Estimating metabolic heat loss in birds and mammals by combining infrared thermography with biophysical modelling

Infrared thermography (IRT) is a technique that determines surface temperature based on physical laws of radiative transfer. Thermal imaging cameras have been used since the 1960s to determine the surface temperature patterns of a wide range of birds and mammals and how species regulate their surface temperature in response to different environmental conditions. As a large proportion of metabolic energy is transferred from the body to the environment as heat, biophysical models have been formulated to determine metabolic heat loss. These models are based on heat transfer equations for radiation, convection, conduction and evaporation and therefore surface temperature recorded by IRT can be used to calculate heat loss from different body regions. This approach has successfully demonstrated that in birds and mammals heat loss is regulated from poorly insulated regions of the body which are seen to be thermal windows for the dissipation of body heat. Rather than absolute measurement of metabolic heat loss, IRT and biophysical models have been most useful in estimating the relative heat loss from different body regions. Further calibration studies will improve the accuracy of models but the strength of this approach is that it is a non-invasive method of measuring the relative energy cost of an animal in response to different environments, behaviours and physiological states. It is likely that the increasing availability and portability of thermal imaging systems will lead to many new insights into the thermal physiology of endotherms.     

Radiated temperature from thermal imaging is related to feed consumption,growth rate and feed efficiency in grower pigs

Individual feed consumption and animal weight were continuously recorded in grower pigs using an automated feeding system. Infrared images were recorded each time a pig entered the feeding system and infrared thermography provided radiated thermal measurements of the dorsal surface of each animal. Feed was withdrawn and the animals fasted for a period of 24 h three times during the growth of the animals at body weights of approximately 35, 65 and 105 kg. There was a significant reduction of 0.28 °C in the maximum surface temperature (Tmax), and 0.48 °C in the average surface temperature (Tmean) during the periods of fasting. Maximum and average pig temperatures exhibited negative correlations to feed consumption and growth variables. There were negative correlations of residual feed intake (RFI) to Tmax and Tmean radiated temperatures. There were positive correlations of residual gain (RG) and residual intake and gain (RIG) with Tmax and Tmean. The Tmax and Tmean temperature responses to fasting were negatively associated with feed consumption and growth variables. Absolute temperature and temperature response variables were positively associated with RFI and negatively associated with residual intake and gain (RIG). These findings provide support for the concept of radiated heat losses as a measure of metabolic activity and a predictor of growth performance.     

Application of infrared thermography to the study of behavioural fever in the desert locust

Infrared (IR) thermography is used increasingly to estimate body temperature in small ectotherms such as insects. We used the thermal behaviour of an agricultural pest, the desert locust, Schistocerca gregaria, as a case study to demonstrate the application of this method to thermal biology. During microbial infection, the desert locust uses environmentally derived heat to elevate its body temperature. This ‘behavioural fever’ delays onset of disease caused by a fungal biopesticide. Understanding the thermal biology of S. gregaria is therefore a prerequisite for the development of a more effective mycoinsecticide. To accurately use IR thermography as a method of temperature measurement, IR data must first be calibrated with body temperature. Here, we identify two major factors which affect the IR data output and hence need to be incorporated into a camera calibration: (1) emissivity, predominantly determined by colour, and (2) the internal temperature of the camera. We demonstrate the limitations of thermocouple-based methods of temperature measurement in comparison to IR thermography. The detail provided by the large data sets revealed for the first time an early onset of fever in S. gregaria during infection with Metarhizium acridum viz. 20–25 h post-inoculation in comparison to the 48 h demonstrated previously.     

