8. N. Nathanson and G. Gole, Adv. Virus Res. 16, 11. P. Petrov, Am. J. Trop. Med. Hyg. 19, 146 The studied population has increased 397 (1970). (1970). 9. Spleens were removed aseptically from donor 12. D. C. Gajudsek, Adv. Neurol. 36, 363 (1982). from 58 to 129, and 15 horses (yearlings rats and placed in petri dishes, and the spleno- 13. We thank H. Becht for helpful discussions. cytes were teased into Dulbecco's minimum Supported by the Deutsche Forschungsgemein- or older) have disappeared. All individ- essential medium containing heparin (5 I.U. per schaft (Sonder forschungsbereich 47). A guest uals have been identified and the ages of milliliter). The cells were passed through a stain- professorship awarded by the DFG to O.N. less steel screen (100-p.m mesh), washed once while on sabbatical at the Giessen Institut fur more than 90 percent are known (6). Of by centrifugation, and injected intraperitoneally Virologie is also acknowledged. the 15 animals that disappeared, the bod- into recipients on a basis of one spleen equiva- * Present address: Departments of Neurology and lent per recipient. Comparative Medicine, Johns Hopkins Univer- ies or skulls of one bachelor male (9 10. D. H. Gilden, G. A. Cole, A. A. Monjan, N. sity School of Medicine, Baltimore, Md. 21205. years), three stallions (15+, 13, and 10 Nathanson, J. Exp. Med. 135, 860 and 874 (1972). 20 January 1983 years), two younger males (2 and 3 years), five females (12, 6, 5, 5, and 4 years), and a male yearling have been located. Remains of nine of these ani- mals (75 percent) were discovered along Ecology and Catastrophic Mortality in Wild Horses: windswept ridgelines (Fig. lA) at about Implications for Interpreting Fossil Assemblages 2600 m. From hair fragments of tails, manes, and fur, the animals were identi- Abstract. The identities, sexes, and reproductive status of groups of wild horses fied as members of two bands, last ob- (Equus caballus) living in the Great Basin Desert ofNorth America were known prior served alive during the summer of 1979. to their deaths on ridgelines. Another group of very young horses died on a One band of six animals (a male, 4 fe- quagmire. Snow accumulation or drought was apparently responsible for the mass males, and ajuvenile) was found in June deaths. These data have implications for reconstructing some aspects of the social 1981, and the other with three horses structure of fossil mammals on the basis of skewed sex or age ratios in bone (two females and a juvenile), which had assemblages. been part of a band of 12, was found in June 1982. Socioecological interpretation of fossil death in groups may be more common Three lines of evidence suggest not mammalian assemblages is a fairly re- than has been assumed. This information only that these two groups perished as a cent field of study (1), although grouped has implications for determining some result of severe winter snowstorms but remains of fossil bison, camels, pigs, patterns of mortality in extant wild horse also that high altitude snow-induced rhinos, prongbucks, horses (2), and dino- populations and assessing the validity of mortality may be more common than has saurs (3) have been used as evidence of paleoethological reconstructions of so- been assumed among some populations on August 2, 2012 complex social structures because of bi- cial groupings. of mountain dwelling wild horses. First, ased sex ratios or adult females situated Horses played prominent roles in com- because of their ages (7), one would not in proximity to young. Such group re- munity dynamics during the middle and expect the dead animals to be in poor mains potentially provide information late Tertiary in North America (4). To- condition, and when last observed alive about social structure of a species. Be- day, feral horses are conspicuous ele- they appeared to be healthy and vigor- cause groups varying in sex or age com- ments of the mammalian fauna of the ous. Second, 80 to 85 percent of the position can become fossilized in the Great Basin, where an estimated 35,000 animals migrated every year from winter same quarries for many different reasons to 40,000 range. Since 1979 I have stud- ranges at low altitudes (around 1400 m) (for example, death traps), realistic ied populations of horses in the Granite to higher altitudes for the summer and www.sciencemag.org grouping patterns may be obscured. Range of northwestern Nevada. Most fall, where some remained into winter. Data on sex, age, and reproductive sta- horses migrate seasonally to the highest The bands observed at high-altitude sites tus of wild (feral) horses (Equus cabal- peaks, some of which exceed 2780 m. (2100 m, 2470 m, and 2560 m) during lus) that died in groups or individually on They are organized into distinct year- December 1980 and January 1981 were high altitude windswept ridges in the round bands, each with at least one not