Water conservation strategies of large African ungulates
No Thumbnail Available
Date
2017
Authors
Strauss, Willem Maarten
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Ecosystems in the southern hemisphere are limited mainly by water
availability, a resource predicted to become increasingly scarce in the
region in the face of anthropogenic climate change. Understanding how
the artiodactyls (e.g., sheep, goats, cattle, antelope and camels), an
important grouping of animals regionally, conserve body water to facilitate
their survival, is therefore of importance. The idea that selective brain
cooling conserves body water has been advanced for more than 20 years,
without any definitive proof that ungulates, naturally making use of
selective brain cooling, actually save body water as a result of
implementing selective brain cooling. Using implanted biologgers I
simultaneously measure carotid arterial and hypothalamic temperature in
Dorper sheep Ovis aries. The concomitant determination of water
turnover, based on the washout rates of the stable hydrogen isotope
deuterium oxide (D2O), allowed me to measure the volume of water that a
dehydrated Dorper sheep conserves during a day when exposed to heat.
Artiodactyls differ in their water requirements though, and may have
selective brain cooling capabilities relative to their level of water
dependency. I therefore undertook the first comparative investigation into
selective brain in free-living artiodactyls by implanting, with the assistance
of colleagues, biologgers to measure carotid arterial and hypothalamic
temperatures in three large, sympatric artiodactyl species with varying
water dependencies. Despite a clear water-dependency gradient across
species, I found no difference in the magnitudes of selective brain cooling,
iii
the proportion of time that selective brain cooling was used, or the
threshold temperatures for selective brain between the gemsbok Oryx
gazella, red hartebeest Alcelaphus buselaphus or blue wildebeest
Connochaetes taurinus in an environment where the animals had free
access to water. I found greater variability in selective brain cooling within
species, than between species and conclude that all three species had the
same underlying ability to make use of selective brain cooling.
Artiodactyls, however, are likely to rely on range of water conservation
mechanisms in the face of climate change. A variable body temperature
and the use of appropriate microclimates are two additional strategies that
could be important in the quest to conserve body water. I therefore
investigated the 24h nychthemeral body temperature rhythms of the
gemsbok, red hartebeest and blue wildebeest, in combination with their
24h microclimate use patterns, during the five months of a southern
hemisphere calendar year most challenging physiologically: the end of the
dry season through the peak of summer. I found no species differences in
the 24h nychthemeral body temperature rhythms of the gemsbok, the red
hartebeest or the blue wildebeest. All three species, however, were
heterothermic over about the first 50 days of the study, as a result of
elevated maximum and depressed minimum body temperatures. Although
subtle differences were detected in microclimate use, all three species
used behavioural thermoregulation and accessed microclimates cooler
than that available in the full sun. Such behaviour was enhanced during
the hottest days, compared to the coolest days.
In conclusion, I have investigated some of the water conservation
strategies relied upon by large African ungulates in their current
environments. These strategies are not mutually exclusive and will all
benefit these species in the face of climate change. The long-term
monitoring of these water conservation strategies will allow us to tease out
the relative contributions that selective brain cooling, heterothermy and
microclimate use makes to body water conservation and how that
translates to individual fitness.
Description
A thesis submitted to the Faculty of Health Sciences, University of the
Witwatersrand, Johannesburg, in fulfilment of the requirements for the
degree of Doctor of Philosophy
Johannesburg, South Africa, 2017