Holocene bison diminution on the Snake River Plain,Idaho,USA

Abstract

Researchers have established a Holocene pattern of Bison spp. diminution on the Great Plains of North America. This pattern, however, is less clear west of the Rocky Mountains. This lack of clarity stems from a relative paucity of paleontological and archaeological bison assemblages sufficiently large enough to understand local bison diminution. To begin filling this important gap in knowledge, we analyze a large bison assemblage from Baker Cave, a Late-Holocene archaeological site located on Idaho’s Snake River Plain. Measurements of humeri, radii, tibiae, metatarsals, and calcanei demonstrate that these animals were significantly smaller than Early-Holocene bison from both the Great Plains and Snake River Plain. Middle-Holocene bison from the Great Plains are generally larger than those from Baker Cave, but this size difference varies by skeletal element. The Baker Cave bison do fall within the range of Late-Holocene morphological variation present in both Snake River Plain and Great Plains bison populations. These results provide a necessary first step for understanding bison morphology in the region, but establishing a pattern of diminution west of the Rocky Mountains will require follow-up studies with other faunas.

Keywords

bison diminution Late Holocene paleoecology paleontology Snake River Plain zooarchaeology

Introduction

North American bison (Bison spp.) declined in body size through the Holocene, a trend well documented by archaeologists and paleontologists working in the Great Plains. Possible factors responsible for diminution include constraints on thermoregulation (Butler et al., 1971), selection for faster maturation rates brought on by anthropogenic exploitation (McDonald, 1981: 258), or the transition from top-down (predator limited) to bottom-up (forage limited) ecosystem controls (Hill et al., 2008). A strong relationship between climate change and bison diminution, along with a lack of evidence for overhunting, has led some paleoecologists to conclude that human predation was not the prime mover behind shifts in bison morphology (Hill et al., 2008). Instead, climate-dependent trends in forage availability and quality appear more likely to have been responsible.

Paleoecologists have documented morphological change in Great Plains bison through a number of biometric studies (Bedord, 1974; Hill, 1996; Hill et al., 2008; Hillerud, 1970; Hughes, 1978). These studies consistently show a pattern of diminution, although the timing of size reduction varies by region and skeletal element (Hill et al., 2008; McDonald, 1981: 259). The trend is also nonlinear. One recent study suggests a slight size increase following the Middle Holocene (8000–5000 cal. BP), although Late-Holocene (5000 cal. BP to present) bison remain far smaller than Terminal-Pleistocene and Early-Holocene (12,000–8000 cal. BP) bison (Hill et al., 2008). The general pattern of decreasing body size is not unique to bison, as researchers have documented Holocene diminution across a range of artiodactyl taxa in North America (e.g. Guthrie, 1984; Lyman, 2006, 2009, 2010; Purdue, 1989; Purdue and Reitz, 1993). However, the sheer number of individuals and sites sampled by Hill et al. (2008) make the bison trend by far the best-documented case of artiodactyl diminution on the continent.

The trajectory of bison diminution is far less clear west of the Rocky Mountains, and researchers have yet to demonstrate the same patterns seen in Great Plains populations to the east. Much of this lack of clarity stems from the relative paucity of bison remains found west of the Rocky Mountains. Although bison were present in the western United States through the Holocene, they existed in relatively low numbers compared with the large populations living on the Great Plains. Recent studies collating the Great Basin and eastern Washington bison records demonstrate that regional archaeofaunas rarely contain more than two or three individuals (Grayson, 2006, 2011: 268–278; Lyman, 2004). In contrast, minimum numbers of individuals documented at Great Plains sites can reach the hundreds. Consequently, the small numbers of bison encountered in western archaeological and paleontological contexts complicate statistical evaluations of morphological change.

One region of the American West, the Snake River Plain (SRP), has produced a continuous bison record spanning the Holocene (Butler, 1978; Plew and Sundell, 2000). Although the majority of dated archaeological and paleontological sites contain no more than one individual, a few sites have produced sufficiently large numbers of remains to document bison diminution west of the Rocky Mountains. Investigations at Baker Caves I and III (10BN154 and 10BN153, henceforth ‘Baker Cave’), located on the eastern SRP, produced one such assemblage.

To begin filling the substantial gaps in our understanding of SRP bison, we report on the morphology of those animals found in the Baker Cave archaeofauna. We begin by briefly reviewing research on Holocene bison morphology and fossil bison from the SRP. We then discuss the SRP environment as well as Baker Cave and its associated archaeofauna. We next outline procedures for measuring select bison elements from Baker Cave. Finally, we compare the Baker Cave bison measurements to those of bison from other contexts, both on the SRP and the Great Plains to the east. We do so to assess the trajectory of bison diminution not only on the SRP, but also more broadly within a trans-North American context.

Holocene bison diminution

Research on the Great Plains has produced a large and well-documented Holocene bison record. This information has allowed researchers to propose and evaluate ideas about the roles of people and climate in shaping animal populations. In one recent look at Great Plains faunas through the Late Quaternary, Hill et al. (2008) assess whether human hunting could account for bison size diminution. They use the timing of human arrival in North American as well as bison mortality profiles to evaluate the human hunting argument. Their results fail to support human predation as a prime mover behind trends in bison morphology. Instead, they find that bison diminution was a punctuated trend apparently unrelated to coincident trends in human demographics. Moreover, shifts in morphology occurred during periods of significant climate change, specifically during the Late Glacial Maximum (18,000–21,000 cal. BP), the Younger Dryas (11,000–13,000 cal. BP), and the Middle-Holocene Thermal Maximum (7000–9000 cal. BP). Bison partially rebound in size during the transition from the Middle to the Late Holocene (6000–3000 cal. BP), although a sparse record from this period makes the timing of this size increase difficult to establish (see Hill et al., 2008: 1759–1760).

