Rapid Body Size Decline in Alaskan Pleistocene Horses Before Extinction

letters to nature state, PEDOT films exhibit conductivities of up to 1 S cm21, with a principal absorption Pleistocene for radiocarbon dating. Here I show that horses edge at 0.6 eV,making it optically absorbing in the infrared but transparent across the visible underwent a rapid decline in body size before extinction, and I spectrum8,9. The thin-film Si diodes were fabricated by low-temperature deposition on a lightweight, stainless steel substrate coated with a 100-nm-thick Al layer that served as the propose that the size decline and subsequent regional extinction cathode. The p–i–n rectifying structures consisted of a grown, 15-nm nþSi/400-nm at 12,500 radiocarbon years before present are best attributed to a undoped (intrinsic) i-Si/25-nm pþSi junction region. To independently investigate the coincident climatic/vegetational shift. The present data do not switching properties of the polymer, an ITO/polymer/metal sandwich device was fabricated support human overkill1 and several other proposed extinction on precleaned, 20 Q per square, ITO-coated glass substrates. All devices are contacted on the 2,3 polymer surface via thermally evaporated, 17 mm2 Au electrodes. The devices were causes , and also show that large mammal species responded 4–6 electrically characterized in air. In our temperature simulations, the thermal impedance somewhat individualistically to climate changes at the end of between the film and substrate is neglected. This implies that within the first ,100 ns of the the Pleistocene. voltage onset, the substrate/film interface remains at room temperature. Caballoid horses in Alaska were probably part of the diverse Received 15 April; accepted 12 September 2003; doi:10.1038/nature02070. Eurasian species complex of the stocky Equus ferus, judging from their 7,8 9 1. Forrest, S. R., Burrows, P. E. & Thompson, M. E. Organic emitters promise a new generation of fossil anatomy and preserved mitochondrial DNA .Incontrast, displays. Laser Focus World, Feb., 99–101 (1995). fossils of the other equid Alaskan species have unusually long gracile 2. Gelinck, G. H., Geuns, T. C. T. & Leeuw, D. M. d. High-performance all-polymer integrated circuits. metapodials, making them similar to the living Equus hemionus from Appl. Phys. Lett. 77, 1487–1489 (2000). 3. Peumans, P., Yakimov, A. & Forrest, S. R. Small molecular weight organic thin-film photodetectors central Asia, and also similar to fossil New World hemione-like forms. and solar cells. J. Appl. Phys. 93, 3693–3723 (2003). So little work has been done with these northern hemione-like 4. Saluel, D., Daval, J., Bechevet, B., Germain, C. & Valon, B. Ultra high density data storage on phase equid fossils that I shall call these hemionids for simplicity, but note change materials with electrical micro-tips. J. Magn. Magn. Mater. 193, 488–491 (1999). that their specific relationships remain uncertain. 5. Asokan, S. Electrical switching in chalcogenide glasses—Some newer insights. J. Optoelectron. Adv. Mater. 3, 753–756 (2001). Alaskan hemionid fossils had not previously been radiocarbon 6. Stocker, H. J. Bulk and thin film switching and memory effects in semiconducting chalcogenide dated. They are comparatively rare in museum collections. In this glasses. Appl. Phys. Lett. 15, 55–57 (1969). study, 19 such fossils were dated, and the fact that none of the 7. Hua, Z. Y., Chen, G. R., Xu, W. & Chen, D. Y. New organic bistable films for ultrafast electric memories. Appl. Surf. Sci. 169, 447–451 (2001). hemionid metacarpals dated after 31,000 radiocarbon years before 8. Heuer, H. W., Wehrmann, R. & Kirchmeyer, S. Electrochromic window based on conducting poly present (31 kyr BP) makes their extinction before the Last Glacial (3,4-ethylenedioxythiophene)poly(styrene sulfonate). Adv. Funct. Mater. 12, 89–94 (2002). Maximum (LGM) seem likely. It is possible that the disappearance 9. Groenendaal, B. L., Jonas, F., Freitag, D., Pielartzik, H. & Reynolds, J. R. Poly(3,4- of this small-bodied hemionid species somehow affected selection ethylenedioxythiophene) and its derivatives: Past, present, and future. Adv. Mater. 