Thermographic evaluation of the lower critical temperature in weanling horses

Accommodating weanling horses in loose housing (sleeping hall with deep-litter bed and paddock) environments in winter at northern latitudes exposes the nonhuman animals to low ambient temperatures. We determined the heat loss of nine weanling horses in a cold environment by infrared thermography to assess their thermoregulatory capacity. The rate of heat loss was 73.5 to 98.7 W/m2 from the neck and 69.9 to 94.3 W/m2 from the trunk. The heat loss was higher at -16 degrees C than at 0 degrees C and -9 degrees C (p相似文献   

Does Thermoregulatory Feeding Occur in Newborn Infants? A Novel View of the Role of Brown Adipose Tissue Thermo genesis in Control of Food Intake

The physiological significance of the extensive deposits of brown adipose tissue (BAT) in newborn human infants has been the subject of much experimentation and discussion. Because of its large thermogenic capacity, its function has usually been viewed as preparing the infant for producing heat in response to cold exposure at birth. Newborn infants are indeed capable of precise thermoregulation for a limited time over a rather limited range of ambient temperatures, from thermoneutrality (32–34°C) down to common “room” temperatures (24–28°C). During such mild “cold-exposure”, in response to a decrease in their skin temperature, their sympathetic nervous system activity increases, and they can more than double their resting metabolic rate, principally by thermo genesis in their BAT. This review puts forward an entirely new role for BAT thermo genesis in the cyclic feeding pattern of newborn infants during their first months of life. BAT thermo genesis is proposed to be an integral element in a physiological thermoregulatory feeding control mechanism in which extended periods of very gradual cooling are interspersed with episodes of increased sympathetic nervous system activity, increased heating via BAT thermo genesis, arousal, and feeding. The cry with which the baby attracts its mother's attention is an integral part of the mechanism, as is the nutritive suckling reflex and the behavior of the mother. Initiation of feeding is attributed to a transient dip in blood glucose concentration that is due to stimulation of glucose utilization in the BAT. Termination of feeding is attributed to the high temperature brought about by the stimulated BAT thermo genesis. The duration of the urge to feed extends from the time of the cry to the time of the peak rise in temperature, when feeding stops. There is no clear Orcadian rhythm in core temperature in newborn infants, and meals occur at fairly frequent intervals both day and night in infants that are fed on demand. These physiological mechanisms are consistent with the limited information on phenomena attending spontaneous feeding in the newborn human infant and with what is known about the physiological control of feeding in rats. In rats, thermoregulatory feeding is defined as a feeding episode that occurs during a transient but marked increase in sympathetic nervous system activity that has several consequences. It stimulates BAT thermo genesis and increases body temperature. It produces a transient decline in blood glucose concentration secondary to the increased uptake of glucose by the stimulated BAT; this signals the initiation of the feeding episode. Subsequently the high temperature induced by BAT thermo genesis signals termination of the feeding episode. The size of the meal is determined by the balance between the capacity for BAT thermo genesis (heat production) and ambient temperature (heat loss). BAT thermo genesis is here viewed as an integral part of a physiological feeding control mechanism that links thermal balance with energy balance. The phenomenon is referred to as thermoregulatory feeding to distinguish it from feeding originating from other causes. As applied to human infants, the thermoregulatory feeding hypothesis supports the current practice of “feeding-on-demand”, i.e., entirely in accordance with the physiological oscillations in body temperature generated by the baby, determined by its thermal environment, mediated by oscillations in its BAT thermo genesis, and signaled by its demand for food. Whether the hypothesis has implications for feeding premature infants housed in incubators, usually fed on schedule rather than on demand, requires investigation.     

Thermographic Evaluation of the Lower Critical Temperature in Weanling Horses

Accommodating weanling horses in loose housing (sleeping hall with deep-litter bed and paddock) environments in winter at northern latitudes exposes the nonhuman animals to low ambient temperatures. We determined the heat loss of nine weanling horses in a cold environment by infrared thermography to assess their thermoregulatory capacity. The rate of heat loss was 73.5 to 98.7 W/m² from the neck and 69.9 to 94.3 W/m² from the trunk. The heat loss was higher at -16 °C than at 0 °C and -9 °C (p ≤ .01), indicating that the lower critical temperature may have been between -9 °C and -16 °C. Surprisingly, the heat loss from the trunk was less at -23 °C than -16 °C (p < .05). The frost on the surface of the hair coat at -23 °C probably disturbed the thermographic examination. Thus, thermography is not necessarily suitable for determining heat loss at very cold temperatures. Our results emphasize the importance of taking the housing temperature into account in planning the feeding of cold-housed weanling horses in northern latitudes.     