Common explanations for artiodactyl diminution through the Holocene have included thermoregulation (Butler et al., 1971), predator release (Geist, 1989; Matheus, 2001), and forage availability (Geist, 1971, 1989; Guthrie, 1984). Butler et al. (1971) proposed Bergmann’s Rule as an explanation for size change in bison after comparing Holocene bison assemblages across North America. However, issues relating to sex determination as well as conflation of latitudinal and temporal variation make Butler et al.’s conclusions problematic (Wilson, 1974: 142–145).

Hill et al. (2008: 1764) detail further problems with applying Bergmann’s Rule to the bison chronocline. These include difficulty in controlling for covariance between temperature and forage availability as well as the lack of a simple linear correlation between body size and temperature through the Quaternary Period. On a more general level, researchers have also demonstrated that models of net primary productivity better explain a number of taxon-specific cases (Huston and Wolverton, 2011; Wolverton et al., 2009). In sum, Bergmann’s rule is an extremely problematic explanation for changes in bison morphology through the Holocene. For an alternative explanation, Geist (1989) and Matheus (2001) argue that the large body masses of Pleistocene megafauna were a response to predation and interspecific competition. After the extinction of Pleistocene carnivores and other megaherbivores, selection for large body size was relaxed. Unfortunately, the uncertain timing of Pleistocene predator extinctions (Grayson, 1991, 2007; Mead and Meltzer, 1984) complicates this hypothesis. The numerous problems with these hypotheses have left changes in forage quality and availability as the best supported explanation for diminution in Holocene mammals.

Hill et al. (2008) see forage quality as playing an important role in bison diminution, although they caution that forage quality alone is probably not the answer. Bison are obligate grazers dependent on grassland forage. Modern studies have shown that more easily digestible, and consequently more nutritious, C₃ grasses tend to dominate in areas characterized by cooler climates, while less nutritious C₄ grasses tend to dominate in warmer environments (Ehleringer, 1978; Epstein et al., 1997; Teeri and Stowe, 1976). Hill et al. (2008) found that the timing of body size reductions tended to occur during warmer periods characterized by expanding dominance of less nutritious, warm weather C₄ grasses. The consumption of more C₄ grasses would have resulted in a lower quality bison diet that would have constrained somatic growth and created a selective pressure emphasizing smaller, less energetically expensive bodies (Hill et al., 2008). If the relationship between forage quality and bison morphology explains the trend in diminution on the Great Plains, then we expect a similar chronocline on the SRP as well.

Geography and paleoecology of the SRP

The SRP sits at the southeastern most extent of the Columbia Plateau, where a unique igneous landscape and drainage system characterize the region (Kuntz et al., 1986, 1992; Malde and Powers, 1962; Smith, 2004; Wood and Clemens, 2002). Much like surrounding areas west of the Rocky Mountains, the SRP contains habitat less suitable than that found on the Great Plains (Mack and Thompson, 1982). This is probably due to climate-dependent effects on forage availability, including severe inter-annual summer drought and winter snow volume (Williams, 2005).

Although the SRP shares these climatic patterns with much of the northwestern USA, the region’s uniquely flat topography provides few obstructions for migrating bison, a factor that likely made the SRP a better bison habitat than other areas west of the Rocky Mountains (Van Vuren, 1987). Moreover, the near uniform sagebrush–steppe biome across the region provides few ecological obstacles for bison populations, such as habitats fragmented by variability in elevation. While the SRP today lacks the forage communities likely to support large bison populations, these conditions have by no means remained consistent over the Holocene. Instead, climate-dependent trends in forage quality and availability mediated the ability of the SRP to support bison populations. These trends likely conditioned local bison diminution in ways similar to those seen on the Great Plains.

Climatic factors such as seasonality, aridity, and temperature condition forage quality and availability. Paleoenvironmental studies suggest that during the Terminal Pleistocene, the SRP was generally wet and cool (Bright, 1966). However, a warming trend began between 10,800 and 10,300 cal. BP, as indicated by pollen cores from Swan Lake, southeastern Idaho (Bright, 1966). This Early-Holocene warming triggered an expansion of sagebrush and the movement of biotic communities to higher elevations. Warming continued into the Middle Holocene, although records indicate conflicting dates for the timing of maximum aridity. The Swan Lake record suggests a period of maximum aridity between 8400 and 3100 cal. BP (Bright, 1966), although packrat middens from the Idaho National Engineering Laboratory indicate this thermal maximum was reached at about 7000 cal. BP (Bright and Davis, 1982). Pollen from Gray’s Lake mirrors the Swan Lake record, suggesting that the swing toward Middle-Holocene aridity began around 8200 cal. BP (Beiswenger, 1991). Grass pollen from Scaredy Cat Cave, located in Craters of the Moon National Park, demonstrates that a highly variable climate better characterizes the Middle Holocene, as opposed to a general state of aridity (Wigand, 1997). Finally, most records indicate that an essentially modern climate was established between 3100 and 2000 cal. BP (Beiswenger, 1991; Bright, 1966; Cummings, 2002).

