George S. Bakken

A heat transfer analysis of animals: unifying concepts and the application of metabolism chamber data to field ecology.

This paper presents a heat transfer analysis of animals which lies between detailed thermal energy budget analysis and the simplified “Newtons Law of cooling” approach. The analysis considers the animal to be composed of two or three thermal layers. Heat transfer by conduction, convection, and thermal radiation are included separately in linearized form. Solar and thermal radiation from the environment and heat production and loss by metabolism and evaporation are included as rates. The solution for body temperature, Tb, is arranged into three parameters: (1) The operative environmental temperature, Te, the effective temperature of the environment for a specific animal, combining conduction, convection, and radiation; (2) the overall thermal conductance, K0, giving the thermal insulation of the animal; (3) the effective dry metabolic heat production, M∗, the algebraic sum of the metabolic heat production and evaporative cooling, corrected for the partitioning of evaporative cooling between the body core and the environment. For Tb constant, Te, and K0 are combined into a fourth parameter, the standard operative environmental temperature, Tes, This parameter is a measure of the value of M∗ required to maintain Tb constant, and allows the sensible thermal stress of different environments (e.g. a metabolism chamber and an outdoor habitat) to be compared. The parameters Te, K0, and M∗ are functionally equivalent to the corresponding parameters in “Newtons Law”. However, this should not be construed to justify the use of “Newtons Law”, since the parameters are not measured correctly by traditional procedures. However, it is possible to make direct measurements of Te, K0, Tes, and M∗ with taxidermic models of the animal. These direct measurements allow a simplified approach to many descriptive problems in thermal physiology and ecology, while obtaining results which may be related to detailed energy budget analysis.

Evolutionary stasis and lability in thermal physiology in a group of tropical lizards

Understanding how quickly physiological traits evolve is a topic of great interest, particularly in the context of how organisms can adapt in response to climate warming. Adjustment to novel thermal habitats may occur either through behavioural adjustments, physiological adaptation or both. Here, we test whether rates of evolution differ among physiological traits in the cybotoids, a clade of tropical Anolis lizards distributed in markedly different thermal environments on the Caribbean island of Hispaniola. We find that cold tolerance evolves considerably faster than heat tolerance, a difference that results because behavioural thermoregulation more effectively shields these organisms from selection on upper than lower temperature tolerances. Specifically, because lizards in very different environments behaviourally thermoregulate during the day to similar body temperatures, divergent selection on body temperature and heat tolerance is precluded, whereas night-time temperatures can only be partially buffered by behaviour, thereby exposing organisms to selection on cold tolerance. We discuss how exposure to selection on physiology influences divergence among tropical organisms and its implications for adaptive evolutionary response to climate warming.

An improved method for determining thermal conductance and equilibrium body temperature with cooling curve experiments

Abstract 1. 1.|The analysis of cooling curves to find the thermal conductance by plotting log(body temperature-air temperature) vs time is generally inaccurate. 2. 2.|Cooling curves are correctly analyzed by plotting log(body temperature-equilibrium body temperature) vs time. 3. 3.|Experiment time may be greatly reduced by using computer analysis to find equilibrium body temperature and the thermal conductance simultaneously. 4. 4.|Two computer programs for cooling curve analysis are described, together with representative results. 5. 5.|Accurate values of thermal conductance may be determined with cooling ranges of less than 5°C.

Thermoregulation is the pits: use of thermal radiation for retreat site selection by rattlesnakes

SUMMARY Pitvipers (Viperidae: Crotalinae) possess unique sensory organs, the facial pits, capable of sensing subtle fluctuations in thermal radiation. Prey acquisition has long been regarded as the sole function of the facial pits. However, the ability to sense thermal radiation could also direct thermoregulatory behavior by remotely sensing nearby surface temperatures. Using a series of behavioral arenas of varying spatial complexity and ecological relevance, we surveyed the ability of the western diamondback rattlesnake Crotalus atrox to direct successful thermoregulatory movements with either functional or disabled facial pits. We found that western diamondback rattlesnakes could base thermoregulatory decisions on thermal radiation cues when their pits were functional, but not when blocked. Our results indicate that the facial pit is part of a generalized sense, and suggest thermoregulation as an alternative hypothesis to prey acquisition for the origin of facial pits.

Thermoregulatory effects of radiotelemetry transmitters on mallard ducklings

Many telemetry transmitter attachments disrupt downy insulation, and may bias survival studies during cold weather by making ducklings more susceptible to chilling. We compared thermal responses of untreated 1-day-old mallards (Anas platyrhynchos) to ducklings carrying external sutured backpack or subcutaneously implanted transmitters. Ducklings carrying external transmitters showed areas of increased surface temperature in thermographic images. However, open-circuit respirometry studies at 5, 10, 15, 20, and 25 C and wind speeds of 0.1, 0.2, 0.5, and 1 m/s indicated no biologically significant differences in total heat production, net heat production, or short-term body mass loss. These results do not exclude the possibility of other negative effects of transmitters on duckling behavior and survival.

