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Article #325 South African Journal of Science )

9,10 Aepyceros The has genus The Research Article 2 ), indicate that they

Aepyceros melampus 3,5 leistocene In particular, the genetic P 3,4,5 a S Afr J Sci

The Shungura species ( 1,8 years) may pass unnoticed in the 3 for pala

– 10 m 2 i also are abundant in the Shungura, 7

) and the eland ( Other species exhibit more complex spatial patterns With apparently almost no speciation during the Plio- the during speciation no almost apparently With Only males possess ridged, thin, S-shaped horns. In 6 genus are known from Lothagam, northern Kenya, from Kenya, northern Lothagam, from known are genus 3,4,5 evidence 11,12 frican

ABSTRACT A INTRODUCTION Aepyceros 2 ) and impala. Damaliscus lunatus genetic

eastern of

Several species co-occurring in southern and eastern Africa, show marked body Several species co-occurring in southern and 2 ), the ( of

Vol. 106 No. 11/12 Page 1 of 7 106 No. 11/12 Vol. Connochaetes taurinus

) had a smaller overall body size and somewhat different horncore morphology. horncore different somewhat and size body overall smaller a had ) The Plio-Pleistocene African fauna, particularly species with extensive geographic ranges, 1,2 Palaeontology typically relies on studies, in particular morphological differences, to reconstruct to differences, morphological particular in studies, fossil on relies typically Palaeontology and interpret patterns of vertebrate evolution. However, genetic studies of population histories of extant species provide data about past population events which are (e.g. equally relevant local to extinctions, palaeontological questions. recolonisations) This study used morphological traits evaluate a to hypothesis based on genetic evidence that southern African impala ( fossil record, and lineages may appear within uninterrupted. a species Instead, may be greater observed, which morphological may variation be misinterpreted or as ecophenotypic reflectingvariation. Combining adata from speciationgenetic studies event, and palaeontology may provide further clues as to how faunal dispersals within Africa shaped the morphological variation in differences. fossil record, and how to best interpret such the are the founder population for all other living African impala populations, after an eastern African extirpation event dating to around 200 000 years ago. Measurements of three metrics and the presence or absence of a particular dental trait were compared across four regional impala samples. Eastern African impala possess a unique combination of have to likely are traits These populations. impala other to compared when entostyles of occurrence larger horns and a significantly higher characterised a small group of founding impala which recolonised this region. This pattern appears consistent with the genetic evidence that a subset of the southern morphological African certain impala in eastern gave African variation rise populations. express to Other to species the with expected complex be population histories, therefore such also as may , and topi eland, extinction local of process The recolonisations. of patterns similar of because record fossil the in traits and subsequent repopulation over shorter timescales (10 evaluation Alcelaphus buselaphus or not variation in morphological traits can be reconciled with current genetic evidence of population history. Pleistocene, they contrast markedly with other, more speciose tribes, . such as the Alcelaphini or the the to assigned specimens earliest The deposits which are dated to between 7 MYA and 4 MYA. Member B () deposits, which date to shungurae 2.95 ± 0.05 MYA. dated palaeomagnetically Makapansgat, Africa, southern in deposit early one from identified been also a species. to approximately three million years old; but these specimens have not been assigned to Impala are widespread in modern times, distribution inhabiting throughout savannah sub-Saharan Africa. environments over the to populations, reintroduced as and, a reserves to confined are discontinuous impala Africa, South present-day eastern coast of (Figure 1). Because this species has been well studied, from behavioural, genetic, palaeontological and morphological perspectives, it provides a useful model to assess whether to the same extent. Genetic studies of living populations provide a non-palaeontological perspective on recent population changes, local extinctions (known as extirpations) evidence and across several recolonisations. species suggests that dramatic late Pleistocene climate changes have profoundly altered their population structures. The suggests a amount late of Pleistocene variation extirpation (ca. in 200 microsatellite KYA) wildebeest ( markers of several species in eastern Africa, including of refugia during the same temporal period; for example, phylogeographic studies ( of the hartebeest survived during this period in small isolated areas of eastern and western Africa. Of all the African subjected to genetic studies to date, the impala stands out as an intriguing Africa in evolution of record unique a offering complexity, and simplicity evolutionary of mix ’, ‘living be to Aepycerotini, the tribe, bovid this consider authors Some years. million several over evolution. their of course the over changed little Studies of fossil assemblages recovered at various sites, have, until very recently, been the only way to reconstruct and interpret patterns of species evolution and community response to large-scale climate shifts. climate pronounced to changes, size body as such responses, mammal regarding data important provide and seasonality shifts. size changes in eastern African conspecifics, relative to southern African counterparts. This pattern has been explained by dramatic climate changes over the last three million affect not did which Africa, east in tectonism and volcanism of degrees different by years, exacerbated further which may have been