Temporal patterns in SRP climates mirror patterns found in nearby areas of the Great Basin and southwestern Wyoming that document broad climate-dependent trends in forage quality and, consequently, artiodactyl reproductive success (Broughton et al., 2008; Byers and Broughton, 2004; Byers and Smith, 2007; Byers et al., 2005). In these instances, trends in artiodactyl abundances mirror trends in effective precipitation, with moister periods characterized by greater abundances of these animals on the landscape. This well-documented relationship, in combination with trends in bison morphology identified by Hill et al. (2008), allow us to make several broad predictions about morphological trends in Holocene SRP bison populations. Simply put, we expect Early-Holocene SRP faunas to contain larger bison, with subsequent size reductions occurring in tandem with increasing Holocene aridity. If the model presented here correctly anticipates the trajectory of SRP bison diminution, then the aridity of the Middle Holocene west of the Rocky Mountains should have selected for smaller individuals and resulted in Late-Holocene bison possessing smaller mean body sizes.

Bison on the SRP

Bison remains have been recovered from numerous contexts throughout the SRP. In fact, one recent review of archaeofaunas from southern Idaho documented bison at 56% of the sites in the area (Plew, 2009). Most of these remains are reported as Number of Identified Specimens (NISP), Minimum Number of Individuals (MNI), or simply as presence or absence data. While bison are common components of SRP archaeofaunas, NISP is typically low and MNIs usually indicate no more than one individual (Gruhn, 1961; Henrikson, 1996; Henrikson et al., 2006; Holmer and Ringe, 1986; McDonald, 2006; Murphey and Crutchfield, 1985; Pavesic and Meatte, 1980; Plew, 1981; Rudolph, 1995). Consequently, archaeologists have argued that the SRP did not support bison populations large enough to allow for prehistoric mass kill events like those documented on the Great Plains (Henrikson, 2003, 2004, 2005; see also Daubenmire, 1985; Mack and Thompson, 1982). Unfortunately, these low numbers also make understanding trends in bison morphology difficult.

Regions bordering the northern and eastern SRP have produced a more substantial bison record. For example, the Late-Holocene (107–757 cal. BP) Rock Springs Site (Arkush, 2002), located in southeastern Idaho, contains an assemblage of 945 specimens representing at least 19 individuals (Walker, 2002). Upland areas north of the SRP also contain evidence for Holocene bison. Swanson (1972) reports at least 128 bison (NISP = 1241) at the Birch Creek Rockshelters. Butler (1971) estimates 20–30 bison the Challis Bison Jump (poor preservation prevented explicit quantification), and at least 11 bison (NISP = 364) at Quill Cave. Although these sites produced fewer bison than often documented in Great Plains assemblages, they nonetheless contain far more bison than seen in most SRP archaeological and paleontological faunas.

Exceptions to the pattern of low bison MNI counts include the Wasden Site (Butler, 1968; Butler et al., 1971; Miller and Dort, 1978) and Baker Cave (Miller, 1987; see section ‘Results’). The Wasden site consists of a cave on the eastern SRP containing a large bison assemblage dating to the Early Holocene (8015–8593 cal. BP). Although a complete quantification of bison remains from the Wasden Site remains unpublished, several investigators have provided MNI estimates. Initial investigations of the locality yielded an MNI of 50 based on lower mandibles (Butler, 1968). Butler et al. (1971) raised this minimum estimate to 60 individuals, and Butler (1978) reports at least 66 individuals. Following continued excavations, Miller and Dort (1978) estimate that 150 bison were present at Wasden (however, it is unclear if this is a minimum estimate). Even the smallest minimum bison estimates for Wasden are far larger than MNI estimates from elsewhere on the SRP.

The large number of bison at the Wasden Site is likely why Butler et al. (1971) selected Wasden to examine the change from Bison antiquus to Bison bison. They concluded their study by suggesting that the next steps for investigating Holocene bison diminution should include establishing age- and sex-dependent variability in modern bison skeletal elements, comparing archaeological and paleontological metrics for aging and sexing bison with established data and developing standardized criteria for determining age and sex. Great Plains research has mostly fulfilled these goals, although these improved methods have gone unused on the SRP. Baker Cave provides an opportunity to apply these methods to a SRP bison assemblage. For the first time since Butler et al.’s (1971) investigation, enough data are available to evaluate the broad trends in SRP bison diminution.

The Baker Cave bison

Excavations at Baker Cave

Baker Cave is an eastern SRP site located about 19 km east of Minidoka, Idaho, and roughly 8 km northeast of Lake Walcott (Figure 1). The site occupies a lava blister that formed during the extrusion of the Wapi Lava Flow. The setting typifies lava fields found throughout the region today, characterized by a sagebrush–steppe biome dispersed across an uneven basalt terrain. Although at a distance from permanent water today, the geological setting provides opportunities for spring rains to pool in seasonal ponds (Henrikson et al., 2006: 45).

Figure 1.

Location of southern Idaho sites that have produced measurable bison specimens.

In an effort to mitigate looting, Boise State University, in conjunction with the Idaho Bureau of Land Management, excavated Baker Cave in 1985 (Plew et al., 1987). Investigations took place in two chambers designated Baker I and Baker III. Plew et al. (1987) report a third chamber (Baker II) that lacked cultural materials and sediment depth, which they did not investigate further. Baker I is a low hanging chamber roughly 3 m deep by 7 m wide. A wall constructed of basalt and juniper branches partially blocked the entrance at the time of investigation (Plew et al., 1987: 13). Baker III consists of two interconnected chambers. The first chamber is roughly 60 m by 9 m and contained the majority of cultural deposits. The second chamber is a long tube roughly 6 m wide and 100 m deep. Plew et al. (1987: 13) report little deposition or evidence for human use in this chamber. Due to time restrictions, Boise State University focused recovery efforts on Baker I and the first chamber of Baker III.

Plew et al. (1987) designed the excavation to recover the maximum amount of cultural materials. Due to the shallow deposition (~15 cm maximum), they treated all sediments as a single component. Most of the excavation was accomplished with brushes and all sediment was passed through 0.3 cm mesh. They excavated 100% of the undisturbed sediments from Baker I and roughly 70% of Baker III. These efforts resulted in the collection of a large cultural assemblage that included substantial numbers of bison bone.

