Tuesday, 26 August 2014

On the feet of elephants




Pools of rain-water in the axils of leaves, in tree hollows, on the surfaces of expanses of rock and in the foot-prints of large herbivores, are ubiquitous everywhere and may number in the millions of habitats for freshwater dwelling organisms; reviewed by  (McLachlan and Ladle 2001). They are inhabited by a great variety of aquatic animals, both invertebrate and vertebrate, sometimes indigenous and at great densities. Indeed, a case might be made that they are by far the principal fresh-water habitats on earth and hence also the source of much of the adaptive evolution among fresh water dwelling organisms. They are therefore a good place to identify adaptation. Because these small bodies of water are numerous and often occur in arid places, they provide an important focus for the activities of a variety of vertebrates, including birds, primates, viverrids, elephants and many others, all seeking drinking water. 

The activity around these puddles lead to an interaction with the pool inhabitants in unexpected ways. It is these interactions that provide the substance of my essay. Because pools typically dry after a short period, a central adaptation is dispersal. Charles Darwin (1859), regards pools as ‘island’ habitats and makes the point that their fauna may need help in achieving dispersal, this being provided by birds, among others, …”dispersal of their seeds by animals, more especially by fresh water birds, which have great powers of flight, and naturally travel from one piece of water to another”.

Interactions between the terrestrial and aquatic ecosystems often involve food. In tropical Africa viverrids (civets and genets), use the pools as lavatories, thus unintentionally providing a rich source of food for invertebrates in what may otherwise be a nutritionally poor habitat. The consequence is a flourishing population of biting midge larvae of the family Ceratopogonidae, not always welcome to humans. Such additions from the terrestrial environment can be too much of a good thing. My colleague Bram Vanschoenwinkel, reporting on the work of Brian Timms, tells me that the droppings of kangaroos, feral goats and human tourists, have lead to the pollution of rain-pools of Ayres Rock in Australia leading to dead pools and the possible total extinction of an indigenous fairy shrimp.

I now arrive at the point I really want to develop – the interactions between rain pool dwellers and elephants. As for as I am aware this relationship has never been formally investigated, but see (Vanschoenwinkel, Waterkeyn et al. 2011). Early in the 1960s it became apparent to me that something interesting followed from the routine bathing activities of elephants. My observations were made on the shores of Lake Kariba in the Zambezi valley.  In the rainy season the puddles of rain water in the foot prints of elephants on soft, wet ground after rain prove attractive to elephants. Their  foot prints are about 18 inches across and perhaps 6 inches deep. Scooping up water and mud to apply to their backs, perhaps to provide relief from heat, tsetse flies etc, deepens the foot print, eventually leading a pool several metres wide and perhaps half a metre deep. Such pools are inhabited by aquatic plants (Marsilia), and fairy shrimps (Chirocephalus), or in some cases ny the shrimp Lepidurus (Fig. 1). As far as I can tell, these organisms are found nowhere else in the Zambezi valley. Thus the activity of elephants leads to the creation of a unique fresh water habitat (Fig. 2, Fig. 3).

Here is an interesting opportunity for someone. I would start with the question of where Lepidurus and Chirocephalus come from. My prediction is that they will turn out to be carried as a resistant egg (Hildrew 1985), just like those on the feet of Darwin’s birds, but  by elephants moving from pools to pool with mud on their feet. Thus elephants may be solely responsible for the existence of this extraordinary habitat. Devising a sampling method to test such a hypothesis will require an inventive mind.








Fig. 1. The biota of elephant ponds. The floating leaves and submerged fruits of Marsilia (left) and the pond shrimps Lepidurus (top right) and Chirocephalus (below right).




Fig. 2. A rain pool created by the elephants shown in Fig. 3 below with Sandra McLachlan making observations in the rainy season 1964.





Fig. 3. The group of elephants observed in pool making activities. The pink colour is due to mud thrown on their backs.



References

Darwin, C. (1859). The Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. Sixth edition, John Murray, London. Popular impression of the corrected copyright edition. 

         
Hildrew, A. G. (1985). "A quantitative study of the life history of a fairy shrimp (Branchiopoda: Anostraca) in relation to the temporary nature of its habitat, a Kenyan rain pool. ." Journal of Animal Ecology 54: 99-110.
           
McLachlan, A. J. and R. Ladle (2001). "Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools." Biological Reviews 76: 377-388.
           
Vanschoenwinkel, B., A. Waterkeyn, et al. (2011). "Passive external transport of freshwater invertebrates by elephant and other mud-wallowing mammals in an African savannah habitat." Freshwater Biology 56: 1606-1619.