extirpation 1 orphological M Honorary Research Publishing. This work is licensed under the Creative Commons Attribution License. © 2010. The Authors. © 2010. Licensee: OpenJournals at: http://www.sajs.co.za 7 pages. DOI: 10.4102/sajs. v106i11/12.325 This article is available evidence for a Pleistocene extirpation of eastern African impala. S Afr J Sci. Art. #325, 2010;106(11/12), How to cite this article: Reynolds SC. Morphological evaluation of genetic Dates: Received: June 20 2010 Accepted: 03 Oct. 2010 2010 Published: Nov. 17 populations; genetic population histories; Pleistocene climate changes Keywords: antelope; entostyles; extirpations; founder Evolution, UniversityEvolution, the of Witwatersrand, Private Bag PO WITS, Johannesburg3, South2050, Africa [email protected] Postal address: Institute for Human Correspondence to: Sally Reynolds email: Witwatersrand, Johannesburg, South Africa 1 Associate, Institute for Human Evolution, University the of Author: Sally C. Reynolds Affiliation: http://www.sajs.co.za Pleistocene impala extirpation Research Article Reynolds

is the founder population for all modern impala.4 The genetic perspective appears at odds with the current fossil evidence, where impala are abundant in many fossil sites in eastern Africa, but are sparsely represented in the southern African sites. In this study, I therefore used horn metrics and the presence or absence of simple folds and basal entostyles in the upper molar teeth of impala samples across their geographic range to test for regional differences, and to establish whether there are morphological traits which could support the genetic hypothesis.

MATERIALS AND METHODS A total of 276 modern impala skulls of both sexes from four African regions were examined (Table 1). Both adult and sub- adult (third molars erupting) specimens were included. Only wild-caught specimens with known country provenance were included. The impala were divided into four groups corresponding to geographic localities, in order to characterise the degree of potential variation from each region. Group 1 (A. m. melampus) represents the from the Republic of South Africa, while Group 2 (A. m. johnstoni) represents those from Zambia, Malawi and Zimbabwe. Group 3 (A. m. suara) represents Uganda, Kenya and Tanzania and includes the subspecies A. m. rendilis, which is considered a for A. m. suara.16 The final group, Group 4 A.( m. petersi), contains the rare black-faced impala from Angola, Namibia and . Specimens were studied at the National Museums of Kenya (Nairobi), the Natural FIGURE 1 History Museum (London), the Powell-Cotton Museum (Kent), Present-day distribution of impala (Aepyceros melampus).7 Dark shaded areas show the confirmed range of this species, and lighter shaded areas the regions the University Museum of Zoology (Cambridge), the University where impala have been reintroduced. The earliest appearances of the genus are at of the Witwatersrand (Johannesburg) and the Northern Flagship the fossil localities of Lothagam (northern Kenya), where deposits date to 4 MYA– 7 Institute (Pretoria). MYA7 and the contemporaneous (Ethiopia) and Makapansgat (South Africa), which date to ca. 3 MYA8,9 Traits examined Modern geographic variation in impala Three horn metrics were measured on 164 male impala Most authors recognise between three and four subspecies of skulls: total horn length, greatest width between horns and impala, (1) the long-horned impala from (Aepyceros width at tips; these metrics were recorded on unbroken horn melampus suara), (2) the common impala from the southern sheaths, using a standard tape measure, to the nearest 3 mm African regions (A. m. melampus), (3) the subspecies A. m. (Figure 2). Means for all groups were tested using the Mann- Whitney U test.17 The presence or absence of (1) simple folds and