subordinate bands and, thus, social Great Basin Desert of North America stallion and a harem (females and young) factors did not appear to explain why indicate that (i) snow and mud are promi- (5). Nonharem males live alone or form they remained in summer ranges into Downloaded from nent forces influencing mortality and (ii) bachelor groups. early winter. Although it is likely that Fig. 1. (A) Skulls and skeletons on a ridge (about 2600 m) in the Granite Range, Nevada. Two of the skulls were removed from underlying vegetation and placed on rocks. (B) Foals in the Owyhee Desert, Nevada. Animals were pulled from the quagmire. [Desert photograph courtesy of the U.S. Department of the Interior] 24 JUNE 1983 1403 these bands had access to high-quality variables such as heavy snow accumula- References and Notes food by remaining at the high altitudes tion apparently increase the susceptibil- 1. A. K. Behrensmeyer and A. P. Hill, Fossils in they were also more likely to encounter of entire the Making (Univ. of Chicago Press, Chicago, ity groups to mortality and may 1980); A. K. Behrensmeyer, D. Western, D. D. severe winter snowstorms and cold. be more debilitating to mountain dwell- Boaz, Paleobiology 5, 12 (1979). 2. T. Downs, Fossil Vertebrates of Southern Cali- Third, the winter of 1979-80 (when these ing Great Basin horses than once be- fornia (Univ. of California Press, Berkeley, bands disappeared) was severe, and each lieved. 1968); 0. A. Peterson, Ann. Carnegie Mus. 12, 149 (1912); M. Voorhies, Contrib. Geol. Spec. of several snowstorms deposited over 65 The data on mortality in Granite Pap. 1 (1969); Nat. Geog. 159, 66 (1981). cm of snow at about 1760 m. Accumula- Range horses suggest that 3. J. H. Ostrom, Palaeogeogr., Palaeoclimatol., generaliza- Palaeoecol. 11, 287 (1972). tion at higher elevations and ridgelines, tions about the social structure of paleo- 4. J. A. Shotwell, Trans. Am. Phil. Soc. 53, 3 where the missing (1963); G. G. Simpson, Horses (Oxford Univ. horses were found, mammals from fossil assemblages should Press, New York, 1951). would be much greater. be made with caution. For instance, if 5. J. Berger, Behav. Ecol. Sociobiol. 2, 131 (1977). It is unlikely that snow was a 6. , in preparation. density- nothing was known about the social or- 7. American Association of Equine Practice, Offi- dependent mortality factor, because of ganization of horses several different cial Guidefor Determining the Age ofthe Horse 42 skeletons located so (Golden, Colo., 1966). Although ages are esti- far, 31 (74 per- conclusions might have arisen had only mated from patterns of tooth wear, they were cent) were situated above 2000 m and 28 one of the assemblages in the Granites judged on known animals within the population and then compared with the guide. (67 percent) were above 2300 m. The rest been found. If the discovery was that of 8. "Horse and burro files," Bureau of Land Man- of the agement, Winnemucca, Nevada, unpublished skeletons were found between the two females and juvenile, it could be files. 1400 and 2005 m. Because most horses argued that females and young formed 9. E. Joubert, Madoqua 8, 49 (1974). spend less time at high the 10. A. R. E. Sinclair, The African Buffalo (Univ. of altitudes (6), and only permanent associations for that Chicago Press, Chicago, 1977), plate 41. all sites were searched for similar peri- species. However, if the stallion and his 11. L. M. Talbot and M. H. Talbot, Wildl. Monogr. 12, 1 (1963). ods of time, the disproportionate number harem were located instead, the conclu- 12. B. L. Penzhorn, thesis, University of Pretoria, of bodies found at higher elevations [X2 sion might be that bands were the pri- South Africa (1975). 13. D. R. McCullough [The George Reserve Deer (1) = 9.52; P < .01] suggests that unpre- mary social units. Thus although skewed Herd (Univ. of Michigan Press, Ann Arbor, dictably heavy snow accumulation is a sex and 1980)] describes the population dynamics of age ratios may indicate some large herbivorous mammals. principal mortality agent in the Granites aspect of sociality, there are limitations 14. I thank C. Cunningham and P. Jancar for assist- as ance in the field; A. Behrensmeyer, L. D. Mar- it may be elsewhere in the Great to the extent to which they can be used tin, S. H. Jenkins, C. M. Wemmer, and M. Basin. For instance, during the winter of to reconstruct the social structure of Bekofffor comments on the manuscript; and the Winnemucca Office of the Bureau of Land Man- 1977 an estimated 300 horses (50 percent fossil mammals. agement for logistical aid. Supported by the of the population) died in the Buffalo Harry Frank Guggenheim Foundation, Bureau JOEL BERGER on August 2, 2012 of Land Management, the National Geographic Hills (8), an insular area 12 km west of Conservation and Research Center, Society, and the Smithsonian Institution's Con- the Granites.