12, 481–494 (2000). 10. Hack, M. & Street, R. A. Analysis of double injection in amorphous silicon p-i-n diodes. J. Appl. Phys. pressures on later Alaskan caballoid horses, promoting reductions 72, 2331–2339 (1992). in body size and increased numbers of the latter during the LGM, 11. Johansson, T., Pettersson, L. A. A. & Inganas, O. Conductivity of de-doped poly(3,4- around 18–20 kyr BP, but at present this seems unlikely given the ethylenedioxythiophene). Synth. Met. 129, 269–274 (2002). 12. Greczynski, G. et al. Photoelectron spectroscopy of thin films of PEDOT-PSS conjugated polymer implied time lag between the hemionid extinction and caballoid blend: a mini-review and some new results. J. Electron Spectrosc. Rel. Phenom. 121, 1–17 (2001). response. 13. Xing, K. Z., Fahlman, M., Chen, X. W., Inganas, O. & Salaneck, W. R. The electronic structure of Compared with the hemionids, caballoid horse fossils from poly(3,4-ethylene-dioxythiophene): Studied by XPS and UPS. Synth. Met. 89, 161–165 (1997). Alaska are much more numerous and much better studied. Previous 14. Pei, Q. B., Zuccarello, G., Ahlskog, M. & Inganas, O. Electrochromic and highly stable poly(3,4- ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue. Polymer 35, dates on over 220 fossil specimens from Alaska and the Yukon 10,11 1347–1351 (1994). Territory revealed no dates later than 12.5 kyr BP. Other studies from Eurasia and Beringia had indicated a size decline among Acknowledgements We thank A. Elschner and H. C. Starck for samples of Baytron P, and horses at some time during the late Pleistocene12,13, but the exact M. Thompson for discussions. We also thank National Renewable Energy Laboratories for supplying the thin-film Si diodes. This work was supported by HP and the National Science timing was uncertain. Unlike previous studies, I chose to date Foundation. metacarpals exclusively in order to better assess any subtle changes in body size with time and how this related to extinction. The Competing interests statement The authors declare that they have no competing financial dramatic size changes recorded just before extinction were most interests. unexpected (Fig. 1). Explanations for these apparent size changes

Correspondence and requests for materials should be addressed to S.R.F. include the possibility that my sampling methods may somehow ([email protected]). have affected this plot and artificially created a drop in apparent metacarpal size. This seems unlikely (see Methods). I suspect that the gap in dates at 28–35 kyr BP may reflect lower population numbers during this time, because previous, non-metacarpal, radiocarbon work on Alaskan horse fossils also showed compara- ...... tively few dates during this pre-LGM period14. Rapid body size decline in Alaskan A second possibility is that a new species or morphotype of smaller horses immigrated into Alaska, creating the illusion of size Pleistocene horses before extinction decline. The apparent high metacarpal variability during the LGM (Fig. 1) as opposed to post-LGM suggests such an interpretation. R. Dale Guthrie Some of this variability may simply be due to the greater abundance of LGM dates. In addition, species in transition from one adaptive Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775, USA peak to another, undergoing rapid evolution, are often more 15 ...... variable . The graph of size/time does not show two overlapping About 70% of North American large mammal species were lost at clusters, as one would expect with species replacement. Instead, the end of the Pleistocene epoch1. The causes of this extinction— smaller variants continue to emerge on the graph over time, and the role of humans versus that of climate—have been the focus of larger variants continue to fall out. This would necessitate multiple much controversy1–6. Horses have figured centrally in that debate, species replacements. In contrast, the explanation of a simple because equid species dominated North American late Pleisto- body-size decline in one widespread population seems much cene faunas in terms of abundance, geographical distribution, more parsimonious. Comparable size declines