Critical to studying caribou antler accumulations is the capacity to confidently identify

Transcription

1 Electronic Supplementary Material ESM Text 1.0: Differentiating female and male caribou antlers. Critical to studying caribou antler accumulations is the capacity to confidently identify their gender (thus, differentiating calving grounds, based on shed female antlers, from breeding/wintering grounds, based on shed male antlers). Adult female and male caribou antlers are strongly sexually dimorphic; while there is some overlap between juvenile males and adult females, male antlers are generally larger (e.g., mass, length) [1]. However, confident gender identification of shed antlers from bone assemblages, which are likely to be incomplete due to gnawing and/or breakage [2, 3], requires quantifiable diagnostic traits. Linear measurements of the antler pedicle attachment surface of shed antlers (major- and minor-axes) and derived estimates of surface area are used as the quantitative framework for gender identity. ESM Text 2.0: Probabilistic gender assignment Gender assignment for unknown (ANWR) antlers was calculated using normal probability density functions fit to the surface area measurements of known (museum) male and female caribou pedicle surfaces (the cranium-antler attachment surface). For each ANWR antler with known pedicle surface area, the probability of it being female was calculated as the proportion of probability density estimates for male and female antlers of that size: p female = density female / (density female + density male ). Where p female is the probability that the antler is female, density female is the probability density estimate based on the known (museum) distribution of female antlers at the pedicle size of the unknown antler, and density male is the probability density estimate based on the known (museum) distribution of male antlers at the pedicle size of the unknown antler. The probability of that same antler being male is the inverse: p male = 1- p female. 1

2 To express results as frequency distributions, probability estimates, and their variability, were summed together (i.e., within size-classes (Figure 2) or weathering stages (Figure 5)). Necessarily, gender assignment is influenced by how well we understand the true underlying size frequency distributions of male and female caribou antlers. By iteratively constructing the male distribution using equal sample-sizes to the female sample, we could incorporate all available knowledge on male caribou antler sizes while simultaneously calculating uncertainty in gender assignment for ANWR antlers. Thus, while the underlying female distribution remains constant for each analytical run, we generate uncertainty in gender assignment of the unknown (ANWR) antlers due to changes in the overlap of probability density functions of known male and female antlers. We note that adding uncertainty to the female dataset (for example bootstrapping all available data for both male and female museum specimens) results in virtually the same mean and 95% confidence intervals of females antlers in ANWR (with bootstrapping yielding expected larger confidence intervals: Mean ANWR Females Sub-sample standardized = 203 [C.I.: 196, 213], Mean ANWR Females Bootstrapped = 203 [C.I.: 188, 216]. However, bootstrapping adds additional uncertainty in gender assignment across the entire distribution of antler pedicle sizes, including very small size classes (which, the data indicate, are clearly in the domain of adult females to the exclusion of adult males). Thus, we feel bootstrapping produces biologically dubious results producing analytical uncertainty where the biology is clear. For calculating uncertainty in caribou gender assignment, subsampling-based sample-size standardization appears to generate sufficient estimates of variability, while maintaining biologically defensible results. ESM Text 3.0: Antler concentrations. 2

3 Based on how bone material accumulates on landscape surfaces, we feel that antler concentrations (antlers/km 2 ) are a natural unit of measurement for this study. This metric also relates well when quantifying agreement between the observed antler accumulation and expected antler accumulation, based on empirical estimates of calving intensity over the last thirty years Figure 4. Using all antlers also alleviates complexities of deciphering the side (left or right) of highly fragmented shed antlers. While this may result in double-counting some individual input (each parturient female will shed two antlers per year), such affects should influence antler accumulations uniformly across all habitats. Additionally, because we are not estimating individuals from the antler accumulation, our analyses and results should not be meaningfully impacted. ESM Text 4.0: Weathering Stages used for bones on Jago Bitty. Weathering stages used to characterize bones and antlers follow standard protocols [4]. To supplement novel arctic conditions, WS 4 was also used to characterize bones and antlers for which lichen encrustation was so advanced that limited bone surfaces (or none) were exposed. ESM Text 5.0: Confidence intervals of gender assignment are small. Ninety-five percent confidence intervals of gender assignment probabilities were low (ranging from 0.01 to 0.19), with most antlers exhibiting lower degrees of gender uncertainty throughout the sub-sample standardization (ESM Figure S1). Consistently small confidence intervals support strong differences between the pedicle sizes of adult male and female caribou and that their geometric differentiation is useful for assigning gender to antlers on landscape surfaces. 3

4 Frequency Confidence Interval Size (probability that an antler is female) ESM Figure S1. Frequency distribution of the sizes of 95% confidence intervals for the probability of gender identity as generated for all ANWR antlers during iterative gender differentiation based on sub-sample standardization of museum specimens. Consistently small confidence intervals support strong differences between the pedicle sizes of adult male and female caribou and that their geometric differentiation is useful for assigning gender to antlers on landscape surfaces. 4

5 ESM Text 6.0: Estimating expected antler concentrations at Jago Bitty. We estimated the expected number of antlers on the Jago Bitty calving ground using annual demographics and calving geography of the Porcupine Caribou Herd from (PCH [5-8]). While calving intensity shifts annually to decadally across the Coastal Plain, for the purposes of this analysis, we are only interested in calving intensity (i.e., density of calving females) at the Jago Bitty calving ground. Using the known calving history of Jago Bitty, we can identify which years it was part of the Concentrated Calving Ground (CCG the most heavily used localities), the Annual Calving Ground (ACG lesser-used areas peripheral to the CCG), or not used at all. In addition, annual surveys provide data on the area (km 2 ) over which different calving intensities occurred, the proportion of calving females in each of these regions, and annual parturition rates. Thus, by combing the above information with estimates of total PCH population and percent females, we can estimate each annual antler contribution to the Jago Bitty bone accumulation through the formula: Annual Antler Contribution [calculated separately for CCG and ACG] = Total Population * Proportion female * Proportion parturient * 2 (number of antlers produced each year by calving females) * Proportion of females calving in either area [CCG or ACG] / area [CCG or ACG] Annual antler contributions are treated as additive (loss rates are unknown and likely small given the short timescale (<30 years) and low-levels of observed gnawing damage) and are summed over all years to produce the total expected antler concentration at the Jago Bitty calving ground. 5

