applicable to large herbivores in temperate climates. Carcass composition data for bison are rare, and much of the following has been drawn from other large mammals, especially caribou and deer.

 While bone and muscle are highly predictable components of large mammal carcasses, fat is the most variable tissue in the body. Factors affecting this variability include genetic potential, sex, age, season and reproductive status. Historically, variability in fat stores was considered to be a survival mechanism, whereby fat is stored when energy is abundant and is expended when no new energy is available. This allows for survival through fluctuations in food supply. It is now believed that the primary role of fat is to foster successful reproduction. Males of most wild species will have maximum fat stores just prior to the rut, and may expend the great majority of fat engaging in the reproductive effort. Truth of the role of fat in reproduction can be seen in arctic mammals where nearly all fat is lost during the rut, which occurs just prior to the onset of an extremely harsh climatic period. This loss of fat (in mature males) just prior to the most demanding climatic period is contrary to expectations if the primary role of fat was to serve as a stored energy source.

So a model for seasonal fat cycles among typical adult males would be as follows: fat is gained rapidly through the summer reaching a peak prior to the rut; most fat is lost during the rut and males begin the fall or early winter with little fat; some fat may actually be gained during late fall and early winter while forage is still available; by mid-winter fat stores are again being mobilized to compensate for low feed levels; and by late winter and early spring, fat levels are again very low, followed by rapid fat gain of early summer.

Fat in female herbivores is also closely correlated with reproduction. Fat animals tend to become pregnant while lean animals do not. This is a natural mechanism that helps insure that females do not put their own lives at risk by carrying a fetus when they do not have sufficient body reserves to guarantee their own survival. Through this mechanism, they will live to breed another day when conditions improve. Pregnancy is associated with high levels of fat, and this is true for nearly all of the gestation term. Contrary to popular rumor, pregnancy does not drain the fat resources of the cow until the very end of gestation. Indeed, for most of this period pregnant cows are by far the fattest animals in a herd.

Lactation, on the other hand, is closely associated with loss of fat stores. The energetic drain of lactation is five times that of late pregnancy. Wet cows lose a large proportion of their body fat in the first six weeks of lactation and then slowly recover. Thus, cows tend to have greatest fat stores at conception and lowest in the weeks immediately following parturition. These two periods may only be separated by a few months, and may not be a sufficient amount of time for the wet cows to recover enough fat to again become pregnant in the following rut. Hence, it is common for cows to skip a year after parturition. They will then be the fattest cows a year later, as the young have been weaned and the energetic drain of lactation has ceased.

Variability in fat stores in cows is more complex than in males because there are essentially two groups of cows; those animals that are fat at the time of the rut and become pregnant, and those animals that are lean at the time of the rut and will fail to become pregnant. Generally, the former group will not have nursed a calf through the summer and hence will not have suffered the great energetic drain of lactation. The latter group will typically be the wet cows whose recovery of fat stores is delayed by lactation and who have less chance of becoming pregnant. Obviously, membership in each of these two groups will switch every other year. It must be stressed that these are not hard and fast rules, and that some cows will breed several years in a row and then take a year off. The important point is that the cow group will consist of members with dramatically different fat content at various points of the season.

A general model for cows then would be as follows: beginning in the summer the cows that were pregnant have calved and are lactating, and as such are at the absolute low point of fat stores. In contrast, cows that did not calve do not suffer the drain of lactation and gain fat rapidly on fresh summer forage. By late summer these dry cows are now fat and will almost certainly take part in the reproductive effort. The nursing cows have recovered some fat stores by late summer and may breed again but many are still lean and will skip this breeding season. After the rut the pregnant cows are very fat and will continue to improve in condition through the fall and early winter. The process of “pregnancy anabolism” seems to promote even greater fat deposition in pregnant cows compared with barren ones. Cows that did not become pregnant in the rut will also improve in condition through the fall and early winter but will always be somewhat leaner than the pregnant cows. By mid-winter both groups of cows start mobilizing fat stores to compensate for poor feed, a process that continues into the late winter. By early spring all cows have suffered considerable loss of fat but since the pregnant cows had so much more fat to begin with they tend to have greater fat content than the non-pregnant cows almost until calving time.

