RESEARCH LETTER

 

Morphological variation in the distal phalanges of the springbok, Antidorcas marsupialis (Zimmermann, 1780) (Mammalia: Bovidae)

 

 

Lloyd Rossouw^{I, II}

^IFlorisbad Quaternary Research Station, National Museum Bloemfontein, Bloemfontein, South Africa
^IIDepartment of Plant Sciences, University of the Free State, Bloemfontein, South Africa

Correspondence

 

 

---

ABSTRACT

A comparative study of distal phalanges belonging to adult springbok individuals shows distinctive morphological differences between the subspecies Antidorcas marsupialis marsupialis and Antidorcas marsupialis hofmeyri, most notably reflected by significant lengthening of the sole of the latter. Results were derived from comparative osteomorphological techniques, using standard anatomical nomenclature for descriptions and parametric statistics for measurements and dimensions. The configuration in A. m. hofmeyri proved useful for distinguishing between the two subspecies. The findings suggest that the osteometrical differences observed in the distal phalanges relate to different habitats occupied by the species.
SIGNIFICANCE:
Infraspecific morphological variation exhibited by distal phalanges in Antidorcas marsupialis is significant. The results lay the groundwork for further testing of relationships between functional morphology of foot bones and substrate in bovids.

Keywords: southern Africa, Antidorcas marsupialis, osteometry, osteomorphology

---

 

 

Introduction

This study was undertaken after cursory examination revealed observable morphological variation in the distal phalanges of the springbok, Antidorcas marsupialis. Springbok are plains-dwelling animals, adapted to arid regions as well as open grasslands; they are confined largely to the inland plateau and the western coastal regions of southern Africa (Figure 1).^1-4 The species is the sole living representative of the genus Antidorcas and the only antelopine species found in southern Africa today. It belongs to a group of antelopes that evolved from a single cluster of gazelline-like antelopes that eventually separated from the genus Gazella during the Pliocene in East Africa to evolve further in isolation in southern Africa.^5-8 A long history of intensive farming activities in South Africa severely depleted wildlife, but following the proclamation of several national parks since the beginning of the last century, ungulates were re-introduced to these reserves in an attempt to approximate the original wildlife composition in those areas.^9,10 Today, the existing populations of springbok in the Republic of South Africa are commercially managed through introduction into nature reserves and game farms resulting in the reoccupation of most of the springbok's historical range.^11,12 Although the species is continuously distributed, differences in body size, size of the postcranial skeleton and other external characteristics have been shown to vary according to locality.¹³¹⁵ Body mass was highly correlated with winter dietary protein, demonstrating significant differences between the nominate subspecies Antidorcas marsupialis marsupialis and two larger northern groups.¹⁶ Three subspecies are recognised: the nominate subspecies Antidorcas marsupialis marsupialis or southern springbok, found in the central interior of South Africa and south of the lower Orange River; Antidorcas marsupialis angolensis, a northwestern springbok group confined mainly to the western parts of southern Angola, the Kaokoveld and the northern Namib Desert; and the Kalahari springbok, Antidorcas marsupialis hofmeyri (Thomas, 1926), which is generally restricted to southern Namibia, southern and western Botswana, and the adjacent parts of the Northern Cape in South Africa north of the Orange River.^17,18 Current opinion on the taxonomic status of A. marsupialis varies, with recent classifications also raising all three subspecies to species rank.¹⁹ The general taxonomy in this study followed Groves' description of the springbok as a polytypic species, based on the geographical division of three subspecies.²⁰

 