at the end of and species variety, yet none survived into the Holocene epoch. the Pleistocene are not unique to horses16. Bison declined more The timing of these equid regional extinctions and accompanying dramatically in an even shorter time span17 but at a later date. evolutionary changes are poorly known. In an attempt to docu- The significance of this size decline among Alaskan horses just ment better the decline and demise of two Alaskan Pleistocene before their regional extinction is that environmental pressures equids, I selected a large number of fossils from the latest provoking smaller body size may well have been the same ones that

240 14 Mammoth 230 12 Horse R2 = 0.4703 10 220

8 210 6 200 4 Metacarpal length (mm) 190 Last date 12,510 ± 130 2 Numbers of dates per millennium 180 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Radiocarbon years BP Radiocarbon years BP x 1,000 Figure 1 Length of Alaskan Pleistocene horse metacarpals plotted against AMS radiocarbon dates, with polynomial trendline. The plot suggests declining body size with Figure 2 Smoothed plot of new Alaskan dates for horses (n ¼ 73) and mammoths time, and local extinction in Alaska around 12.5 kyr BP. Error bars are 1 j. The latest dates (n ¼ 72) in radiocarbon years. Plot covers the past 24,000 years (dates summed within for horses in this study are: AMNHI34BX36, Upper Cleary Creek, AA26830, 12,482 ^ 80 1,000-year intervals). The clusters of dates, which are potentially the result of higher (21.2); AA26819, 12,510 ^ 130 (221.1); AMNH6159, Fox, Alaska (AK), AMNH144- population densities, change significantly with time. The serial correlations of lagged 9422, Ester, AK, AA26810, 12,560 ^ 170 (221.6); AA26828, 12,580 ^ 140 (221.2); values31 were 0.57 for horse, and 0.17 for mammoth. The latest mammoth dates in this AMNH126-6846, Lower Goldstream Creek, AA2682, 12,710 ^ 170 (221.2); study are: AK A-1011, Galena, AK, AA22573, 11,500 ^ 160 (222.9); Charles Holmes AMNH4339, Fairbanks Creek, AA26829, 12,860 ^ 140 (221.2). Data are as follows: (Swan Point Site), Delta, AK, AA17601, 11,540 ^ 140 (220.8); NMC25941, Dawson AMNH (Frick Collection) are museum collection numbers, and AA numbers are Arizona AMS area, Yukon Territory (YK), AA17559, 11,860 ^ 120 (221.2); UNM6684, Cape dating laboratory designations; then date is given in uncalibrated radiocarbon years with 1j; Lisbourn, AK, AA26006, 11,910 ^ 130 (221.2); CMN777, YK 1915, AA17562, and then (in parentheses) d13C values (in per mil) are given. These and other radiocarbon 11,990 ^ 130 (221.3). See Fig. 1 legend for data presentation. The mammoth dates data on Alaskan horses and hemionids are available as Supplementary Information. are drawn from part of a more extended data set (R.D.G., manuscript in preparation). ultimately resulted in their extinction. Alaska at levels below archaeological visibility when horses became Details of extinction timing are crucial in assessing competing extinct at 12.5 kyr BP, the idea that such a low density of hunters theories about Pleistocene extinctions. Models positing explosive could have caused horse extinction requires an unlikely scale of events, such as cross-species virulent disease, require the disap- overkill performance. That overkill theory must further address the pearance of species to coincide fairly closely, at least on a regional paradox of why super-able horse hunters should coexist with basis. Significant disjunctions in the timing of extinctions are also Alaskan mammoths for another 1,300 calendar years. a challenge to theories that portray numerous species being What, on the other hand, do we know about environmental rapidly ‘overhunted’ by a colonizing wave of human hunters. The changes in Alaska at the time of these extinctions? Broadly speaking, apparent extinction of Alaskan hemionids (youngest date at the last glacial was a time in which the cold/arid northern Mam- 31,400 ^ 1,200 yr BP), coinciding with the early phases of the last moth Steppe was most extreme, though still capable of supporting a glacial, is so early that few researchers would argue that humans rich diversity of large mammals22. Dune fields, thick loess