The Boise State University excavations exposed several archaeological features containing charcoal suitable for radiocarbon dating. The features include an S-shaped rock alignment in Baker III as well as three hearths in Baker I and III (Plew et al., 1987: 21). Boise State University collected five radiocarbon dates from the hearths that place occupation of the cave at 685–908 cal. BP (Plew et al., 1987: 22). These dates disagree with two obsidian hydration dates from the cave, which indicate occupation around 1341–1541 BP and 1264–1394 BP (Plew et al., 1987: 17). Plew et al. (1987: 22) suggest that the hearths and obsidian artifacts indicate two different human occupations, although Plew et al.’s coarse-grained excavation methods failed to document any stratigraphic separation between materials in the cave. Rather, the excavated sediments most likely contain a palimpsest of several hundred years of cultural and natural deposition. Moreover, recent critiques of obsidian hydration dating suggest that a number of environmental factors can bias dates generated by this method (Anovitz et al., 1999). Regardless, while we acknowledge some imprecision in the dating of Baker Cave, the relatively coarse-grained temporal scales researchers have used to examine bison diminution through the Holocene (e.g. the 1000-year bins used by Hill et al. (2008: 1760)) obviate the issue of time averaging over a 350- to 850-year period.

Bison remains from Baker Cave

Baker Cave produced a large archaeofauna containing artiodactyls, lagomorphs, rodents, canids, snakes, and birds. Of these specimens, we identified 591 as adult B. bison (MNI = 37, based on the distal right tibia). The assemblage also contains 431 fetal bison specimens (MNI = 7, based on the right radial diaphysis). Specimens lacking features that distinguish Bos taurus from B. bison were classified as large bovid (NISP = 179). An additional 3814 specimens fell into the size range of elk (Cervus canadensis), moose (Alces alces), bison (B. bison), and domestic cattle (B. taurus), but lacked taxonomically diagnostic features. These specimens are mostly comprised of diaphysis fragments. We classify them as large artiodactyl here. The lack of large artiodactyls other than bison suggests that these fragments also represent this taxon.

Taphonomic processes can bias archaeofaunas against certain age and sex classes. A number of factors may affect the representation of less dense bone. This is important to consider for the Baker Cave assemblage since volume mineral density of bison bone significantly, but weakly, predicts the frequencies of element portions in this fauna (r_s = 0.282, p = 0.005; volume mineral density values from Kreutzer (1992)). A lack of low-density proximal element portions, including those of humeri, tibiae, and femora appears to drive this trend. The absence of these skeletal portions is probably due to factors such as carnivore ravaging and weathering. Carnivore damage is present on 7.24% of B. bison, large bovid, and large artiodactyl specimens (NISP = 332). In situ chemical weathering probably accounts for some attrition as well. While 72.89% of specimens (NISP = 3342) have at least one unweathered surface (no cracking or flaking), 54.86% of specimens (NISP = 2515) display at least one surface with evidence for exposure to weathering agents. Of the large artiodactyl specimens, 2.09% (NISP = 97) are completely weathered down to fibrous bone or are actively disintegrating.

Despite evidence suggesting that several taphonomic processes have conditioned the collection, a sufficiently large and representative sample of elements has survived for metric analysis. Given this robust, relatively well-preserved bison assemblage, Baker Cave provides a unique opportunity to assess a Late-Holocene bison population west of the Rocky Mountains. This favorable taphonomic context, in combination with the large adult bison MNI, allows a confident size distribution to be determined from a robust osteometric dataset.

Material and methods

Adult B. bison specimens were measured to the nearest 0.5 mm using Pittsburgh^® Model 47257 6″ digital calipers and an osteometric board designed and built in-house. To control for maturational variation, we only included specimens displaying complete fusion. We also only recorded complete dimensions that could be precisely measured. This excluded specimens with carnivore ravaging, rodent gnawing, severe cortical weathering, or other damage along measured points.

We follow Todd’s (1987) protocol for humerus, radius, and tibia measurements; Lewis et al.’s (2005) protocol for metatarsal measurements; and Hill’s (1996) protocol for calcaneus measurements. We measure the greatest length (CL1) and greatest width (CL4) of calcanei. For humeri, we measure the width of the distal articular surface (HM7) and the greatest medial depth of the distal end (HM11). We take measurements on the greatest proximal width (ProxW) and greatest proximal depth (ProxD) of metatarsals. We also measure the greatest proximal articular surface width (RD4) and greatest proximal depth (RD9) of radii. We measure the greatest distal width (TA7) and depth (TA10) of tibiae.

Results

The results presented here derive from 245 measurements taken from 111 specimens of bison bone. These measurements indicate that the Baker Cave bison fauna contains between 70% (based on the calcaneus) and 87.5% (based on the distal tibia) females (Table 1; see also Supplementary Tables 1–5). Metrics from the most commonly measured skeletal part, the distal humerus, indicate that 77% of the mature individuals are female. This high frequency of females is not surprising given the presence of numerous fetal remains in context with adult specimens. Although sex ratios varied between metrics, they all suggest a female-dominated assemblage.

Table 1.

Descriptive statistics for Baker Cave bison specimens.

