Drone-based effective counting and ageing of hippopotamus (Hippopotamus amphibius) in the Okavango Delta in Botswana

Autoři: Victoria L. Inman aff001;  Richard T. Kingsford aff001;  Michael J. Chase aff002;  Keith E. A. Leggett aff001
Působiště autorů: Centre for Ecosystem Science, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, NSW, Australia aff001;  Elephants Without Borders, Kasane, Botswana aff002;  Fowlers Gap Arid Zone Research Station, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, NSW, Australia aff003
Vyšlo v časopise: PLoS ONE 14(12)
Kategorie: Research Article
doi: https://doi.org/10.1371/journal.pone.0219652


Accurately estimating hippopotamus (Hippopotamus amphibius) numbers is difficult due to their aggressive nature, amphibious lifestyle, and habit of diving and surfacing. Traditionally, hippos are counted using aerial surveys and land/boat surveys. We compared estimates of numbers of hippos in a lagoon in the Okavango Delta, counted from land to counts from video taken from a DJI Phantom 4TM drone, testing for effectiveness at three heights (40 m, 80 m, and 120 m) and four times of day (early morning, late morning, early afternoon, and late afternoon). In addition, we determined effectiveness for differentiating age classes (juvenile, subadult, and adult), based on visual assessment and measurements from drone images, at different times and heights. Estimates in the pool averaged 9.18 (± 0.25SE, range 1–14, n = 112 counts). Drone counts at 40 m produced the highest counts of hippos, 10.6% higher than land counts and drone counts at 80 m, and 17.6% higher than drone counts at 120 m. Fewer hippos were counted in the early morning, when the hippos were active and most likely submerged, compared to all other times of day, when they tended to rest in shallow water with their bodies exposed. We were able to assign age classes to similar numbers of hippos from land counts and counts at 40 m, although land counts were better at identifying juveniles and subadults. Early morning was the least effective time to age hippos given their active behaviour, increasingly problematic with increasing height. Use of a relatively low-cost drone provided a rigorous and repeatable method for estimating numbers and ages of hippos, other than in the early morning, compared to land counts, considered the most accurate method of counting hippos.

Klíčová slova:

Aircraft – Botswana – Cameras – Covariance – Lagoons – Surveys – Wildlife – Shallow water


1. Lewison RL, Pluháček J. Hippopotamus amphibius. The IUCN Red List of Threatened Species. 2017. Available: http://dx.doi.org/10.2305/IUCN.UK.2017-2.RLTS.T10103A18567364.en

2. Eksteen JJ. The determination of acceptable hippo densities in the Crocodile River, outside Kruger National Park. MSc Thesis, University of the Witwatersrand. 1993.

3. Karstad EL, Hudson RJ. Census of the Mara River hippopotamus (Hippopotamus amphibius), southwest Kenya, 1980–1982. Afr J Ecol. 1984;22(2):143–7.

4. Kujirakwinja D. The status and conservation of common hippopotamuses in Virunga National Park, Democratic Republic of Congo. M.Sc. Thesis, University of Cape Town, 2010.

5. Smart AC. The density of Hippopotamus amphibius, Linneaus at Lake Naivasha Kenya. Trop Freshw Biol. 1990;2(2):241–7.

6. Stoner C, Caro T, Mduma S, Mlingwa C, Sabuni G, Borner M, et al. Changes in large herbivore populations across large areas of Tanzania. Afr J Ecol. 2007;45(2):202–15.

7. Alonso LE, Nordin LA, editors. A rapid biological assessment of the aquatic ecosystems of the Okavango Delta, Botswana: high water survey. Washington: Conservation International; 2003.

8. Eltringham SK. The afrotropical hippopotamuses (Hippopotamus and Hexaprotodon): review of priorities for conservation action and future research on hippopotamuses. In: Oliver WL, editor. Pigs, peccaries and hippos: status survey and conservation action plan. Gland: IUCN; 1993. p. 61–5.

