Browsing by Author "Mckechnie, Andrew Edward."
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Item Patterns, mechanisms and evolution of avian facultative hypothermic responses : a southern African perspective.(2001) Mckechnie, Andrew Edward.; Lovegrove, Barry Gordon.Recent evidence suggests that avian facultative hypothermic responses are more common than previously thought. Traditionally, several categories of avian hypothermic responses have been recognized, and are frequently differentiated on the basis of minimum body temperature (T[b]) The available data suggest that the capacity for shallow hypothermia (rest-phase hypothermia) occurs throughout the avian phylogeny, but that the capacity for pronounced hypothermia (torpor) is restricted to certain taxa. However, there are currently too few data to test hypotheses concerning the evolution of avian hypothermic responses. Facultative hypothermia occurs over most of the avian body mass (M[b]) range, but is most common in small species. Minimum body temperature during hypothermia (T[min]) is continuously distributed from 4.3 °C to ca. 38°C. The continuous T[min] distribution, as well as recent evidence that the T[b] ranges of different avian physiological states may overlap, question the biological reality of specific T[b] limits. Pattens of thermoregulation during avian hypothermic responses are relatively variable, and do not necessarily follow the entry-maintenance-arousal patterns that characterize mammalian responses. Avian hypothermic responses are determined by a suite of ecological and physiological determinants. I investigated normothermic thermoregulation and hypothermic responses to restricted food in the speckled mousebird Colius striatus in the context of the distinction between normothermia, rest-phase hypothermia, and torpor. The lowest T[b] recorded in a bird which was able to arouse spontaneously was 18.2°C. However, I was unable to clearly discern between normothermic, hypothermic and torpor T[b] ranges. Furthermore, hypothermic responses did not accord with the patterns typically observed in birds and mammals. Metabolic suppression normally associated with entry into torpor and the defence of a torpor T[b] setpoint was largely absent. Laboratory data for C. striatus, as well as published data for Colius colius suggest that clustering behavior plays an important thermoregulatory role in mousebirds. Hence, I investigated thermoregulation under semi-natural conditions in C. striatus. In particular, I was interested in the interaction between clustering behavior and hypothermic responses during energy stress (restricted feeding). In contrast to clustering birds, rest-phase thermoregulation in single birds was characterised by linear decreases in T[b] and the birds did not appear to defend a specific T[b] setpoint. During restricted feeding, both clustering and single birds exhibited significant decreases in rest-phase T[b]. The extent of these facultative hypothermic responses was greater in single birds than in clustering birds, supporting the prediction that clustering behavior moderates the use of facultative hypothermia. I also tested the prediction that in free-ranging C. colius, the use of heterothermy should be rare, even at the coldest time of the year. I recorded mid-winter rest-phase body temperatures (T[b]) in a flock of free-ranging C. colius in an arid habitat in the Karoo, South Africa. The mousebirds' rest-phase T[b] was fairly labile, but was maintained above 33°C, despite T[a]s as low as -3.4 °C. The mousebirds showed no evidence of torpor under natural conditions; a facultative hypothermic response, during which T[b] was reduced to 29 - 33°C, was only observed on one occasion. The observed patterns of thermoregulation supported my predictions, and suggest that thermoregulation in clustering C. colius in the wild is significantly different to that of single birds under laboratory conditions. My results also suggest that the pronounced capacity for heterothermy usually associated with mousebirds is not necessarily representative of their patterns of thermoregulation under natural conditions. The capacity for avian torpor appears to be dependent on phylogeny. To investigate phylogenetic constraints on the capacity for torpor, I measured metabolic responses to food deprivation in a small, arid-zone passerine, the red-headed finch (Amadina erythrocephala). I observed significant reductions in rest-phase energy expenditure and body temperature (T[b]) in response to restricted feeding. The maximum extent of T[b] suppression (ca. 5°C) and energy savings (ca. 10%) were consistent with those reported for a number of other passerines. The lowest T[b] I observed in a bird able to arouse spontaneously was 34.8°C. My data support the hypothesis that the capacity for heterothermy in passerines is phylogenetically constrained, and that the majority cannot employ torpor in response to energetic stress. Selection for the capacity for torpor is presumably similar to the selection pressures acting on other avian energetic traits, such as basal metabolic rate (BMR). I tested the generality of a recent model linking the slow-fast mammalian metabolic continuum to global patterns of climatic predictability using BMR data for 219 non-migratory bird species. Avian BMR varied significantly between zoogeographical zones, with Afrotropical, Indomalayan and Australasian species generally exhibiting lower BMR than Holarctic