Patterns of thermoregulation and seasonal metabolic adjustments in small owls in an arid environment
Date
2009-07-08T11:45:59Z
Authors
Smit, Ben
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Abstract
A high, approximately constant normothermic body temperature is associated with
elevated maintenance energetic requirements. However, many small endotherms offset
the energetic demands of thermoregulation through heterothermy (e.g., daily torpor and
hibernation) and/or phenotypic adjustments in maintenance metabolic requirements. In
terms of their significance in energy conservation, there are limited data on the above
mentioned physiological responses in birds, compared to mammals.
Torpor appears to be common in species that rely on unpredictable food
resources. However, to better understand the adaptive value and evolution of avian
torpor, a better understanding of the phylogenetic distribution of this trait is needed. The
first aim of this dissertation was to investigate the occurrence of torpor in free-ranging
Pearl-spotted Owlets (Glaucidium perlatum) and African Scops-Owls (Otus
senegalensis). Although heterothermic responses have been recorded in a number of
avian taxa, the occurrence of this phenomenon in owls owls (Strigidae) remains unknown.
I investigated winter patterns of thermoregulation in the crepuscular 80-g Pearlspotted
Owlet and the strictly nocturnal 61-g African Scops-Owl by obtaining telemetric
measurements of skin temperature (Tskin) from free-ranging individuals in the Kalahari
Desert of southern Africa. Pearl-spotted Owlets remained normothermic (Tskin > 37°C)
throughout the study period, whereas African Scops-Owls routinely used shallow torpor,
with Tskin reduced by 3° to 9°C below normothermic levels for 1.5 to 5 h after sunrise.
The mean lowest Tskin in three African Scops-Owl individuals was 29.0° ± 0.1°C. The
reductions in Tskin in African Scops-Owls are, to the best of my knowledge, the most pronounced Tskin reductions yet observed in owls. The thermoregulatory differences
between these two species may be related to their diets and activity patterns. African
Scops-Owls are almost exclusively insectivorous and experience a marked reduction in
food availability on cold winter nights, and are therefore likely to undergo energy stress.
In contrast, Pearl-spotted Owlets have more flexible activity patterns and include larger
and/or diurnal vertebrate prey in their diet, which is a more reliable food resource.
On the other hand, there is increasing evidence that birds can adjust their
minimum maintenance metabolic requirements (i.e. basal metabolic rate, BMR) over
short time scales in response to changing environmental conditions. However, there has
been very little emphasis on the role of metabolic adjustments at reducing energy
requirements in birds. Reductions in BMR would facilitate greatest energy savings during
winter when increased thermoregulatory demands coincide with low food availability.
The second aim of this dissertation was to investigate seasonal adjustments in basal
metabolic rate of birds resident year round in the Kalahari Desert of southern Africa. Our current understanding on the role of these adjustments is largely limited to small birds
inhabiting cold temperate regions in the northern Hemisphere, where winter BMR is
typically elevated.
I measured winter and summer BMR in the field for five species resident in the
Kalahari Desert of southern Africa, using a portable respirometry system. I found that
winter BMR was generally lower than in summer. Mass-specific BMR was significantly
lower in winter in the nocturnal African Scops-Owl (Otus senegalensis; 23%), the diurnal
Fork-tailed Drongo (Dicrurus adsimilis; 35%), Crimson-breasted Shrike (Laniarius
atrococcinneus; 29%), White-browed Sparrow-Weaver (Plocepasser mahali; 17%), and the crepuscular Pearl-spotted Owlet (Glaucidium perlatum; 30%). In African Scops-
Owls, however, the lower winter BMR (23%) was largely explained by lower winter
body mass (12%). These results suggest that seasonal adjustments in BMR in both
nocturnal and diurnal species are related to winter energy savings, and possibly also
water savings, rather than cold tolerance.
The limited data on seasonal BMR adjustments that are currently available
suggest that whereas most small northern Hemisphere birds up-regulate winter BMR, the
opposite is generally true in southern Hemisphere species. However, north-south
comparisons are complicated by differences in body mass and latitudes of
acclimatization, and the finding that mid-winter temperatures explained 57% of the
variation in global patterns of seasonal BMR adjustments. More data are therefore
required before global patterns can be reliably identified. Nevertheless, my data reveal a
dichotomy in the direction of seasonal BMR adjustments, and raises the possibility that in
regions where winters are milder, adjustments are related to energy savings in winter. In summary, I found that African Scops-Owls use shallow torpor, presumably to
reduce energy requirements. These data represent the first description of shallow torpor in
free-ranging individuals of a member of the suborder Strigi. I also found that African
Scops-owls, Pearl-spotted Owlets and three diurnal bird species exhibit significant BMR
reductions during winter. These winter reductions in BMR are opposite in direction to
those exhibited by most northern Hemisphere species.