deposits were involved. But even if we discount the Alaskan hemionid event and occasional lakes from this period point to arid and windy as ‘background extinction’, we cannot do that for the caballoid conditions with a treeless, short grass-sedge-sage sward14,23,24. horses. Horses figure too importantly in extinction models. Although the region’s large mammals were evidently adapted to Could the extinction of Alaskan horses and their apparent size handle cold/arid extremes, each species was evolutionarily fine- decline before extinction be the result of human hunting18? Note tuned to different optimal diets and habitats. Key Mammoth Steppe that this new array of Alaskan dates shows a gap of 1,000 radio- grazers—bison and woolly mammoth—were widespread during the carbon years (close to 1,300 calendar years) between the last dated LGM, though fossil evidence suggests that their relative abundance horse and mammoth fossils (Fig. 2). There is accumulating archaeo- may have declined during this climatic extreme11. logical evidence of human presence in Alaska beginning around 12 Proxy data show the ecological transition to Holocene conditions kyr BP or slightly later19. So with Alaskan mammoths persisting at did not occur smoothly. A dramatic pollen shift occurred around 25–27 28 least until 11.5 kyr BP, it now appears that humans and mammoths 13–12.5 kyr BP ; this was also a pivotal time for large mammals coexisted for at least a few hundred years. A vastly longer overlap and for invertebrates, like insects29. Landscape changes included occurred across much of Asia20 and Europe21. the creation of lakes, bogs, shrub tundra, forests, soil paludification, In contrast, there is a hiatus of over 500 radiocarbon years low-nutrient soils, and plants highly defended against herbivory. between the last dated Alaskan horses and the earliest undisputed Vegetation in the north now supports a relatively small biomass of human artefacts. Unless unquestionable human archaeological sites large herbivores, and almost no grazers. Northern tundra is are found in Alaska that date to the LGM, neither the rapid body especially marginal for significant numbers of wild horses14. diminution of Alaskan caballoid horses beginning at least around The greater abundance of both fossil bones and radiocarbon the LGM, nor the apparent decline in their relative numbers soon dates for horses in relation to mammoths (Fig. 2), and most other after LGM, can be reasonably attributed to the impact of selective grazers, across the LGM period in Alaska is noteworthy. Virtually all human hunting on the megafauna, as some have suggested18. The the fossils in question come from the same sites, so changes in present Alaskan data, showing that mammoths, the keystone abundance between species may well be a very rough indicator of grazer2, survived more than a millennium after horses, runs counter relative abundance rather than a taphonomic artefact. Horses are to extinction scenarios based on ecological repercussions following almost obligatory grazers, and these northern horses appear to have human over-hunting of keystone grazer species2,22. been the most specialized grassland-dependent component of the Even if we allow that human hunters may have been present in large mammal fauna.

240 Gold Hill). Two of my dates of 230-mm metacarpals conﬁrmed these previous assessments, and I deleted these specimens with inﬁnite dates from the above plots. The majority of Alaskan horse metacarpals in museum collections are under 230 mm. 200 These constitute some 82% of 1,020 specimens that I examined. I selected horse metacarpals at random from this class. If small animals (,210 mm, say) had existed before 160 the LGM they would have been captured by the sampling technique, as would have larger animals (.210 mm) that existed after the LGM. Had no actual size decline been occurring, 120 all the means of dated cohorts before, during and after the LGM should theoretically be similar. This was not the case. The 19 Alaskan hemione metacarpals were selected at 80 random from the small numbers in museum collections. Received 4 February; accepted 23 September 2003; doi:10.1038/nature02098.