species. In addition, the magnitude of differences between arid and mesic species varied between zones. In the Nearctic, these differences were pronounced, whereas no significant differences were evident for Afrotropical or Australasian species. A slow-fast metabolic continuum similar to that described in mammals appears to exist for birds, with higher BMR associated with predictable, seasonal environments and lower BMR with less predictable environments, in particular those affected by the El Niño Southern Oscillation. I constructed a generalised, conceptual model which attempts to predict the occurrence of torpor using phylogeny, M[b] constraints, a trade-off between energetic benefits and potential ecological costs, and specific ecological factors. A recent hypothesis suggests that endotherm heterothermy is monophyletic, and predicts that torpor should be more widespread in phylogenetically older taxa. Once phylogeny is considered, the most important determinant of avian torpor is M[b]. I used an existing model of endotherm torpor to predict the relationship between M[b] and minimum T[b] during torpor. The available data show that the lower limit of torpor T[b] is determined by the M[b]-dependent costs of rewarming following a torpor bout. Finally, I constructed a model based on the assumption that torpor is adaptive if the energetic benefits exceed the potential ecological costs. The model predicted that torpor should be more prevalent in species near the extremes of the avian metabolic continuum. The available data provide tentative support for this prediction. In addition to generalised factors such as phylogeny and M[b], specific aspects of a particular species' ecology need to be considered when predicting the occurrence of avian torpor.Item Thermoregulation and nocturnal heterothermy in the white-backed mousebird (Colius colius)(1998) Mckechnie, Andrew Edward.; Lovegrove, Barry Gordon.Thermoregulatory patterns, the use of rest-phase heterothermy, the energetic significance of clustering behaviour, and the role of sunning behaviour in thermoregulation were assessed in the white-backed mousebird Colius colius, an southern African arid zone species. I hypothesised that C. colius makes significant rest-phase energy savings by means of clustering behaviour, and has the ability to reduce diurnal energy costs by utilising solar radiation. I tested these hypotheses using indirect calorimetry and surgically implanted temperature-sensitive telemeters, and by simulating solar radiation in a constant-environment chamber. Rest-phase body temperature (T[b]s) was highly labile, with rest-phase T[b]s of down to 26 °C being recorded. During the rest-phase, T[b] was not regulated with respect to a setpoint temperature, as typically occurs in endotherms. Rather, rest-phase T[b] patterns were characterised by periods of linear decreases (passive cooling) at a rate dependent on ambient temperature and the number of individuals in a group. I did not observe any instances of torpor, as described in the literature. When T[b] depression did occur, it appeared to be the result of passive heat loss, and not the metabolic down-regulation which typically precedes a torpor bout. These findings, together with the phylogenetic position of the Coliidae, raise questions regarding the evolution of torpor. The inability of individual mousebirds to maintain a rest-phase setpoint T[b] suggests that in C. colius the physiological mechanism responsible for the defence of a rest-phase setpoint T[b] is replaced by a behavioural mechanism, namely clustering behaviour. The birds in this study showed a basal metabolic rate 40% below the predicted allometric values, and a circadian amplitude of body temperature (T[b]) of 5.2°C, 195% of the predicted value. The use of linear independent contrasts revealed that these deviations from the expected allometric values are not due to the phylogenetic relationships of the Coliiformes, and hence are likely to represent adaptation. These conservative metabolic traits offer opportunities for significant energy savings and are presumed to be adaptive in the unpredictable habitat of this species. The birds were able to make significant energy savings by means of huddling behaviour. At an ambient temperature of 15°C, the energy expenditure of birds in a group of six was 50% of that of single birds. The energy savings the birds were able to make were dependent on ambient temperature and the number of birds in the group. The ability to lower energy requirements by means of huddling behaviour is likely to be highly adaptive when dealing with low nocturnal temperatures in an environment where food supplies are spatially and temporally unpredictable. When allowed access to simulated solar radiation, individual mousebirds showed 15 - 30% reductions in their energy expenditure, while maintaining a constant body temperature. These reductions in energy expenditure hence represent thermoregulatory savings. My results support the hypothesis that solar radiation may be important in the energy budgets of some bird species. The ability to reduce food requirements by means of absorbing solar radiation is likely to be strongly selected for in the arid areas inhabited by C. colius, which are characterised by large circadian fluctuations in ambient temperature, in addition to resource unpredictability. Thermoregulation in C. colius appears to involve an interaction of behavioural patterns and physiological traits.