Sometime around April the non-pregnant cows may become superior in fat content, while the pregnant cows begin calving and lactating. As the summer begins and the cycle repeats the non-pregnant / non-lactating cows begin to fatten rapidly while the calving and lactating cows are quickly losing much of their fat.

But how do the female groups compare with the males on a seasonal basis? Bison, like nearly all large herbivores, exhibit pronounced sexual dimorphism. Adult males tend to be some 25 – 35% larger than adult females. Clearly, males will have much greater weight of tissue such as muscle and bone as these tissues are strongly correlated with overall body size. But fat is only poorly correlated with overall body size. Females (of many species) have a greater genetic potential to accumulate fat. As a result, females will often be proportionally fatter than males. That is, fat may make up a greater percentage of the female carcass than it does in the more muscular males. But does this mean that females have an absolute greater weight of fat than males? It has often been assumed that the greater size of males would result in them always having a greater absolute weight of fat tissue; however, this does not appear to be the case.

Studies from a number of large herbivores, including bison, indicate that females actually have a greater absolute weight of fat than males at certain times of the year. Specifically, after the rut when the cows are in prime condition and the bulls have lost most of their fat the females (especially pregnant cows) have a greater fat weight than the males. This condition persists through the fall and winter months. By spring the fat advantage of the pregnant cows is declining and that of the bulls and the dry cows is increasing. Evidence suggests that the dry cows and bulls are about equally fat in late spring, but by early summer the bulls take a decided advantage in fat weight over all females and that this lasts until the rut. Thus, despite the considerable size advantage of adult male bison, certain females actually have a greater weight of fat for about eight to nine months of the year, from September until either April or May.

My interest in differential bison condition stems from evidence found at bison kill sites used by Plains Indians. This evidence indicates that different sexes of bison were hunted and butchered differently, although the reasons for differential treatment have been unknown. The importance of fat in the diet of Aboriginal hunting cultures is well documented. Not only was fat highly desired for its taste, it was also an essential element for survival. The historic literature pertaining to the Plains Indians documents a clear preference for fat animals and fat parts of the carcass. My research question focused on whether or not Aboriginal bison hunters exploited the differences in male and female bison condition in accordance with the bioenergetic processes discussed above. To investigate this, dozens of historical accounts of bison hunting on the Northern and Central Plains were searched for information on the preferential killing of bison of certain age, sex, and reproductive status and in specific seasons.

Results of this search confirm that during the Historic Period both Aboriginal and European bison hunters made clear, conscious decisions as to which animals to kill. Animals deemed lean or “poor” were systematically avoided while animals presumed fat were targeted. Of greatest interest was the fine level of distinction that early bison hunters made in animal selection, and the evidence that these decisions were based on clear knowledge of processes that govern carcass composition. That is, males were the preferred targets during the late spring and summer months but ceased to be so during and after the rut. Cows were preferred most of the rest of the year, but differences in cow groups were recognized. Pregnant cows (from the time they could be recognized) were highly prized all during late fall and winter. Near calving time historic bison hunters commented that the pregnant cows were lean and useless as food, as they were during the first few months of nursing. Hunting attention in the spring turned to dry, barren cows, sometimes even in preference to males.

Historic Period hunting of bison shows close parallels with differential composition of bison as governed by bioenergetic processes. Traditional hunters were clearly aware of subtle but important changes in bison of different age, sex and reproductive status. Hunting seldom had the goal of maximizing the recovery of sheer tissue weight; had this been the case males would have been preferred most of the time. Rather, fat was the factor that dictated patterns of killing and carcass processing.