Measurement	Female				Male
	n	Range	$\bar{x}$	s	n	Range	$\bar{x}$	s
CL1	17	138.0–151.0	144.3	4.4	5	151.5–161.0	156.7	3.9
CL4	17	44.0–50.5	47.6	1.7	5	54.0–58.5	55.4	2.0
HM7	28	73.0–84.0	79.2	2.7	4	91.5–99.5	93.6	3.9
HM11	20	86.0–98.5	91.1	3.2	3	104.0–114.5	108.2	5.6
ProxW	12	47.0–53.5	51.0	2.2	1	55.0–55.0	55.0	0.0
ProxD	12	46.5–50.5	48.9	1.8	1	52.0–52.0	52.0	0.0
RD4	14	75.5–87.5	80.9	3.3	1	99.5–99.5	99.5	0.0
RD9	14	42.0–52.0	45.4	3.0	1	58.0–58.0	58.0	0.0
TA7	26	63.0–71.0	67.1	1.9	3	75.0–77.5	76.5	1.3
TA10	26	47.0–52.5	49.5	1.5	3	53.0–60.5	56.7	3.8

Calcaneus, humerus, and tibia dimensions all break down into discrete sex distributions (Figure 2a–c). Radius dimensions plot as one cluster and one outlier, which we interpret as a single male specimen (Figure 2d). In contrast, metatarsals group into more ambiguous clusters than the other elements discussed here. Figure 2e appears to show two different clusters with one large outlier. This distribution presents the possibility that the outlier is male and the two clusters are female. An alternative interpretation is that each cluster originates from a different sex and the outlier is an exceptionally large male.

Figure 2.

Bivariate plots of Baker Cave bison metrics: (a) calcaneus, (b), humerus, (c) tibia, (d) radius, and (e) metatarsal.

To evaluate these alternative interpretations, we consider data on modern bison metatarsals originating from individuals of known sex. Lewis et al. (2005) show that proximal metatarsal width is 57.8 ± 4.4 mm (s) in modern males and 50.8 ± 2.8 mm (s) in modern females. They also show that proximal metatarsal depth is 54.9 ± 2.8 mm (s) in modern males and 48.5 ± 2.3 mm (s) in modern females. These data overlap neatly with the Baker Cave metrics. When considering Lewis et al.’s measurements, the two clusters fall within the female group and the outlier falls within the male group. Therefore, we interpret the two clusters as female and the outlier as male. The gap in the female cluster is likely a sampling problem.

We note a potential problem with using modern bison from a different spatial context as a standard for sexing the Baker Cave bison: geographic variability in morphology. Others have identified known latitude-controlled differences in bison body size through the Holocene, although these become less pronounced in the Late Holocene (Hill et al., 2008: 1760). The modern bison used by Lewis et al. (2005) are primarily northern Great Plains and zoo specimens. Using the latitudinal distinctions defined by Hill et al. (2008: 1760), the northern Great Plains fall within the same range as southern Idaho (although Lewis et al. (2005) do not specify the geographic origin of the zoo specimens). Although this modern sexed sample may not overlap perfectly with the Baker Cave sample, we expect minimal temporal and spatial variation in the size of males and females between the samples. Therefore, it is likely a good standard for identifying male and female metatarsals at Baker Cave.

Discussion

To put the Baker Cave bison in a broader context, we compare the Baker Cave size data to bison metrics from previously reported faunas from the SRP and Great Plains (Table 2). Specifically, we compare this dataset to other SRP assemblages, with the expectation that on-average the Baker Cave bison will be smaller than Early-Holocene SRP bison and similar in size to those found in other Late-Holocene bison assemblages. We also compare the SRP bison data with datasets derived from Great Plains bison to provide insight into continental-scale spatial variability in bison morphology. The data on SRP bison morphology presented here support a diminution trend similar to the one documented on the Great Plains. Furthermore, comparisons between SRP bison and those from Great Plains assemblages suggest that similar times in both regions contained similarly sized bison, suggesting parallel chronoclines in diminution (see map in Hill et al. (2008: 1756) for Great Plains site locations).

Table 2.

Descriptive statistics for female bison from archaeological and paleontological contexts.

Metric	Site	Range	$\bar{x}$	s	n	Reference
CL1	Baker Cave	138.0–151.0	144.3	4.4	17	Results section
	Glenrock	128.0–155.0	142.3	6.4	108	Hill et al. (2008)
	Big Goose Creek	134.0–146.0	142.0	4.1	8	Hill et al. (2008)
	Big Bone Lick	129.0–146.0	140.0	4.3	21	Hill et al. (2008)
	Mavrakis-Bentzen-Roberts	132.0–154.0	146.1	5.2	37	Hill et al. (2008)
	Hawken	140.0–155.0	149.6	5.1	19	Hill et al. (2008)
	Mill Iron	156.0–163.0	159.5	3.5	4	Hill (1996)
CL4	Baker Cave	44.0–50.5	47.6	1.7	17	Results section
	Glenrock	42.0–55.0	47.4	2.7	109	Hill et al. (2008)
	Big Goose Creek	42.0–49.0	45.8	2.7	8	Hill et al. (2008)
	Big Bone Lick	36.0–47.0	41.5	3.1	21	Hill et al. (2008)
	Mavrakis-Bentzen-Roberts	44.0–54.0	48.8	2.5	40	Hill et al. (2008)
	Hawken	47.0–63.0	51.6	4.7	20	Hill et al. (2008)
	Mill Iron	46.0–55.0	51.7	4.9	3	Hill (1996)
HM7	Baker Cave	73.0–84.0	79.2	2.7	28	Results section
	Rock Springs	78.0–79.0	78.7	0.6	3	Walker (2002)
	Logan Creek	76.3–96.4	83.9	6.5	11	Hill et al. (2008)
	Spring Creek	75.7–91	82.2	4.6	10	Hill et al. (2008)
	Horner	80.0–87.0	83.4	2.3	19	Todd (1987)
HM11	Baker Cave	86.0–98.5	91.1	3.2	20	Results section
	Rock Springs	87.0–91.0	89.3	2.1	3	Walker (2002)
	Logan Creek	91.1–97.5	93.5	3.1	4	Hill et al. (2008)
	Spring Creek	86.6–94.4	91.5	3.4	4	Hill et al. (2008)
	Horner	85.0–100.0	95.5	3.5	19	Todd (1987)
ProxD	Baker Cave	46.5–52	48.9	1.8	12	Results section
	Wasden^a	48.0–53.0	50.3	1.6	23	Butler et al. (1971; Section 8)
	Wasden^b	48.0–61.0	53.2	3.9	36	Butler et al. (1971)
RD4	Baker Cave	75.5–87.5	80.9	3.3	14	Results section
	Horner	82.0–89.0	84.9	2.4	18	Todd (1987)
RD9	Baker Cave	42.0–52.0	45.4	3.0	14	Results section
	Horner	45.0–52.0	48.8	2.4	18	Todd (1987)
TA7	Baker Cave	63.0–71.0	67.1	1.9	26	Results section
	Rock Springs	65.0–69.0	67.3	2.1	3	Walker (2002)
	Horner	65.0–75.0	71.1	2.6	21	Todd (1987)
TA10	Baker Cave	47.0–52.5	49.5	1.5	26	Results section
	Rock Springs	47.0–52.0	48.7	2.9	3	Walker (2002)
	Horner	48.0–56.0	52.9	2.2	21	Todd (1987)