正常乳腺温度分布的有限元分析

基于正常乳腺的解剖学结构和生理学特征,建立了一个乳腺组织多维热传递模型。该模型考虑了代谢产热、血液灌注和动静脉血管与组织间热的相互作用,采用有限元分析方法求解热传导方程,数值模拟正常乳腺的稳态温度分布,着重研究血液灌注和代谢产热对正常乳腺组织温度分布的影响。研究结果可为乳腺疾病的热图像分析提供重要参考。     

Ejaculation increases scrotal surface temperature in bulls with intact epididymides

The objective of the study was to determine the effects of ejaculation on scrotal surface temperature (SST) measured with infrared thermography in bulls. In 18 Holstein bulls (18 mo old), sexual stimulation and spontaneous ejaculation (into an artificial vagina) increased SST at the bottom of the scrotum (0.9 degrees C; P < 0.0001). In 11 Angus bulls (1 yr old) electroejaculation increased both bottom and average SST (1.7 degrees C; P < 0.005 and 0.9 degrees C, P < 0.05), while in 12 Simmental cross bulls (2 yr old) electroejaculation significantly increased top, bottom and average SST (1.0, 1.2 and 1.1 degrees C, respectively). However, there was no significant increase in SST following electroejaculation in 15 Simmental cross bulls (2 yr old) with caudal epididectomies. The increase in SST was attributed to a localized increase in SST over the cauda epididymides, perhaps due to heat produced by contraction of the cauda epididymides during ejaculation. The results support the hypothesis that spontaneous ejaculation or electroejaculation increases SST and that this response is mediated by the cauda epididymides. Infrared thermography of the scrotum for evaluation of scrotal/testicular thermorégulation for clinical or research purposes should be performed before semen collection since thermography conducted soon after ejaculation may be misleading.     

Thermoregulatory control of feeding and sleep in premature infants

Objective: The aim of the present study was to test the thermoregulatory feeding control hypothesis in sleeping, premature infants. Research Methods and Procedures: In premature infants, the energy supply from food intake is crucial for (in order of importance): organ operation, body homeothermia, and optimal growth. The Himms‐Hagen model of thermoregulatory feeding control involving activation of heat production by brown adipose tissue (BAT) was formulated on the basis of work in (awake) rats. This hypothesis has also been put forward for the human neonate, which can also use BAT to produce metabolic heat. According to the model, feeding episodes occur during a transient increase in body temperature. Feeding is initiated by a dip in blood glucose concentration after sugar uptake by activated BAT. Results: In 14 neonates (bottle‐fed on demand), food intake always took place during an increase in skin temperature (+0.19 ± 0.21 °C). Awakening occurred 18 ± 17 minutes after the minimum skin temperature level had been reached. When feeding time was imposed, feeding was not necessarily situated during an increase in skin temperature, and the sleep duration after food intake increased significantly (+43%). This could be considered as an adaptive response to the short‐term sleep deprivation and/or stress elicited by an imposed feeding rhythm. Discussion: The validity of the model supports the use of on‐demand feeding in neonatal care units, in accordance with the infant's physiological body temperature oscillations.     

Understanding and evaluating bovine testes

The objective is to briefly review bovine testes and how they are assessed, with an emphasis on articles from Theriogenology. Scrotal circumference (SC) is the most common method to assess testicular size; it varies among individual bulls and breeds and is highly heritable. In general, a large SC is associated with early puberty, more sperm, a higher percentage of morphologically normal sperm, and better reproductive performance in closely related females. Consequently, there are minimum requirements for SC for breeding soundness. In prepubertal bull calves, there is an early rise (10–20 weeks of age) in LH, which is critically related to onset of puberty and testicular development. Feeding bulls approximately 130% of maintenance requirements of energy and protein from approximately 8 to 30 weeks of age increased LH release during the early rise, hastened puberty (approximately 1 month), and increased mature testis size and sperm production (approximately 20%–30%). However, high-energy diets after weaning (>200 days) often reduced sperm production and semen quality. A bull's testes and scrotum have opposing (complementary) temperature gradients, which keep the testicular temperature 2 °C to 6 °C cooler than core body temperature for production of fertile sperm (increased testicular temperature reduces semen quality). Infrared thermography, a quick and noninvasive method of assessing scrotal surface temperature, may be beneficial for evaluations of breeding soundness. The primary clinical use of ultrasonography in assessment of reproductive function in the bull is characterization of grossly detectable lesions in the testes and scrotum. In conclusion, testis size and function are critical for bull fertility, affected by nutrition, and readily assessed clinically.     