6 Total populations of the PCH are surveyed as weather conditions allow and surveys are not possible every year. For years with missing population estimates, linear regression of population estimates between sampled years provides population estimates used in these calculations. Additionally, demographic data on the proportion of male to female caribou is not broadly available. Data from a 2009 rutting survey [7] is used here as the long-term estimate of the proportion of adult females in the PCH. ESM Text 7.0: No rodent gnawing on antlers from Jago Bitty. Rodent damage was not observed on antlers recovered from the Jago Bitty. Two antlers showed marks on tine tips similar to rodent gnaw-marks, but were rounded and likely resulted from scraping the antler through rocks and dirt while foraging prior to shedding [9] ESM Text 8.0: Weathering Stage 5 bones on PCH calving grounds. To continue sampling the PCH calving grounds, the highly-used calving area on the delta of the Turner River, AK was surveyed in Sampling procedures followed those of this manuscript, though survey geometries were primarily defined by those of the habitat, allowing smaller habitat patches to be sampled. During bone surveys, two bones in Weathering Stage 5 were observed: one shed antler (T ), and one mammalian bone fragment (T ). Both were uncovered from openly-vegetated Dryas-dominated river terraces. While rare, the observation of WS 5 antlers shows that the full range of bone weathering stages is possible on the ANWR Coastal Plain. 6

7 Supplementary References 1. Johnson D.R., Nagorsen D.W Evaluation of cranial and antler characteristics to determine sex of moutain caribou, Rangifer tarandus. Can Field Nat 104, McCabe R.A Observations on the disappearance of shed caribou antlers. J Mammal 38(2), Wika M Antlers - a mineral source in Rangifer. Acta Zoologica 63(1), Behrensmeyer A.K Taphonomic and ecologic information from bone weathering. Paleobiology 4(2), Caikoski J Porcupin Caribou Herd - Fall 2010 Composition Survey. (p. 6, Alaska Department of Fish and Game. 6. Griffith B., Douglas D.C., Walsh N.E., Young D.D., McCabe T.R., Russell D.E., White R.G., Cameron R.D., Whitten K.R The Porcupine Caribou Herd. In Biological Sciences Report (eds. Douglas D.C., Reynolds P.E., Rhode E.B.), pp Reston, VA, U. S. Geological Survey, Biological Resources Division, Biological Sciences Report USGS/BRD/BSR Caikoski J., Taras B Porcupine Caribou Herd - Fall 2009 Composition Survey. (p. 7, Alaska Department of Fish and Game. 8. Caikoski J Porcupine Caribou Herd calving and post-calving surveys, June-July Alaska Department of Fish and Game, Jin J.J.H., Shipman P Documenting natural wear on antlers: A first step in identifying use-wear on purported antler tools. Quatern Int 211, Miller J.H Ghosts of Yellowstone: Multi-decadal histories of wildlife populations captured by bones on a modern landscape. Plos One 6(3), e doi: /journal.pone Western D., Behrensmeyer A.K Bone assemblages track animal community structure over 40 years in an African savanna ecosystem. Science 324(5930),

8 Antlers of the Arctic Refuge (Electronic Supplementary Material) Miller et al. Frequency Frequency log(area) ESM Figure S2. Log surface area for caribou pedicle surfaces from the ANWR bone assemblage (top) and museum collections (bottom). Female antlers are white, males are gray. Error bars are 95% confidence intervals of gender assignment probabilities. The distributions for all 10,000 iterative male sub-samples are shown (bottom), showing low variability in resulting probability density functions (leading to small 95% confidence intervals (top; ESM Figure S1)). Adult male and female caribou antlers are distinguishable. The Jago Bitty calving ground is dominated by female antlers. 8

9 ESM Table S1. Bones of neonatal caribou observed on the Jago Bitty calving ground. Most of the neonatal MNI are composed of multiple body regions (e.g., paired-forelimbs or cranium with associated limb elements), suggesting individuals were not widely dispersed postmortem. MNI were calculated for each sample plot, based on skeletal element and weathering stage [4, 10, 11]. Geographic proximity among skeletal elements was also used when assigning bones to each constituent MNI. The Number of Identifiable Specimens (NISP) provides a bone count (not including teeth) and indication of carcass completeness. Weathering Stage (WS) designations are made based on the most-weathered bone of the individual [4, 10, 11]. Details of each MNI are provided and organized below by their alphabetical MNI Index. Body-portion abbreviations: C (cranium), M (mandible), A (axial-skeleton: vertebrae and ribs), F (forelimb, including scapula), H (hind-limb, including innominate), p-f (paired Forelimbs), p-h (paired Hind-limbs). Terrain Sample Plot MNI Index NISP WS Body-portions Tussock Tundra T01-03 A 9 2 C, A, F, H Riparian Terrace T01-02 A 13 2 C, A, p-f, p-h T01-02 B 7 2 C, M, p-f, H T01-02 C 1 4 M T01-04 A 2 3 p-f 9