Female specimens identified through a bivariate plot of proximal metatarsal dimensions.

Female specimens identified through bivariate method specified in Butler et al. (1971).

While the lack of bison metrics from SRP assemblages complicates within-region comparisons, data published from two other southern Idaho localities allows for some insights into morphological trends. Butler et al.’s (1971) investigation of SRP bison morphology using the Early-Holocene Wasden Site generated a large dataset of metatarsal measurements. Unfortunately, Butler et al. did not take the same suite of measurements as those recorded from Baker Cave. Nonetheless, both studies share one metric in common, proximal depth.

We differ from Butler et al. (1971) in our interpretation of the sex distribution within this dataset. Butler et al. (1971: 136) identify 36 females by plotting the width and length of metatarsals. We instead use Lewis et al.’s (2005) protocol for metatarsal measurements to interpret the Wasden sex distribution through clustering in proximal metatarsal width and depth. Doing so results in all three specimens that Butler et al. (1971) identify as male falling within the larger group of specimens that they identify as female (Figure 3a). Recognizing this issue, we reclassify the Wasden bison and find 23 females and 16 males (Figure 3b).

Figure 3.

Bivariate plots of proximal width and depth of the Wasden Site metatarsals: (a) sex interpreted through Butler et al.’s (1971) method and (b) sex interpreted through simple observation of point clustering (this paper).

Figure 3 demonstrates the large differences between Butler et al.’s method and the one we use here. We suggest that identifying simple clusters gives a more accurate picture of the Wasden Site sex ratio. One might also interpret these points as three clusters, with the middle cluster that we identify as female here reclassified as male. We do not consider this interpretation since our method provides a more conservative measure that already treats only the smallest cluster as female. If this small female cluster contains specimens larger than those at Baker Cave, then reclassifying the middle cluster as female would only increase the mean size difference between the Wasden and Baker Cave metatarsals.

When considering specimens sexed through our simple clustering approach, female Wasden specimens ( $\bar{x}$ = 50.45 mm, s = 1.50 mm, n = 23) appear significantly larger than female Baker Cave specimens (t = −2.354, p = 0.029; Figure 4). This difference is especially striking when considering that the mean for females derived Butler et al.’s (1971) method results in a proximal depth that is almost 3 mm larger than the mean value that we obtained ( $\bar{x}$ = 53.19 mm, s = 3.85 mm, n = 36). Not surprisingly, using Butler et al.’s (1971) numbers also results in a significant size difference with Baker Cave (t = −3.696, p = 0.001). Although additional bison metrics from Wasden would make comparisons between the sites more robust, these data support the hypothesis that SRP bison underwent a dramatic diminution between the Early and Late Holocene.

Figure 4.

Ratio plot comparing female metatarsals from Wasden with those from Baker Cave. Prehistoric values standardized relative to modern female calculated from data in Speth (1983: Appendix). Positive values indicate individuals larger than the modern average and vice versa. Lyman (2004) outlines the specifics of constructing a ratio plot.

Despite a small sample from the Rock Springs site, t-tests suggest that bison from this site were similar in size to those from Baker Cave. For example, no inter-site differences are seen in measurements of either humeri (HM7: t = 0.895, p = 0.384; HM11: t = 1.246, p = 0.287) or tibiae (TA7: t = −0.188, p = 0.865; TA10: t = 0.492, p = 0.669). These data, in combination with the Wasden site metrics, suggest two conclusions regarding the Baker Cave bison anticipated by bison studies focused on Great Plains populations. First, Early-Holocene SRP bison were on-average larger than those dating to the Late Holocene. Second, Late-Holocene bison from different areas of southern Idaho were morphologically similar.