How many equivalent black-body temperatures are there?

Abstract 1. 1.|Several thermal indices, including “equivalent black-body temperature”, “effective blackbody ambient temperature” and “operative environmental temperature”, are identical with operative temperature and should be so designated. 2. 2.|“Standard operative environmental temperature”, essentially identical to standard operative tempature, is a distinct but related thermal index. 3. 3.|Operative temperature is an index of the potential driving heat flow, and must be used with a thermal resistance or conductance term. Standard operative temperature is a direct index of heat flow. 4. 4.|Both operative and standard operative temperature can either be calculated from animal and environmental heat-transfer parameters or be measured with taxidermic mounts.

Heat in evolution's kitchen: evolutionary perspectives on the functions and origin of the facial pit of pitvipers (Viperidae: Crotalinae)

SUMMARY Pitvipers (Viperidae: Crotalinae) possess thermal radiation receptors, the facial pits, which allow them to detect modest temperature fluctuations within their environments. It was previously thought that these organs were used solely to aid in prey acquisition, but recent findings demonstrated that western diamondback rattlesnakes (Crotalus atrox) use them to direct behavioral thermoregulation, suggesting that facial pits might be general purpose organs used to drive a suite of behaviors. To investigate this further, we conducted a phylogenetic survey of viperine thermoregulatory behavior cued by thermal radiation. We assessed this behavior in 12 pitviper species, representing key nodes in the evolution of pitvipers and a broad range of thermal environments, and a single species of true viper (Viperidae: Viperinae), a closely related subfamily of snakes that lack facial pits but possess a putative thermal radiation receptor. All pitviper species were able to rely on their facial pits to direct thermoregulatory movements, while the true viper was unable to do so. Our results suggest that thermoregulatory behavior cued by thermal radiation is a universal role of facial pits and probably represents an ancestral trait among pitvipers. Further, they establish behavioral thermoregulation as a plausible hypothesis explaining the evolutionary origin of the facial pit.

A two-dimensional operative-temperature model for thermal energy management by animals

Abstract 1. 1.|The directional nature of sum and wind produces two-dimensional variations in the thermal load on the surface of an animal. 2. 2.|Corresponding distributions of colour and insulation are seen on many animals. 3. 3.|An operative-temperature-based heat-transfer model is presented for analyzing the thermoregulatory significance of such patterns. 4. 4.|A test calculation shows that coordinated patterns of thermal load and surface properties have significant thermoregulatory effects not correctly predicted by one-dimensional models. 5. 5.|Standard isothermal laboratory enclosures may not accurately assess these thermoregulatory effects, especially for combinations of strong wind and solar radiation.

Wind speed dependence of the overall thermal conductance of fur and feather insulation

Abstract 1. 1. The effect of wind on heat loss from animals is usually described by a relation of the form a + bu c . 2. 2. I used data from heated taxidermic mounts of 6 avian species and 4 mammal species to determine the exponent c for the functional relation between heat transfer and wind speed, u . 3. 3. For overall thermal conductance, 0.54 c 4. 4. For overall thermal resistance, 0.16 c 5. 5. Using an incorrect value of c makes the slope of the relation, b , dependent on the range of wind speeds used. 6. 6. There is considerable variation in c among mounts of different species.

The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis

SUMMARY It is commonly assumed that the facial pit of pitvipers forms relatively sharp images and can detect small differences in environmental surface temperatures. We have visualized the temperature contrast images formed on the facial pit membrane using a detailed optical and heat transfer analysis, which includes heat transfer through the air in the pit chambers as well as via thermal infrared radiation. We find the image on the membrane to be poorly focused and of very low temperature contrast. Heat flow through the air in the pit chambers severely limits sensitivity, particularly for small animals with small facial pit chambers. The aperture of the facial pit appears to be larger than is optimal for detecting small targets such as prey at 0.5 m. Angular resolution (i.e. sharpness) and image strength and contrast vary complexly with the size of the pit opening. As a result, the patterns of natural background temperatures obscure prey items and other environmental features, creating false patterns. Consequently, snakes cannot simply target the strongest signal to strike prey. To account for observed behavioral capabilities, the sensory endings on the pit membrane apparently must respond to temperature contrasts of 0.001°C or less. While neural image sharpening likely enhances imaging performance, it appears important for foraging snakes to select ambush sites offering uniform backgrounds and strong thermal contrasts. As the ancestral facial pit was likely less sensitive than the current organ, objects with strong thermal signals, such as habitat features, were needed to drive the evolution of this remarkable sense.