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
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.