Materials and methods

Testing variance in size and morphology in the distal phalanx of the subspecies A. m. marsupialis and A. m. hofmeyri was based on a sample comprising 19 marsupialis and 16 hofmeyri individuals from both sexes. Measurements for the 19 marsupialis individuals were obtained from a comparative osteomorphological study of Antidorcas marsupialis and grey rhebok, Pelea capreolus (Forster, 1790) published in 1992.¹⁵ The 16 hofmeyri individuals come from a single springbok population, culled several years ago in the Kalahari Gemsbok National Park and curated at the Florisbad Quaternary Research Station near Bloemfontein in the Free State Province, South Africa (Supplementary table 1). The hofmeyri specimens were measured with a slide caliper to the nearest 0.1 mm. Osteomorphological descriptions follow the nomenclature proposed by the International Committee on Veterinary Gross Anatomical Nomenclature.²¹ Dimensions included greatest diagonal length of the sole (DLS), greatest height in the region of the extensor process (processus extensorius, HP), length of the dorsal surface (Ld) and greatest breadth of the proximal articular surface (facies articularis proximalis, BFp)²² (Figure 2, Supplementary table 2). The dimensions were statistically tested for subspecies and sex-based effects using the parametric Student's t-test (Table 1).

 

Results

The shape of the distal phalanx in the springbok is typical of the family Bovidae, resembling a triangular-shaped and mediolaterally flattened body (Figure 2). Both subspecies exhibited a prominent extensor process (Figure 2, Attr. 1), situated proximally on the dorsal surface of the body and above a posteriorly extended proximal articular surface that ends in a well-developed flexor tubercle (Figure 2, Attr. 2). The phalanges also revealed a characteristically large nutrient foramen on the axial side at the base of the extensor process (Figure 2, Attr. 3). Compared to A. m. marsupialis, the distal phalanx in A. m. hofmeyri appeared to be distally elongated, resulting in a comparably more peg-like appearance (Figure 2, Attr. 4). This morphological trait remained noteworthy in overall proportion when compared metrically to that of A. m. maisupialis. As expected, variables DLS, Ld, HP and BFp were larger in A. m. hofmeyri for both sexes with A. m. marsupialis and A. m. hofmeyri falling into two discrete and non-overlapping clusters (Figure 3). This also applied to the log-transformed values. Even though sex-based variability varied from significant in A. m. hofmeyri to not in A. m. marsupialis, it had no effect on subspecies, which were significant in all the dimensions (n=66, p <0.05). Disproportionate lengthening along the anteroposterior axis of the corpus was indicated by significantly higher DLS:HP (p<0.001) and DLS:BFp (p=0.009) ratios for A.m. hofmeyri (Supplementary figure 1). The results were supported by simple trigonometrical analysis, based on mean values of dimensions DLS, Ld and HP, showing comparatively lower dorsal to palmar (manus) and dorsal to plantar (pedis) angles in A. m. hofmeyri (Figure 4, Supplementary table 3).

 

Concluding remarks

Findings from this study indicate that configuration of the distal phalanx in A.m. hofmeyri is at variance with that of the nominate subspecies and that the measurements, and the indices based on them, proved useful for distinguishing between the distal phalanges of the two subspecies. In this case, a significant increase in the DLS:HP ratio as merely a function of increased body size in A.m. hofmeyri is not consistent with the assumption that two subspecies of different size, but sharing a common morphology, will also have the dimensions of their distal phalanges in the same ratio.

Selection pressures that operate within a specific environment have been linked to the functional expression of postcranial characteristics in modern African bovids, e.g. broad-level correlations between bovid postcrania and open or closed habitats.^23-26 While Antidorcas marsupialis is a plains-living species, the osteometrical differences observed in the distal phalanges relate to the different habitats that the species occupy. Being generally restricted to southern Namibia, southern and western Botswana and the adjacent parts of the Northern Cape Province north of the Orange River, the natural habitat of A. m. hofmeyri is underlain by a distinctive and homogeneous substrate, made up of thick surface sands of different ages that were established during periods of widespread aridity.²⁷ Deposits vary from extensive areas covered by linear dunes to gently undulating sand sheets that extend from southern Angola and western Zambia in the north to the Orange River in the south.^28-30 With the lower Orange River acting as a natural barrier, it is readily perceived that the distinct morphology exhibited by A. m. hofmeyri's distal phalanges, could have resulted from an allopatric isolation event sometime in the past, following continual locomotion on unconsolidated dune and sand sheet accumulations in the region. Further investigations, using a larger, more inclusive data set, will be required to test these assumptions, but for now, it is postulated that in addition to dietary effects¹⁶, abiotic conditions like substrate may also be a driver of morphological alteration within the polytypic springbok.