Article #325 johnstoni, from a geographically intermediate region – Zamibia, Malawi and Zimbabwe and (4) the rare black-faced subspecies (2) entostyles on teeth of both sexes also was recorded (Figure (A. m. petersi).13 The latter subspecies is known only from a small 3). Where both traits occurred, these were scored once only as region of northern Namibia and south-western Angola (Figure ‘entostyles present’. Sexes from each regional subsample were first compared separately, and then as pooled data. TheChi- 1) and is clearly distinguishable from other impala subspecies 2 South African Journal of Science square (x ) goodness-of-fit test was used to compare data to a based on both pelage and mitochondrial DNA differences.14 null distribution of equal numbers of simple folds and entostyles Within the continuous distribution of common impala from across impala groups.18 eastern and southern African regions, the two subspecies (A. m. suara and A. m. melampus) are recognised primarily by horn size differences.7 Male eastern African impala horns are reported to RESULTS be significantly longer than southern African conspecifics.15,16 The large horn dimensions of the eastern African impala, Apart from horn size differences, detailed cranial and dental A. m. suara, are considered by several authors to be a diagnostic studies could find almost no size differences between southern feature of this subspecies.12 However, while the eastern African African and eastern African impala samples.2 subspecies does possess the largest means of all three horn metrics examined (Table 2), the total length of the horns of Several previous studies have examined geographic variation east African impala (Group 3) were not significantly different of impala, using both craniodental and horn morphology,2,15,16 to that of the South African (Group 1) impala (p > 0.05, Mann- other studies have examined how pelage differences relate Whitney U test; Tables 2 and 3, Figure 4). The only metric which to previously recognised subspecies. Horn dimensions differ differed significantly between these two groups was the greatest between impala populations (subspecies) and this ‘diversity width between horns, with the eastern African impala having a may have genetic, evolutionary, ecological or biogeographic significantly greater mean width between horns (p < 0.05, Mann- importance’16. Whitney U test; Tables 2 and 3, Figure 5). The overlap between Group 1 (A. m. melampus) and Group 3 (A. m. suara) therefore Lorenzen and colleagues used their study of genetic diversity renders this trait less useful as an indicator of a particular and spatial population structuring to deduce that southern Africa subspecies in every case (Figures 4 and 5). However, for the

TABLE 1 The number of specimens of four separate Aepyceros melampus subspecies examined in this study Subspecies Countries of origin Female Male Group 1 (A. m. melampus) South Africa 68 94 Group 2 (A. m. johnstoni) Zambia, Zimbabwe, Malawi 26 24 Group 3 (A. m. suara/ rendilis) Uganda, Kenya, Tanzania 7 24 Group 4 (A. m. petersi) Angola, Namibia, Botswana 11 22 Total 112 164

S Afr J Sci Vol. 106 No. 11/12 Page 2 of 7 http://www.sajs.co.za Article #325 South African Journal of Science ) was c b Research Article A. m. johnstoni S Afr J Sci (Group 4), had the second highest second the had 4), (Group and has been identified for several 2,15 DISCUSSION A. m. petersi m. A. ) from all other subspecies (Figures 4 and 5). The b) Simple folds on upper M3 of Simple folds b) melampus) impala (Aepyceros A. m. johnstoni ( horn metrics of Group 4, the similar black-faced to impala, the were most southern (Table 3). being significantly different metrics not African impala, with all three horn For the dental traits, teeth of both sexes had simple entostyles, folds although and these traits were not between equally males distributed and females (Table 4). Increased may clarify whether or sample not there are differences sizes between sexes, commonly most were Entostyles differ. may traits such why and observed in the eastern African impala, occurring in 51.6% specimens. In of contrast, only 16.7% of the South African impala differentiated genetically The 4). (Table entostyles had 1) (Group impala, black-faced occurrence of entostyles at 36.4% (12 of a total of 33 specimens). The occurrence of entostyles in Group 2 ( intermediate between the southern and eastern African groups, most were folds Simple 4). (Table specimens of 26% in occurring common in Groups 2 and 3 (occurring in 16.0% specimens, and respectively; 12.9% Table of 4). They occurred least often in the South African group, with only 5 of 130 specimens having simple folds. (3.1%) Variation in both metric and non-metric traits is often observed in geographically widespread species. One of the most common forms of geographic variation is body size clines, with larger- bodied conspecifics found in South Africa and smaller-bodied individuals in eastern Africa. This pattern has been interpreted as a selection for larger body sizes in more African seasonal southern environments c) Entostyles present on upper present c) Entostyles melampus) M2 of impala (Aepyceros a FIGURE 3 of entostyles of the second upper molar Vol. 106 No. 11/12 Page 3 of 7 106 No. 11/12 Vol. 16 FIGURE 2 modern impala subspecies variation a) Absence of simple folds on upper of simple folds a) Absence melampus) M2 and M3 of impala (Aepyceros at tips – are comparable with measurements used in other studies of measurements – greatest width between horns, total length and width Three measurements taken on undamaged male impala horns. These Upper molar dental traits recorded in all adult impala individuals: a complete a) absence of simple folds or entostyles, the b) presence of simple enamel folds and the c) presence horn metrics were found to be significantly smaller than in their than smaller to be significantly were found horn metrics counterparts from either eastern or southern Africa and (Tables 3) – a 2 pattern which strongly differentiates this subspecies Group 2 impala (from Malawi, Zimbabwe and Zambia), all http://www.sajs.co.za Pleistocene impala extirpation Research Article Reynolds

