The effects of rainfall unpredictability on the circadian energy rhythms of the four-striped field mouse, Rhabdomys pumilio.
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Date
1998
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Abstract
Rainfall is the most important factor limiting primary production, and hence food
availability, in ecosystems. This study tests the thesis that, under conditions where
rainfall is unpredictable and food supply varies spatially and temporally from year-to-year,
physiological energy-saving mechanisms should evolve in animals for which
migration is not an option to avoid periods of low habitat productivity. Apart from the
normally erratic rainfall patterns characteristic of arid regions, unpredictable seasonal
rainfall may also occur in mesic zones as a consequence of climatic phenomena such
as the El Niño Southern Oscillations (ENSO). Using two hypothetical models, this study
attempts to determine whether the respective evolutionary history of both aridity and
ENSO can be detected in the various phenotypic physiological traits of Rhabdomys
pumilio from across southern Africa. Model 1 predicts that a gradient of conservatism
of physiological traits related to metabolism, will be detectable along an aridity gradient.
Model 2 predicts an increase in the conservatism of similar traits correlated with both
the aridity gradient and the gradient of negative rainfall anomalies that are associated
with the ENSO.
Measurements of circadian rhythms of body temperature and oxygen
consumption of individuals of R. pumilio collected from six sites across southern Africa
showed that the geographic variation in the physiological traits and proximate responses
of this species was explained better by Model 2 than by Model 1. Furthermore, for
some traits and proximate responses of this species it was shown that, although the
stresses associated with unpredictable rainfall occur less frequently in non-desert
ENSO-zones, they are nevertheless sufficient to mimic the selection pressures of rainfall
unpredictability in arid zones.
This study also highlighted the low basal rates of metabolism and the other
conservative physiological traits of this species as a whole. The basal metabolic rate
of 0.819 mlO₂g/h and the minimum wet thermal conductance of 0.110 mlO₂/g/h/ºC are
lower than that predicted by allometry. The circadian amplitude of body temperature of
2.52°C and the circadian amplitude of oxygen consumption of 1.39 mlO₂/g/h were higher
than those predicted by allometric equations. These deviations from predicted values
were attributed to the adaptedness of this species to the physiological energy stresses
associated with unpredictable resource availability in southern Africa.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.
Keywords
Mice--physiology., Rhabdomys pumilio--Physiology., Circadian rhythms., Energy metabolism., Theses--Zoology.