Infrared thermography cannot be used to approximate core body temperature in wild primates

Understanding the physiological processes that underpin primate performance is key if we are to assess how a primate might respond when navigating new and changing environments. Given the connection between a mammal's ability to thermoregulate and the changing demands of its thermal environment, increasing attention is being devoted to the study of thermoregulatory processes as a means to assess primate performance. Infrared thermography can be used to record the body surface temperatures of free-ranging animals. However, some uncertainty remains as to how these measurements can be used to approximate core body temperature. Here, we use data collected from wild vervet monkeys (Chlorocebus pygerythrus) to examine the relationship between infrared body surface temperature, core body (intra-abdominal) temperature, and local climate, to determine to what extent surface temperatures reflect core body temperature. While we report a positive association between surface and core body temperature—a finding that has previously been used to justify the use of surface temperature measurements as a proxy for core temperature regulation—when we controlled for the effect of the local climate in our analyses, this relationship was no longer observed. That is, body surface temperatures were solely predicted by local climate, and not core body temperatures, suggesting that surface temperatures tell us more about the environment a primate is in, and less about the thermal status of its body core in that environment. Despite the advantages of a noninvasive means to detect and record animal temperatures, infrared thermography alone cannot be used to approximate core body temperature in wild primates.     

Thermal Imaging of Aye-Ayes (Daubentonia madagascariensis) Reveals a Dynamic Vascular Supply During Haptic Sensation

Infrared thermography (IRT) is used to visualize and estimate variation in surface temperatures. Applications of IRT to animal research include studies of thermofunctional anatomy, ecology, and social behavior. IRT is especially amenable to investigations of the somatosensory system because touch receptors are highly vascularized, dynamic, and located near the surface of the skin. The hands of aye-ayes (Daubentonia madagascariensis) are thus an inviting subject for IRT because of the prominent middle digit that functions as a specialized haptic sense structure during percussive and probative foraging. It is a vital sensory tool that is expected to feature a high density of dermal mechanoreceptors that radiate heat and impose thermal costs under cool temperatures. Here we explore this premise by acquiring IRT images of 8 aye-ayes engaged in a variety of passive and probative behaviors. We found that the middle digit was typically 2.3°C cooler than other digits when the metacarpophalangeal (MP) joint was extended, and that it warmed an average of 2.0°C when the MP joint was flexed during active touching behavior. These changes in digital surface temperature, which were sometimes as much 6.0°C, stand in sharp contrast with the profoundly invariant temperatures of the other digits. Although the physiological mechanisms behind these temperature changes are unknown, they appear to reveal a uniquely dynamic vascular supply.     

The automated analysis of clustering behaviour of piglets from thermal images in response to immune challenge by vaccination

An automated method of estimating the spatial distribution of piglets within a pen was used to assess huddling behaviour under normal conditions and during a febrile response to vaccination. The automated method was compared with a manual assessment of clustering activity. Huddling behaviour was partly related to environmental conditions and clock time such that more huddling occurred during the night and at lower ambient air temperatures. There were no positive relationships between maximum pig temperatures and environmental conditions, suggesting that the narrow range of air temperatures in this study was not a significant factor for pig temperature. Spatial distribution affected radiated pig temperature measurements by IR thermography. Higher temperatures were recorded in groups of animals displaying huddling behaviour. Huddling behaviour was affected by febrile responses to vaccination with increased huddling occurring 3 to 8 h post-vaccination. The automated method of assessing spatial distribution from an IR image successfully identified periods of huddling associated with a febrile response, and to changing environmental temperatures. Infrared imaging could be used to quantify temperature and behaviour from the same images.     

