Megaherbivore browsers vs. tannins: is being big enough?

Abstract

Megaherbivores have been of particular interest to scientists because of the physiological and ecological challenges associated with their extreme body size. Yet, one question that has seldom been explored is how browsing megaherbivores cope with plant secondary metabolites (PSMs), such as tannins, found in their food. It is possible that the sheer body size of these megaherbivores allows them to ingest tannins with no deleterious effects. However, it is plausible that megaherbivores must rely on other mechanisms to cope with PSMs, such as the production of salivary tannin-binding proteins. Thus, we aimed to determine whether megaherbivore browsers produce tannin-binding proteins to further reduce the consequences of ingesting a tannin-rich diet. Using a series of laboratory assays, we explored whether elephants, black rhinoceros, and giraffe had tannin-binding proteins in their saliva. We tested for the presence of proline-rich proteins in the saliva using two different approaches: (1) SDS-PAGE using Laemmli’s (Laemmli, Nature 227:680–685, 1970) destaining method, and (2) comparative SDS-PAGE gels using Beeley et al.’s (Beeley et al. Electrophoresis 12:493–499, 1991) method for staining and destaining to probe for proline-rich proteins. Then, to test for the tannin-binding affinity of their saliva, we performed an inhibition assay. We did not observe proline-rich proteins in any of the megaherbivore species, but they did have other protein(s) in their saliva that have a high tannin-binding affinity. Our results highlight that, despite their large body sizes, and their abilities to tolerate low-quality food, browsing megaherbivores have likely evolved tannin-binding proteins as a way of coping with the negative effects of tannins.

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Acknowledgements

We thank S. Hensman, M. Hensman, J. Crosby, and the staff at Adventures with Elephants and the Rory Hensman Conservation and Research Unit for allowing us to collect samples from the elephants on their premises. Furthermore, we would like to thank Dr. S. Pfitzer (WildVET), I. Colenbrander (Chui Wildlife Services), Dr. G. Bauer (Gondwana Wildlife Services), Dr. D. Grobler, Dr. R. van Deventer (RVD Veterinary and Wildlife Services), and Dr. K. Stears for their assistance with sample collection. We would also like to thank Dr. R. Krause for laboratory support and guidance and Dr. K. Stears for constructive criticisms and comments on earlier drafts.

Funding

This work was funded by the National Research Foundation of South Africa (grant numbers: 90448, 97262, 77582) and the Gay Langmuir Bursary from the University of KwaZulu-Natal, School of Life Sciences awarded to MH Schmitt.

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MHS, AMS, and DW conceived and designed the experiments. MHS performed the experiments. MHS and DW analysed the data. MHS, AMS, and DW wrote the manuscript.

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Correspondence to Melissa H. Schmitt.

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The authors declare no conflicts of interest.

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All aspects of this study were approved by the University of KwaZulu-Natal’s Animal Ethics Committee (095/13/Animal). Moreover, none of the animals were harmed or put under any additional stress during the study. Due to the non-invasive and voluntary nature of the saliva collection procedure from the human subjects (i.e., the authors and two additional postgrads), the BioMedical Research Ethics Committee indicated that they did not need to review that experimental design or provide ethical approval.

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Communicated by Graeme Shannon.

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Schmitt, M.H., Shrader, A.M. & Ward, D. Megaherbivore browsers vs. tannins: is being big enough?. Oecologia 194, 383–390 (2020). https://doi.org/10.1007/s00442-020-04784-9

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Keywords

  • Tannin-binding proteins
  • Elephant
  • Rhinoceros
  • Giraffe
  • Saliva