Why do elephant uproot trees?

Scientists know a relatively good amount about African bush elephant (Loxodonta africana), which is no great surprise given the disproportionate effect elephant have on savanna systems [1]; the ongoing conflict between crop-raiding elephant and rural human populations [2], and the devastating effects of illegal harvest on elephant populations [3]. And then of course is our species’ fascination with charismatic megafauna!

jumbo.retro.treeA glance through the past scientific literature on elephant is interesting in that it tracks not only the rise of African ecology, but also the threats facing elephant; many papers published recently address highly topical issues such as the ‘elephant problem’, human-elephant conflict, trophy hunting and illegal harvest. The ‘elephant problem’ is – crudely – that (growing) elephant populations are now restricted to protected areas, and dispersal during times of resource scarcity is restricted. One consequence is a high rate of tree loss [4], and the solution is not easy given that culling is no longer acceptable. The very high rates of illegal harvest experienced by bush elephants (and forest elephants Loxodonta cyclotis) in central and west Africa have not yet reached southern Africa [5], although that will likely change.

I digress. The reason I write this is because I recently read a lot of literature around the ‘elephant problem’ while preparing a proposal, and I came across an interesting question – why do bull elephants push over large trees? Remarkably this is not fully understood. One would imagine that large trees are pushed over for forage; fruit or green leaves. But this has been questioned – bulls only take small mouthfuls of forage once a tree has been uprooted [see Guy 1976 in 6].

Midgley et al [6] provided a few hypotheses as to why bulls uproot trees; elephant ‘farming’, non-adaptive or deviant behaviour and sexual/social reasons. Elephant ‘farming’ or coppicing implies that elephant may damage trees with the long-term goal of securing good forage through re-growth, but this implies high levels of spatio-temporal memory, that long-term gains that may not be beneficial at an individual level, and ignores elephant’ reliance on grass during the wet season. Non-adaptive behaviour suggests that uprooting is an unintended consequence of vigorous foraging, although bulls have been observed to be selective when foraging. Deviant behaviour suggests tree pushover or damage as a consequence of high stress levels among persecuted animals or where density-dependent effects are high – but overlooks the fact that uprooting occurs in areas where animals are not persecuted, and or at low densities. Sexual-social hypotheses suggest that males may uproot trees as a form of display although the eyesight of elephants is poor, and it is doubtful that trees are only damaged in the presence of other elephants. Finally, male elephants may uproot trees as form of strength training; deliberately selecting large trees to test their ability and build confidence. This is perhaps the most plausible hypothesis – and could be tested through observational approaches.

None of the ideas put across here are my own – I’ve paraphrased Midgley et al [6]. But I wonder how many people are aware of little we actually know. And just how can we maintain viable bush elephant populations in savanna systems and keep those lovely marula trees?

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