 

Acknowledgements

The National Museum in Bloemfontein provided financial support for the project (ethical clearance number NMB ECC 2019/5). The author is indebted to Estie Rossouw (independent graphic designer) for creating the drawings.

Competing interests

I have no competing interests to declare.

 

References

1. Nagy KA, Knight MH. Energy, water, and food use by springbok antelope (Antidorcas marsupialis) in the Kalahari Desert. J Mammal. 1994;75(4):860-872. https://doi.org/10.2307/1382468        [ Links ]

2. Smithers RHN. The mammals of the southern African subregion. Pretoria: University of Pretoria; 1983.         [ Links ]

3. Skinner JD, Louw GN. The springbok: Antidorcas marsupialis (Zimmermann, 1780;. Transv Mus Monogr. 1996:10.         [ Links ]

4. Bigalke RC. Observations on the behaviour and feeding habits of the springbok, Antidorcas marsupialis. Zool Afr. 1972;7(1):333-359. https://doi.org/10.1080/00445096.1972.11447448        [ Links ]

5. Vrba ES. Two species of Antidorcas Sundevall at Swartkrans (Mammalia: Bovidae). Ann Transv Mus. 1973;28:287-351.         [ Links ]

6. Hendey QB, Hendey H. New Quaternary fossil sites near Swartklip, Cape Province. Ann S Afr Mus. 1968;52:43-78.         [ Links ]

7. Gentry AW. Bovidae. In: Werdelin L, Sanders WJ, editors. Cenozoic mammals of Africa. Berkeley, CA: University of California Press; 2010. p. 741-796. https://doi.org/10.1525/california/9780520257214.003.0038        [ Links ]

8. De Ruiter DJ. Revised faunal lists for Members 1-3 of Swartkrans, South Africa. Ann Transv Mus. 2003;40:29-41.         [ Links ]

9. Penzhorn BL. A summary of the re-introduction of ungulates into South African national parks (to 31 December 1970). Koedoe. 1971;14:145-159. https://doi.org/10.4102/koedoe.v14i1.725        [ Links ]

10. De Graaf G, Penzhorn BL. The re-introduction of springbok Antidorcas marsupialis into South African National Parks - a documentation. Koedoe, 1976;19:75-82. https://doi.org/10.4102/koedoe.v19i1.1184        [ Links ]

11. Bigalke RC. Springbok management - what's new? Pelea. 1994;13:23-29.         [ Links ]

12. Du Plessis S. The past and present geographical distribution of the Perissodactyla and Artiodactyla in southern Africa [MSc thesis]. Pretoria: University of Pretoria; 1969.         [ Links ]

13. Robinson TJ. A comparative study of the three subspecies of springbok, Antidorcas marsupialis marsupialis (Zimmermann, 1780), A. m. hofmeyri (Thomas, 1926) and A. m. angolensis (Blaine, 1922) [MSc thesis]. Pretoria: University of Pretoria; 1975.         [ Links ]

14. Bigalke RC, Hartl BG, Berry MPS, Van Hensbergen HJ. Population genetics of the springbok Antidorcas marsupialis - a preliminary study. Acta Theriol 1993;38(2):103-111. https://doi.org/10.4098/AT.arch.93-45        [ Links ]