FIGURE 4 Means and 95% confidence intervals of total horn length of the different subspecies of impala. Impala from Malawi (A. m. johnstoni) have the shortest horns, while the eastern African subspecies (A. m. suara) have the longest horns Article #325 South African Journal of Science

FIGURE 5 Means and 95% confidence intervals of the greatest width between the horns of the different subspecies of impala. Impala from Malawi (A. m. johnstoni) have the narrowest width between horns, and the eastern African subspecies (A. m. suara) have the largest, followed closely by those of the genetically differentiated black-faced impala from Namibia (A. m. petersi)

S Afr J Sci Vol. 106 No. 11/12 Page 4 of 7 http://www.sajs.co.za Article #325 South African Journal of Science is a separate, Research Article Absent Most derived population Aepyceros melampus suara Tanzania; (Kenya; Uganda ) 41.7 (10) 14.3% (1) 54.5% (6)* 37.5% (9)* 35.5% (11) 50.0% (11)* 51.5% (17)* 88.2% (60)* 74.5% (70)* 76.9% (20)* 58.0% (29)* Zambia, Zimbabwe; Malawi A. m. johnstoni 80.2% (130)* A. m. petersi S Afr J Sci ounder population: F Aepyceros m. melampus Aepyceros southern Africa A. m. petersi FIGURE 6 Genetically differentiated Black-Faced impala (Namibia; Botswana and Angola) population of the black-faced impala 12.1 (4) 8.3% (2) 4.5% (1) 2.9% (2) 3.2% (3) 3.1% (5) 7.7% (2) 28.6% (2) 12.9% (4) 27.3% (3) 25.0% (6) 16.0% (8) Upper molar traits Simple folds present Schematic representation of how the is recolonisation hypothesised from the founder to populationnorthwest have and progressed: to the derived two northeast. population of separate Theeastern Africa, northeast with groups largeofentostyles. branchhorn This model sizes dispersedcannot presentlyand culminates a explain highwhy the impala prevalencefrom inMalawi to and the Zimbabwe the most and Zambia differ equally populations. from eastern The African and dispersal southern African to the northwest leads to the genetically isolated Overall horn (Group 1) and the eastern African impala (Group 3) metrics are similar, for while the dental the non-metric traits strongly southern differentiate groups these (Table 4). The African entostyle evidence is founder impala consistent population with a scenario, and entostyles of where numbers high possessed region African impala eastern recolonising the black-faced the of metrics horn three All dimensions. horn larger impala do not differ significantlyimpala from(Tables 2 and 3). However, the black-faced impala show the southern African found are than entostyles and folds simple of incidence higher a in southern African impala (Table 4). This findingconfirm appears studies to which suggest that TABLE 4 TABLE = 15) n (cm) 40.73 41.39 34.71 0.103 0.520 0.669 0.100 0.797 0.666 43.16* 0.001* 0.003* 0.050* Total length Total a) and plains Group 4 ( Vol. 106 No. 11/12 Page 5 of 7 106 No. 11/12 Vol. 8.8% (6) 15.4% (4) 18.2% (2) 37.5% (9)* 57.1% (4)* 22.3% (21) 16.7% (27) 26.0% (13) 45.5% (10) 36.4% (12) 50.0% (12)* 51.6% (16)* Incidence of upper molar traits, by sex and by geographical group Entostyles present -values - - - p 0.118 0.436 0.006* 0.000* 0.012* 0.000* Group 3 (cm) 21.63 24.34 17.00 25.52* Crocuta crocut Width at tips Width - - -