The similarity in temporal trends between SRP and Great Plains bison suggests that the two populations experienced similar morphological trajectories. If so, morphological similarity should exist between SRP and Great Plains bison through time as well. To evaluate similarities and differences in bison between the two regions, we next compare the metric data from the three SRP bison records presented here with Early-, Middle-, and Late-Holocene bison data from the Great Plains (Figure 5 and Table 3). We focus on comparisons of two skeletal elements, calcanei and humeri from female animals, since measurements of these bones were most often reported and females are most common. The Early-Holocene records we use include the Mill Iron (10,838–11,722 cal. BP) and Horner sites (9255–9511 cal. BP). Great Plains Middle-Holocene bison are represented by data from the Hawken (7131–7374 cal. BP), Logan Creek-Zone B (6980–7480 cal. BP), and Spring Creek sites (6940–7160 cal. BP). Finally, the Glenrock (221–298 cal. BP), Big Goose Creek (404–600 cal. BP), and Mavrakis-Bentzen-Roberts sites (2536–2684 cal. BP) provide measurements documenting the size of Late-Holocene bison on the Great Plains.

Figure 5.

Ratio plots comparing Baker Cave females with those from selected SRP and Great Plains sites: (a) calcaneus and (b) humerus metrics. Prehistoric values standardized relative to modern female average presented in Hill et al. (2008). Positive values indicate individuals larger than the modern average and vice versa. Lyman (2004) outlines the specifics of constructing a ratio plot.

Table 3.

Intersite comparison of calcanei, humeri, and tibiae.

		Baker	Cave	Rock Springs		Glenrock		Big Goose Creek		Mavrakis-Bentzen-Roberts		Spring Creek		Logan Creek		Hawken		Horner		Mill Iron
Site	Metric	t	p	t	p	t	p	t	p	t	p	t	p	t	p	t	p	t	p	t	p
Baker Cave	CL1	0.000	1.000	–	–	1.627	0.115	1.292	0.216	−1.312	0.2	–	–	–	–	−3.352	0.002	–	–	−7.366	<0.001
	CL4	0.000	1.000	–	–	0.518	0.619	1.846	0.095	−2.030	0.05	–	–	–	–	−4.435	0.002	–	–	−1.397	0.293
	HM7	0.000	1.000	0.895	0.384	–	–	–	–	–	–	−1.970	0.074	−2.323	0.040	–	–	−5.758	<0.001	–	–
	HM11	0.000	1.000	1.246	0.287	–	–	–	–	–	–	−0.243	0.820	−1.446	0.215	–	–	−4.174	<0.001	–	–
	TA7	0.000	1.000	−0.188	0.865	–	–	–	–	–	–	–	–	–	–	–	–	−5.934	<0.001	–	–
	TA10	0.000	1.000	0.492	0.669	–	–	–	–	–	–	–	–	–	–	–	–	−6.029	<0.001	–	–
Rock Springs	HM7	0.895	0.384	0.000	1.000	–	–	–	–	–	–	−2.407	0.037	−2.641	0.024	–	–	−7.693	<0.001	–	–
	HM11	1.246	0.287	0.000	1.000	–	–	–	–	–	–	−1.049	0.343	−2.149	0.084	–	–	−4.302	0.012	–	–
	TA7	−0.188	0.865	0.000	1.000	–	–	–	–	–	–	–	–	–	–	–	–	−2.868	0.065	–	–
	TA10	0.492	0.669	0.000	1.000	–	–	–	–	–	–	–	–	–	–	–	–	−2.444	0.116	–	–
Glenrock	CL1	1.627	0.115	–	–	0.000	1.000	0.195	0.849	−3.615	0	–	–	–	–	−5.58	<0.001	–	–	−9.243	0.001
	CL4	0.518	0.619	–	–	0.000	1.000	1.688	0.130	−3.024	0	–	–	–	–	−3.819	0.001	–	–	−1.494	0.272
Big Goose Creek	CL1	1.292	0.216	–	–	0.195	0.849	0.000	1.000	−2.464	0.029	–	–	–	–	−4.128	0.001	–	–	−7.708	<0.001
	CL4	1.846	0.095	–	–	1.688	0.130	0.000	1.000	−2.999	0.014	–	–	–	–	−4.099	<0.001	–	–	−1.973	0.163
Mavrakis-Bentzen-Roberts	CL1	−1.312	0.198	–	–	−3.615	0.001	−2.464	0.029	0.000	1.000	–	–	–	–	−2.418	0.021	–	–	−6.831	0.001
	CL4	−2.030	0.049	–	–	−3.024	0.003	−2.999	0.014	0.000	1.000	–	–	–	–	−2.442	0.022	–	–	−0.997	0.420
Spring Creek	HM7	−1.970	0.074	−2.407	0.037	–	–	–	–	–	–	0.000	1.000	0.702	0.491	–	–	−0.777	0.453	–	–
	HM11	−0.243	0.820	−1.049	0.343	–	–	–	–	–	–	0.000	1.000	0.870	0.418	–	–	−2.125	0.095	–	–
Logan Creek	HM7	−2.323	0.040	−2.641	0.024	–	–	–	–	–	–	0.702	0.491	0.000	1.000	–	–	0.257	0.802	–	–
	HM11	−1.446	0.215	−2.149	0.084	–	–	–	–	–	–	0.870	0.418	0.000	1.000	–	–	−1.158	0.302	–	–
Hawken	CL1	−3.352	0.002	–	–	−5.58	<0.001	−4.128	0.001	−2.418	0.021	–	–	–	–	0.000	1.000	–	–	−4.689	0.003
	CL4	−4.435	0.002	–	–	−3.819	0.001	−4.099	<0.001	−2.442	0.022	–	–	–	–	0.000	1.000	–	–	−0.038	0.972
Horner	HM7	−5.758	<0.001	−7.693	<0.001	–	–	–	–	–	–	−0.777	0.453	0.257	0.802	–	–	0.000	1.000	–	–
	HM11	−4.174	<0.001	−4.302	0.012	–	–	–	–	–	–	−2.125	0.095	−1.158	0.302	–	–	0.000	1.000	–	–
	TA7	−5.934	<0.001	−2.868	0.065	–	–	–	–	–	–	–	–	–	–	–	–	0.000	1.000	–	–
	TA10	−6.029	<0.001	−2.444	0.116	–	–	–	–	–	–	–	–	–	–	–	–	0.000	1.000	–	–
Mill Iron	CL1	−7.366	<0.001	–	–	−9.243	0.001	−7.708	<0.001	−6.831	0.001	–	–	–	–	−4.689	0.003	–	–	0.000	1.000
	CL4	−1.397	0.293	–	–	−1.494	0.272	−1.973	0.163	−0.997	0.420	–	–	–	–	−0.038	0.972	–	–	0.000	1.000