15. Peters J, Brink JS. Comparative postcranial osteomorphology and osteometry of springbok, Antidorcas marsupialis (Zimmerman, 1780) and grey rhebok, Pelea capreolus (Forster, 1790) (Mammalia: Bovidae). Nav Nas Mus Bfn.1992;8(4):161-207.         [ Links ]

16. Robinson TJ. Influence of a nutritional parameter on the size differences of the three springbok subspecies. S Afr J Zool. 1979;14:13-15.         [ Links ]

17. Bigalke RC. The springbok. Nat Hist NY 1966;75(6):20-25.         [ Links ]

18. Meester JAJ, Rautenbach IL, Dippenaar NJ, Baker CM. Classification of southern African mammals. Transv Mus Monogr 5. Pretoria: Transvaal Museum; 1986.         [ Links ]

19. Groves C, Grubb P Ungulate taxonomy. Baltimore, MD: The Johns Hopkins University Press; 2011.         [ Links ]

20. Groves CP Subspecies and clines in the springbok (Antidorcas). Z Saugetierkd. 1981;46:189-197.         [ Links ]

21. International Committee on Veterinary Gross Anatomical Nomenclature, World Association of Veterinary Anatomists. Nomina Anatomica Veterinaria. 6th ed. Hannover/Ghent/Columbia, MO/Rio de Janeiro: Editorial Committee; 2017.         [ Links ]

22. Von den Driesch A. A guide to the measurement of animal bones from archaeological sites. Peabody Museum Bulletin 1. Cambridge, MA: Peabody Museum of Archaeology and Ethnology, Harvard University; 1976.         [ Links ]

23. Kappelman J, Plummer IT, Bishop L, Duncan A, Appleton S. Bovids as indicators of Plio-Pleistocene paleoenvironments in east Africa. J Hum Evol. 1997;32:229-256. https://doi.org/10.1006/jhev.1996.0105        [ Links ]

24. DeGusta D, Vrba E. Methods for inferring paleohabitats from the functional morphology of bovid phalanges. J Archeol Sci. 2005;32:1099-1113. https://doi.org/10.1016/j.jas.2005.02.010        [ Links ]

25. DeGusta D, Vrba E. Methods for inferring paleohabitats from discrete traits of the bovid postcranial skeleton. J Archeol Sci. 2005;32:1115-1123. https://doi.org/10.1016/j.jas.2005.02.011        [ Links ]

26. Schellhorn R, Pfretzschner H. Analyzing ungulate long bones as a tool for habitat reconstruction. Mammal Res. 2015;60:195-205. https://doi.org/10.1007/s13364-015-0218-0        [ Links ]

27. Grove AT. Landforms and climatic change in the Kalahari and Ngamiland. Geogr J. 1969;134:194-208. https://doi.org/10.2307/1792436        [ Links ]

28. Lancaster N. Palaeoenvironmental implications of fixed dune systems in southern Africa. Palaeogeogr Palaeoclimatol Palaeoecol. 1981;33:327-346. https://doi.org/10.1016/0031-0182(81)90025-0        [ Links ]

29. Bullard JE, Thomas DSG, Wiggs GFS, Livingstone I. Analysis of linear sand dune morphological variability, southwestern Kalahari Desert. Geomorphology. 1995;11:189-203. https://doi.org/10.1016/0169-555X(94)00061-U        [ Links ]

30. Thomas DSG, Knight M, Wiggs GFS. Remobilization of southern African desert dune systems by twenty-first century global warming. Nature. 2005;435(7046):1218. https://doi.org/10.1038/nature03717        [ Links ]

 

 

Correspondence:
Lloyd Rossouw
Email: lloyd@nasmus.co.za

Received: 21 Mar. 2021
Revised: 02 Nov. 2021
Accepted: 15 Dec. 2021
Published: 29 Mar. 2022

 

 

EDITORS: Bettine van Vuuren Sydney Moyo
FUNDING: National Museum Bloemfontein

 

 

Supplementary Data

The supplementary data is available in pdf: [Supplementary data]