0.006* 0.000* 0.000* 0.000* 0.000* 0.003* Group 2 TABLE 2 TABLE TABLE 3 TABLE subspecies ). While the geographic variation in (cm) 33.63 34.19 27.86 37.12* = 21) = 24) = 69) = 24) = 25) = 69) = 21) = 25) = 67) n n n n n n n n n Greatest width between horns Group 1 ( Group 2 ( Group 3 ( Group 2 ( Group 3 ( Group 1 ( Group 2 ( Group 3 ( Group 1 ( = 33) = 50) = 31) = 162) = 15) n n n n = 23) n = 11) = 26) = 7) = 68) n Equus burchelli Comparison of the mean horn metrics for four impala subspecies Comparison of the mean horn n n n n = 21) = 22) = 24) = 24) = 94) n ifferences between regions, (2) slight horn size differences n n n n Results of the pairwise comparisons of three horn metrics between four impala Results of the pairwise comparisons of three horn metrics between = 69) n 4: Angola, Namibia, 4: Botswana ( *, Highest values. Malawi ( 3: Uganda, Kenya, ( Tanzania 1: South Africa 1: South ( 2: Zambia, Zimbabwe, Group Males ( Combined ( percentages. Note: Percentages of group total and numbers of specimens are given. Numbers of specimens are indicated in brackets after the *, Highest totals. Combined ( Group 4 Females ( Combined ( Group 3 Females ( Males ( Group 2 Females ( Males ( Group 1 Females ( Males ( Combined ( Group *, Significant values. Total length Total (cm) Width at tips (cm) Greatest width between horns (cm) plains zebra. The local extirpation and recolonisation pattern has pattern recolonisation and extirpation local The zebra. plains for account may and species two other the for observed been not visible in impala. the complex regional patterns (except for the greatest width between horns) and (3) a greater number of entostyles in the eastern African impala. These data do suggest that the populations of impala may have undergone and hyaena spotted the to relative histories population different zebra ( these species body is of lack a (1) easier of consisting pattern to a show impala differences, understand in terms of seasonality size d species including spotted hyaena ( http://www.sajs.co.za Pleistocene impala extirpation Research Article Reynolds

distinct subspecies, most closely related to the common, variation can be interpreted. In the light of the genetic evidence,4,5 southern African impala populations.14 differentiation of horn and upper molar traits can be interpreted as a result of the dispersal of impala populations from a founder Possible dispersal routes of impala population elsewhere. The high occurrence of entostyles in the eastern African region suggests that the founder population High genetic diversity indicates that impala dispersed that recolonised this region possessed high frequencies of this northwards from a founder population in southern Africa.4 Low trait. While entostyle loss is considered a derived trait20 here dispersal distances presumably aided the genetic differentiation, the entostyles in the eastern African impala do not represent especially in the black-faced impala.19 With no significant a ‘primitive’ population; instead the trait appears to dominate differences in horn metrics between Group 1 and Group 4 impala this subspecies because it was more prevalent in the population and only slight differences in upper molar traits, it is plausible which dispersed to eastern Africa. that the founder population gave rise directly to the black-faced form (Figure 6). However, other authors have proposed that the In the fossil record, such processes of local extinction and subspecies is a relict population of an earlier westward range repopulation of areas over shorter timescales (in this case, expansion, before populations became geographically separated approximately 200 KYA) may be virtually palaeontologically as a result of Pleistocene climate shifts.4 invisible, apart from cases similar to this one, where population genetics is able to contribute an alternative perspective on The A. m. johnstoni subspecies from Malawi, Zambia and species changes in the past. Instead, fossil lineages may appear Zimbabwe is a unique group: it has similar dental traits to the continuous, but with possible variability in certain traits, black-faced impala subspecies, but also has the smallest horn which could be interpreted as speciation, or ecophenotypic metrics of any impala subspecies examined here. The sample, adaptation to different environments. This pattern may be albeit small, shows possible in dental even more relevant for extinct species, where palaeontological traits (Table 4). Whether these traits were characteristic of the and palaeoclimatic evidence alone must be used to develop impala which colonised this region, or whether they represent scenarios of adaptation, endemism and extinction. Extant ecophenotypic adaptation to the region is unclear, and should be species with similarly complex population histories would also the focus of future studies. The observed high genetic diversity be expected to show some morphological variation as a result of of this subspecies has been ascribed to its intermediate position population changes after the Pleistocene climate shifts. Species in the gene flow between populations.4 such as the wildebeest, eland, topi and hartebeest, may possess somewhat different morphological traits in the fossil record. Implications for understanding the fossil record The combination of genetic population studies and morphology While the presence of entostyles have been inferred to be a studies can potentially provide further clues as to how mammal primitive trait,20 within the context of this study, it appears that dispersals within Africa shaped the morphological variation characteristics such as horn size and entostyles, may increase observed in the fossil record. and decrease through time in response to the extirpation and recolonisations of certain subspecies. As a result, even though the ACKNOWLEDGEMENTS geographic range is seemingly continuous, the localised effects This research received support from the SYNTHESYS Project of other variables, such as disease outbreaks, inbreeding effects, http://www.synthesys.info/ which is financed by European or climate changes, may act over smaller spatial and temporal Community Research Infrastructure Action under the FP6 scales. In essence, regional populations may function as separate Article #325 ‘Structuring the European Research Area’ Programme. Part evolutionary entities, influencing how morphological variation of this research derived from my doctoral studies at the appears in the fossil record. Liverpool John Moores University, Liverpool, United Kingdom. Meredith Robinson kindly assisted with the data collection in The recent population history of impala clearly illustrates how