9. Durrheim DN, Leggat PA. Risk to tourists posed by wild mammals in South Africa. J Travel Med. 1999;6(3):172–9. doi: 10.1111/j.1708-8305.1999.tb00856.x 10467154

10. Dunham KM, Ghiurghi A, Cumbi R, Urbano F. Human–wildlife conflict in Mozambique: a national perspective, with emphasis on wildlife attacks on humans. Oryx. 2010;44(2):185–93.

11. Chomba C, Senzota R, Chabwela H, Mwitwa J, Nyirenda V. Patterns of human—wildlife conflicts in Zambia; causes, consequences and management responses. J Ecol Nat Environ. 2012;4(12):303–13.

12. Klingel H. Hippopotamus amphibius Common Hippopotamus. In: Kingdon J, Hoffman M, editors. Mammals of Africa: Volume VI: Pigs, Hippopotamuses, Chevrotain, Giraffes, Deer and Bovids. London: Bloomsbury Publishing; 2013. p. 68–77.

13. Prinsloo AS. Aspects of the spatial and behavioural ecology of Hippopotamus amphibius in the Saint Lucia Estuary, KwaZulu-Natal, South Africa. M.Sc. Thesis, University of Cape Town. 2016.

14. Viljoen PC. Changes in number and distribution of hippopotamus (Hippopotamus amphibius) in the Sabie River, Kruger National Park, during the 1992 drought. Koedoe. 1995;38(2):115–21.

15. Viljoen P, Biggs H. Population trends of hippopotami in the rivers of the Kruger National Park, South Africa. In: Dunstone N, Gorman M, editors. Behaviour and ecology of riparian mammals. Cambridge: Cambridge University Press; 1998. p. 251–79.

16. Zisadza P, Gandiwa E, van der Westhuizen H, van der Westhuizen E, Bodzo V. Abundance, distribution and population trends of hippopotamus in Gonarezhou National Park, Zimbabwe. South African J Wildl Res. 2010;40(2):149–57.

17. Chomba C. Factors affecting the Luangwa (Zambia) hippo population dynamics within its carrying capacity band–insights for better management. Int J Biodivers Conserv. 2013;5(3):109–21.

18. Scholte P, Nguimkeng F, Iyah E. Good news from north-central Africa: largest population of Vulnerable common hippopotamus Hippopotamus amphibius is stable. Oryx. 2016;51(2): 218–221.

19. Bouché P. Méthodes d’inventaire de la grande faune à l’usage des ZCV. Bangui: ECOFAC; 2008.

20. Timbuka CD. The ecology and behaviour of the Common hippopotamus, Hippopotamus amphibius L. in Katavi National Park, Tanzania: responses to varying water resources. PhD Thesis, University of East Anglia. 2012.

21. Michez A. Etude de la population d’hippopotames (Hippopotamus amphibius L.) de la rivière Mouena Mouele au Parc National du Loango-Sud (Gabon). PhD Thesis, University of Liège. 2006.

22. Vermeulen C, Lejeune P, Lisein J, Sawadogo P, Bouché P. Unmanned aerial survey of elephants. PLoS One. 2013;8(2):e54700. doi: 10.1371/journal.pone.0054700 23405088

23. Hodgson A, Kelly N, Peel D. Unmanned aerial vehicles (UAVs) for surveying marine fauna: a dugong case study. PLoS ONE. 2013;8(11):1–15.

24. Linchant J, Lisein J, Semeki J, Lejeune P, Vermeulen C. Are unmanned aircraft systems (UASs) the future of wildlife monitoring? A review of accomplishments and challenges. Mammal Rev. 2015; 45(4):239–52.

25. Jachmann H. Comparison of aerial counts with ground counts for large African herbivores. J Appl Ecol. 2002;39(5):841–52.

26. Olivier RCD, Laurie WA. Habitat utilization by hippopotamus in the Mara River. Afr J Ecol. 1974;12(4):249–71.

27. Delvingt W. Ecologie de l’hippopotame (Hippopotamus amphibius L.) au Parc National des Virunga (Zaïre). PhD Thesis, Faculté des sciences agronomiques de l'état Gembloux. 1978.