Number of metacarpals 40 ------1. Martin, P.S. in Quaternary Extinctions (eds Martin, P.S. & Klein, R. G.) 345–403 (Univ. Arizona Press, 0 Tucson, 1984). 180 190 200 210 220 230 240 250 260 270 2. Zimov, S. A. et al. Steppe–tundra transition: A herbivore driven biome shift at the end of the Metacarpal length (mm) Pleistocene. Am. Nat. 146, 765–794 (1995). 3. MacPhee, R. D. E. & Marx, P. A. in Natural Change and Human Impact in Madagascar (eds Goodman, Figure 3 Smoothed plot of numbers of Alaskan horse metacarpals available in collections S. M. & Patterson, B. D.) 169–217 (Smithsonian Institution Press, Washington, 1997). 4. Guthrie, R. D. in Megafauna and Man (eds Agenbroad, L. D., Mead, J. I. & Nelson, L. W.) 42–54 (The against their lengths summed in 5-mm increments. The dashed line represents the span Mammoth Hot Springs Scientific Papers, Vol. 1, North Dakota, 1984). from which I drew my sample (190 to 230 mm). 5. Graham, R. W. & Lundelius, E. L. Jr in Quaternary Extinctions (eds Martin, P. S. & Klein, R. G.) 223–250 (Univ. Arizona Press, Tucson, 1984). 6. Guthrie, R. D. in Quaternary Extinctions (eds Martin, P. S. & Klein, R. G.) 259–298 (Univ. Arizona Press, Tucson, 1984). The present data suggest that Alaskan horses prospered during 7. Eisenmann, V. & Karchoud, A. Analyses multidimensionelles de me´tapodes d’Equus sensu lato the LGM, and appear to have been particularly well adapted to the (Mammalia, Perissodactyla). Bull. Mus. Natl Hist. Nat. Paris Ser. 4, 75–103 (1982). more intense versions of the cold/arid Mammoth Steppe. The data 8. Forsten, A. Equus lambei the Yukon Canada wild horse not ass. J. Mamm. 67, 422–423 (1982). further suggest that Alaskan horses were the first steppe species to 9. Vila`,C.et al. Widespread origins of domestic horse lineages. Science 291, 474–477 (2001). 10. Harington, C. R. Annotated Bibliography of Quaternary Vertebrates of Northern North America (Univ. drop out at 12.5 kyr BP as the Mammoth Steppe began to unravel Toronto Press, Toronto, 2003). and be replaced by more mesic vegetation. I further hypothesize that 11. Matheus, P., Kunz, M. & Guthrie, R. D. in Abstr 3rd Int. Mammoth Conf. 3–5 (Yukon Territorial some of the diminution observed in these horse fossils during post- Government, Dawson, Canada, 2003). LGM times can be attributed to the shift away from intensely arid 12. Forsten, A. Size decrease in Pleistocene–Holocene true or caballoid horses in Europe. Mammalia 55, 407–420 (1991). steppic conditions. That woolly mammoth, a slightly less obligate 13. Guthrie, R. D. in People and Wildlife in Northern North America (eds Gerlach, C. & Murray, M.) 32–49 steppic species, should persist in Alaska for over another millen- (Br. Archaeol. Rep., Int. Ser. 944, 2001). nium is congruent with this hypothesis. Perhaps the declining body 14. Guthrie, R. D. Frozen Fauna of the Mammoth Steppe (Univ. Chicago Press, Chicago, 1990). size of Alaskan horses and their extinction relate not only to the 15. Guthrie, R. D. Variability in characters undergoing rapid evolution, an analysis of Microtus molars. Evolution 19, 214–233 (1965). absolute decline of their access to optimal food resources, but also to 16. Guthrie, R. D. in Contributions in Vertebrate Paleontology (eds Genoways, H. H. & Dawson, M. R.) increasing competition with other large mammals possessing the 482–510 (Carnegie Museum of Natural History Special Publication 8, Pittsburgh, 1964). physiological capacities to thrive on the vegetation characteristic of 17. Wilson, M. C. Late Quaternary vertebrates and the opening of the ice-free corridor, with special reference to bison. Quat. Int. 32, 97–105 (1996). this 13–10 kyr BP, end-Pleistocene, transition. 18. McDonald, J. in Quaternary Extinctions (eds Martin, P. S. & Klein, R. G.) 409–439 (Univ. Arizona As radiocarbon chronologies and proxy data of palaeoclimate Press, Tucson, 1984). gain refinement, our reconstructions of this dynamic end-Pleisto- 19. Holmes, C. E. Tanana River Valley archaeology circa 14,000 to 9,000 B.P. Arctic Anthropol. 38, 154–170 cene time have become less simple. Although global climatic drivers (2001). were at play in the terminal Pleistocene, some of their effects are 20. Kuznetsova, T. V., Sulerzhitsky, L. D. & Siegert, Ch. in The World of Elephants (eds Cavarretta, G., Gioia, P., Mussi, M. & Palombo, M. R.) 289–292 (Universita` degli Studi de Roma “La Sapienza”,2001). remarkably particular. For example, during this time we note the 21. Stuart, A. J., Sulerzhitsky, L. D., Orlova, L. A., Kuzmin, Y. V. & Lister, A. M. The latest woolly general replacement of caecalids—species that use a large caecum mammoths (Mammuthus primigenius Blumenbach) in Europe and Asia: A review of the current gut-diverticulum in food processing (for example, woolly mam- evidence. Quat. Sci. Rev. 21, 1559–1569 (2002). moth, woolly rhino and horses) by ruminants (for example, bison, 22. Alroy, J. A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction. Science — 292, 1893–1896 (2001). 