Comparisons of later Late-Holocene calcanei (<2000 cal. BP) from both regions suggests that Great Plains and SRP bison were indistinguishable in size during this period (Table 3 and Figure 5a). Size differences become apparent as recently as 2536–2684 cal. BP at the Mavrakis-Bentzen-Roberts site, where calcaneus breadth is significantly larger than calcanei from more recent assemblages (<2000 cal. BP), including the Baker Cave materials. Middle-Holocene bison also appear larger than Late-Holocene examples, although the strength of the statistical difference depends on the element considered. The Spring Creek and Logan Creek humeri are larger than those at Baker Cave and Rock Springs, although this difference is only statistically significant for HM7 (with the exception of the HM7 comparison between Spring Creek and Baker Cave). The lack of statistical differences between HM11 measurements likely results from the small sample sizes under test, as Spring Creek and Logan Creek only have four cases each for this measurement. Moving earlier into the Middle Holocene, the calcanei measurements from Hawken provide an especially strong contrast with those from Baker Cave, suggesting the Hawken bison were larger.

Finally, we compare measurements from Early-Holocene Great Plains bison with those from Late-Holocene SRP bison. With the exception of tibia metrics, measurements from Horner are uniformly greater than those from either Baker Cave or Rock Springs (Table 3). Calcanei from Mill Iron show a similar pattern. Both CL1 and CL4 are larger at Mill Iron than Baker Cave, although this difference is only significant for CL1. It is notable that the Mill Iron site only yielded three female calcanei that were measured along CL4, introducing a potential sample size problem. Nonetheless, metric comparisons between sites from both regions demonstrate that bison became smaller through the Holocene.

These data suggest that southern Idaho bison followed a diminution trend similar to the one identified on the Great Plains. This agreement between regions suggests that the bottom-up ecosystem controls acting on Great Plains bison morphology likely also conditioned diminution in southern Idaho. Furthermore, Late-Holocene bison in both regions are morphologically indistinguishable, pointing to a lack of geographic variability on each side of the Rocky Mountains. This is interesting since the low quality of SRP forage might potentially limit somatic growth relative to the Great Plains. Instead, it appears that the two regions’ environments differentially conditioned population numbers, rather than morphology, across the Holocene. The morphological similarity between the bison from the two regions may be due to gene flow or parallel trends in growing season length (or both). If both southern Idaho and the northern Great Plains had similar forage growth seasons, then bison may have experienced similar somatic responses to the intra-annual length of forage availability (Guthrie, 1984).

Conclusion

Punctuated changes in bison morphology occur alongside climatic events on the Great Plains, suggesting that diminution results from climate-dependent trends in forage quality and availability (Hill et al., 2008). Similar climatic trends characterize western North America (Broughton et al., 2008; Byers and Broughton, 2004; Byers and Smith, 2007; Byers et al., 2005), including the SRP. Therefore, we expected to see a similar pattern of bison diminution on the SRP. To test this hypothesis, we measured specimens from Baker Cave, a Late-Holocene SRP site, and compared those specimens to bison from other geographic and temporal contexts. Bison from the Wasden Site, located on the eastern SRP, suggest that Early-Holocene individuals were larger than those from the Late Holocene. In contrast, our study demonstrates that Late-Holocene bison from multiple SRP contexts shared similar morphologies. These comparisons also hold when the Baker Cave bison are compared with Late-Holocene assemblages from the Great Plains (Glenrock and Big Goose Creek). However, Great Plains bison from as recently as 2500 cal. BP were larger than the Baker Cave animals. Moreover, measurements taken on specimens from the Hawken, Spring Creek, and Logan Creek sites all demonstrate that Middle-Holocene Great Plains bison were larger than the Late-Holocene individuals from Baker Cave. These differences become even greater when the Baker Cave materials are compared with Early-Holocene Great Plains bison recovered from the Horner and Mill Iron sites.

Our study confirms that SRP and Great Plains bison experienced similar trends in morphological change across the Holocene. However, some questions remain open. It is still unknown if morphological similarity with Great Plains populations extends back through the Middle and Early Holocene. Additionally, it is unknown if the pattern of SRP diminution follows the punctuated pattern documented on the Great Plains. Resolving these problems will require further metric studies of SRP bison. Several large bison assemblages remain unanalyzed and temporal gaps in the record could be further filled with smaller assemblages from across the region. Finally, paleoecologists should collect more data from as of yet uninvestigated localities across the SRP. Completing these steps would create a dataset that allows for a detailed look at Holocene bison diminution west of the Rocky Mountains. Currently, such a dataset does not exist in western North America. We hope to address these gaps in knowledge in the near future.

Footnotes

Acknowledgements

We thank Craters of the Moon National Park, the Idaho Museum of Natural History, and the Idaho Bureau of Land Management (Burley Field Office) for supporting this project. Ken Cannon, Patricia Lambert, and L. Suzann Henrikson provided important insights and suggestions that contributed to this manuscript. R. Lee Lyman and one anonymous reviewer provided thoughtful comments that greatly improved this paper.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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