28. Tembo A. Population status of the hippopotamus on the Luangwa River, Zambia. Afr J Ecol. 1987;25(2):71–7.

29. Balole-Bwami E, de Merode E, Kujirakwinja D, Tchouamo IR. Conservation de l’hippopotame commun (Hippopotamus amphibius) au Parc National des Virunga: population actuelle et mesures de protection. Parcs & Réserves. 2014;69(3):27.

30. Perry LR. Observations of hippopotamus H. amphibius in the Little Scarcies River of Sierra Leone and arguments for their conservation based on roles they play in riverine grasslands and nutrient loading. Masters Thesis, Michigan Technological University. 2015.

31. Bere R. Queen Elizabeth National Park, Uganda: the hippopotamus problem and experiment. Oryx. 1959;5(3):116–24.

32. Scotcher JSB. Hippopotamus numbers and movements in Ndumu Game Reserve. The Lammergeyer. 1978;24:5–12.

33. Ansell WFH. Hippo census on the Luangwa River. Puku. 1965;32(3):647–55.

34. Junda J, Greene E, Bird DM. Proper flight technique for using a small rotary-winged drone aircraft to safely, quickly, and accurately survey raptor nests. J Unmanned Veh Syst. 2015;3(4):222–36.

35. Schofield G, Katselidis KA, Lilley MKS, Reina RD, Hays GC. Detecting elusive aspects of wildlife ecology using drones: new insights on the mating dynamics and operational sex ratios of sea turtles. Funct Ecol. 2017;31(12):2310–9.

36. Koski WR, Allen T, Ireland D, Buck G, Smith PR, Macrender AM, et al. Evaluation of an unmanned airborne system for monitoring marine mammals. Aquat Mamm. 2009;35(3):347–57.

37. Watts AC, Perry JH, Smith SE, Burgess MA, Wilkinson BE, Szantoi Z, et al. Small unmanned aircraft systems for low-altitude aerial surveys. J Wildl Manage. 2010;74(7):1614–9.

38. Christie KS, Gilbert SL, Brown CL, Hatfield M, Hanson L. Unmanned aircraft systems in wildlife research: current and future applications of a transformative technology. Front Ecol Environ. 2016;14(5):241–51.

39. Lhoest S. Contribution au monitoring de populations d’Hippopotame Commun (Hippopotamus amphibius L.) par l’utilisation de la technologie drone (Parc National de la Garamba, Republique Democratique du Congo). PhD Thesis, University of Liège. 2015.

40. Lhoest S, Linchant J, Quevauvillers S, Vermeulen C, Lejeune P. How many hippos (HOMHIP): algorithm for automatic counts of animals with infra-red thermal imagery from UAV. Int Arch Photogramm Remote Sens Spat Inf Sci. 2015;40(3):355–62.

41. Linchant J, Lhoest S, Quevauvillers S, Lejeune P, Ngabinzeke JS, Belanganayi BL, et al. UAS imagery reveals new survey opportunities for counting hippos. PLoS ONE. 2018;13(11): e0206413. doi: 10.1371/journal.pone.0206413 30427890

42. Karstad EL, Hudson RJ. Social organization and communication of riverine hippopotami in southwestern Kenya. Mammalia. 1986;50(2):153–64.

43. Mekonen S, Hailemariam B. Ecological behaviour of common hippopotamus (Hippopotamus amphibius, Linnaeus, 1758) in Boye Wetland, Jimma, Ethiopia. Am J Sci Ind Res. 2016;7(2):41–9.

44. Karstad EL. The ecology of hippopotami (Hippopotamus ampibius) in southwestern Kenya. MSc Thesis, The University of Alberta. 1984.

45. Ngog Nje J. Contribution à l’étude de la structure de la population des hippopotames (Hippopotamus amphibius L.) au Parc National de la Bénoué (Cameroun). Mammalia. 1988;52(2):149–58.