South African Journal of Science Johannesburg. I thank the curators of the Northern Flagship regional populations (or subspecies) do not share the same Institute (formerly the Transvaal Museum, Pretoria), the Bernard evolutionary trajectory, with one population surviving for Price Institute, University of the Witwatersrand (Johannesburg), ca. 200 KYA in the south, while another failed to survive in the Natural History Museum, (London), the National Museums eastern Africa. Given that the Aepyceros lineage stretches back to of Kenya (Nairobi), Powell-Cotton Museum (Kent) and the between 4 MYA and 7 MYA, at Lothagam, in northern Kenya,7 University Museum of Zoology (Cambridge) and the museums it is reasonable to assume that this scenario of population for access to their comparative collections. extirpations and recolonisations would have been repeated in different places within the temporal and geographical range of this genus. For instance, it is possible, given the dearth of REFERENCES impala material in southern Africa, that the Makapansgat 1. Bobe R, Eck GG. Responses of African bovids to Pliocene impala population may have been extirpated and later replaced climatic change. Paleobiology Memoirs 2001;27(2):1–48. by a different population of southward-dispersing impala, 2. Reynolds SC. Mammalian body size changes and Plio- possibly originating in eastern Africa. In order to test this type of Pleistocene environmental shifts: Implications for scenario, it may be more informative to consider entostyles and understanding hominin evolution in eastern and southern other distinguishing morphological characteristics as population Africa. J Hum Evol. 2007;53(5):528–548. markers, rather than traits which are uniformly ‘primitive’ or 3. Arctander P, Johansen C, Coutellec-Vreto M-A. ‘derived’. The presence and absence of such traits could indicate Phylogeography of three closely related African bovids possible genetic relationships between regional groups through (Tribe Alcelaphini). Mol Biol Evol. 1999;16:1724–1739. time, even beyond the timescales for which genetic population 4. Lorenzen ED, Arctander P, Siegismund HR. Regional genetic studies can provide information. structuring and evolutionary history of the impala Aepyceros melampus. J Hered. 2006;97(2):119–132. CONCLUSIONS 5. Lorenzen ED, Masembe C, Arctander P, Siegismund HR. A Two traits which differ among the four impala populations have long-standing Pleistocene refugium in southern Africa and a been identified. The A.m. suara subspecies from eastern Africa mosaic of refugia in East Africa: Insights from mtDNA and have the largest horn metrics measured in this study, as well the antelope. J Biogeogr. 2010;37:571–581. as the highest occurrence of entostyles (51.6%). While it is not 6. Vrba ES. Evolutionary pattern and process in the sister- possible to know exactly why the distribution of entostyles group Alcelaphini-Aepycerotini (Mammalia: ). In: and specific horn sizes differ among regional populations, the Eldredge N, Stanley SM, editors. Living fossils. New York: recent genetic studies provide a framework within which such Springer, 1984; p. 62–79.

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