4,10,11 moose and reindeer) . The patterns of the radiocarbon dates 23. Guthrie, R. D. Origin and causes of the mammoth steppe: A story of cloud cover, woolly mammal reported here highlight how some Alaskan large mammal species tooth pits, buckles, and inside-out Beringia. Quat. Sci. Rev. 20, 549–574 (2001). responded to changes during the Pleistocene–Holocene transition 24. Goetcheus, V. G. & Birks, H. H. Full Glacial upland tundra vegetation preserved under tephra in the Beringia National Park, Seward Peninsula, Alaska. Quat. Sci. Rev. 20, 135–147 (2001). in different ways, and suggest that this individualistic response is a 25. Anderson, P. M. Late Quaternary pollen records from the Kobuk and Noatak River drainages, general phenomenon—that within a general framework, ultimately, northwestern Alaska. Quat. Res. 29, 263–276 (1988). each species had its own unique story. A 26. Lozhkin, A. V. et al. Late Quaternary lacustrine pollen records from Southwestern Beringia. Quat. Res. 39, 314–324 (1993). Methods 27. Lamb, H. F. & Edwards, M. E. in Vegetation History (eds Huntley, B. & Webb, T. III) 519–556 (Kluwer Academic, Dordrecht, 1988). Alaskan fossils of late Pleistocene large mammals are generally collected from thawed 28. Sher, A. V. Past and future rapid environmental changes. Glob. Environ. Change 1 47, 319–339 (1997). permafrost (some bones still contain white marrow), and thus have a very high degree of 29. Lowe, J. et al. Climatic changes in areas adjacent to the North Atlantic during the last Glacial– collagen preservation for reliable dating. Dates were run at the US National AMS Interglacial transition (14–9 ka BP): A contribution to the IGCP-235. Quat. Sci. 9, 185–198 (1994). (Accelerator Mass Spectrometer) Radiocarbon Laboratory at Tucson, Arizona, with their 30. Brown, T. A., Nelson, D. E., Vogel, J. S. & Southon, J. R. Improved collagen extraction by Longin 30 usual pretreatment . I chose to use metacarpals in this study for several reasons. method. Radiocarbon 30, 171–177 (1988).. Metacarpals are recognized as one of the best postcranial bones for distinguishing co- 31. Kachigan, S. K. Multivariate Statistical Analysis (Radius, New York, 1991). existing equid species, such as hemiones and caballoids7. The cortex of these bones is so unusually dense that they are rarely used by scavengers, which probably helps make them Supplementary Information accompanies the paper on www.nature.com/nature. abundant in the fossil record. Metacarpals are a critical weight-carrying structure, so their size is a good indicator of horse body mass (in this study, metacarpal width declined in the Acknowledgements I am grateful to R. Tedford for sampling access to these Alaskan equid and same pattern as metacarpal lengths.) Unlike woolly mammoths and most other mammoth fossils. Similar thanks go to R. Gangloff and R. Harington for dating access to Pleistocene large mammals, horses are barely sexually dimorphic, so I did not have to mammoth fossils from Alaska and the Yukon Territory. M. L. Guthrie, A. Lister and P. Matheus address the complex issue of sexual identity. Also unlike mammoths, bone growth among made suggestions that improved the manuscript. AMS dates were done at the AMS Laboratory, horses is quite determinate, with epiphyses fusing at adulthood, so maturity could be University of Arizona. This work was supported by the National Science Foundation. assessed by this fusion—only metacarpals of mature animals were used. To make best use of funding allocated for radiocarbon dating, I did not date the largest metacarpals (Fig. 3). Earlier studies12,13 had shown that the largest metacarpals, ranging Competing interests statement The author declares that he has no competing financial interests. from 230 to 260 mm in length, tended to fall beyond radiocarbon dating range. These larger metacarpals are normally associated in Alaska with sites where Early to Middle Correspondence and request for materials should be addressed to the author Pleistocene sediments, as dated by tephra horizons, predominate (Cripple Creek Sump, ([email protected]).

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