46. DJI. Phantom 4. 2019. Available: www.dji.com/au/phantom-4.

47. VIZf/x. DJI Phantom 4 lens cCalculations. 2016. Available: http://projects.vizfx.com/dji/DJI_P4_Lens_convert_camera.pdf

48. Barklow WE. Amphibious communication with sound in hippos, Hippopotamus amphibius. Anim Behav. 2004;68(5):1125–32.

49. VLC media player. VLC media player homepage. 2014. Available: http://www.videolan.org/vlc/.

50. Rasband WS. ImageJ. 2009–2013. Bethesda: U. S. National Institutes of Health; Available: http://rsb.info.nih.gov/ij/

51. Martin RB. Transboundary species project, background study, Hippopotamus. Windhoek: Namibia Nature Foundation; 2005.

52. Laws RM, Clough G. Observations on reproduction in the hippopotamus Hippopotamus amphibius Linn. Comp Biol Reprod Mamm. 1966;15:117–40.

53. Dittrich L. Age of sexual maturity in the hippopotamus. Int Zoo Yearb. 1976;16(1):171.

54. Graham LH, Reid K, Webster T, Richards M, Joseph S. Endocrine patterns associated with reproduction in the Nile hippopotamus (Hippopotamus amphibius) as assessed by fecal progestagen analysis. Gen Comp Endocrinol. 2002;128(1):74–81. doi: 10.1016/s0016-6480(02)00066-7 12270790

55. Pienaar U de V., Van Wyk P, Fairall N. An experimental cropping scheme of hippopotami in the Letaba River of the Kruger National Park. Koedoe. 1966;9:1–33.

56. Laws RM. Dentition and ageing of the Hippopotamus. Afr J Ecol. 1968;6(1):19–52.

57. Skinner JD, Scorer JA, Millar RP. Observations on the reproductive physiological status of mature herd bulls, bachelor bulls, and young bulls in the hippopotamus Hippopotamus amphibius amphibius Linnaeus. Gen Comp Endocrinol. 1975;26(1):92. doi: 10.1016/0016-6480(75)90218-x 1132668

58. Brooks ME, Kristensen K, van Benthem KJ, Magnusson A, Berg CW, Nielsen A, et al. glmmTMB balances speed and flexibility among packages for zero-inflated generalized linear mixed modeling. R J. 2017;9(2):378–400.

59. Lenth R. emmeans: Estimated Marginal Means, aka Least-Squares Means. R package version 1.3.3. 2019. Available: https://CRAN.R-project.org/package=emmeans.

60. Eltringham SK. The hippos: natural history and conservation. London: Academic Press; 1999.

61. R Core Team. R: A language and environment for statistical computing. Vienna: R Foundation for Statistical Computing; 2018. Available: https://www.R-project.org/.

62. Attwell RIG. Surveying Luangwa hippo. Puku. 1963;1:153–65.

63. Onyeanusi AE. Some behavioural characteristics of common hippopotamus (H. amphibius Linn. 1758) in Nigeria’s Kainji Lake National Park. Int J Agric Rural Dev. 2004;5(1):27–35.

64. Callaghan CT, Brandis KJ, Lyons MB, Ryall S, Kingsford RT. A comment on the limitations of UAVs in wildlife research—the example of colonial nesting waterbirds. J Avian Biol. 2018;49(9):e01825.

65. Norton-Griffiths M. Counting animals. Nairobi: African Wildlife Leadership Foundation; 1978.

66. Bhima R. Census of hippopotamus (Hippopotamus amphibius (L)) in the Upper Shire River, Malawi. Afr J Ecol. 1996;34:83–5.

67. Chase M, Schlossberg S, Sutcliffe R, Seonyatseng E. Dry season aerial survey of elephants and wildlife in northern Botswana, July—October 2018. Kasane: Elephants Without Borders; 2018.

68. Marshall PJ, Sayer JA. Population ecology and response to cropping of a hippopotamus population in eastern Zambia. J Appl Ecol. 1976;13(2):391–403.

69. Jones G, Pearlstine L, Percival HF. An assessment of small unmanned aerial vehicles for wildlife research. Wildl Soc Bull. 2006;34(3):750–8.

70. Lyons MB, Brandis KJ, Murray NJ, Wilshire JH, McCann JA, Kingsford RT, et al. Monitoring large and complex wildlife aggregations with drones. Methods Ecol Evol. 2019;00:1–12.

Článek vyšel v časopise


2019 Číslo 12
Nejčtenější tento týden