Heat transfer in elephants: thermal partitioning based on skin temperature profiles

The elephant with its low surface-to-volume ratio presents an interesting problem concerning heat dissipation. To understand how such large mammals remain in thermal balance, we determined the major avenues of heat loss for an adult African elephant and an immature Indian elephant. Because conventional physiological measurements are difficult for these animals, the present study used a non-invasive technique, infrared thermography, to measure skin temperatures of each elephant. Detailed surface temperature profiles and surface area measurements of each elephant were used in standard equations for convective, conductive and radiant heat transfer. Results demonstrated that heat transfer by free convection and radiation accounted for 86% of the total heat loss for the elephants at T _a= 12·6 °C. Heat transfer across the ears, an important thermal window at high ambient temperatures, represented less than 8% of the total heat loss. Surface area of the animals, and metabolic heat production calculated from total heat loss of the African elephant, scaled predictably with body mass. In contrast, the thermal conductance of the elephants (71·6 W /°C, African; 84·5 W /°C, Indian) was three to five times higher than predicted from an allometric relationship for smaller mammals. The high thermal conductance of elephants is attributed to the absence of fur and appears to counteract reduced heat transfer associated with a low surface-to-volume ratio.     

An assessment of skin temperature gradients in a tropical primate using infrared thermography and subcutaneous implants

Infrared thermography has become a useful tool to assess surface temperatures of animals for thermoregulatory research. However, surface temperatures are an endpoint along the body's core-shell temperature gradient. Skin and fur are the peripheral tissues most exposed to ambient thermal conditions and are known to serve as thermosensors that initiate thermoregulatory responses. Yet relatively little is known about how surface temperatures of wild mammals measured by infrared thermography relate to subcutaneous temperatures. Moreover, this relationship may differ with the degree that fur covers the body. To assess the relationship between temperatures and temperature gradients in peripheral tissues between furred and bare areas, we collected data from wild mantled howling monkeys (Alouatta palliata) in Costa Rica. We used infrared thermography to measure surface temperatures of the furred dorsum and bare facial areas of the body, recorded concurrent subcutaneous temperatures in the dorsum, and measured ambient thermal conditions via a weather station. Temperature gradients through cutaneous tissues (subcutaneous-surface temperature) and surface temperature gradients (surface-ambient temperature) were calculated. Our results indicate that there are differences in temperatures and temperature gradients in furred versus bare areas of mantled howlers. Under natural thermal conditions experienced by wild animals, the bare facial areas were warmer than temperatures in the furred dorsum, and cutaneous temperature gradients in the face were more variable than the dorsum, consistent with these bare areas acting as thermal windows. Cutaneous temperature gradients in the dorsum were more closely linked to subcutaneous temperatures, while facial temperature gradients were more heavily influenced by ambient conditions. These findings indicate that despite the insulative properties of fur, for mantled howling monkeys surface temperatures of furred areas still demonstrate a relationship with subcutaneous temperatures. Given that most mammals possess dense fur, this provides insight for using infrared imaging in thermoregulatory studies of wild animals lacking bare skin.     

The value of infrared thermography for research on mammals: previous applications and future directions

• 1 Infrared thermography (IRT) involves the precise measurement of infrared radiation which allows surface temperature to be determined according to simple physical laws. This review describes previous applications of IRT in studies of thermal physiology, veterinary diagnosis of disease or injury and population surveys on domestic and wild mammals.
• 2 IRT is a useful technique because it is non‐invasive and measurements can be made at distances of <1 m to examine specific sites of heat loss to >1000 m to count large mammals. Detailed measurements of surface temperature variation can be made where large numbers of temperature sensors would otherwise be required and where conventional solid sensors can give false readings on mammal coats. Studies need to take into account sources of error due to variation in emissivity, evaporative cooling and radiative heating of the coat.
• 3 Recent advances in thermal imaging technology have produced lightweight, portable systems that store digital images with high temperature and spatial resolution. For these reasons, there are many further opportunities for IRT in studies of captive and wild mammals.

     

Heat increment of feeding and its thermoregulatory benefit in the muskrat (Ondatra zibethicus)

The calorigenic effect of feeding and its potential benefit in defraying thermoregulatory costs and attenuating immersion hypothermia of adult muskrats were investigated. A single session of feeding on aquatic vegetation was sufficient to raise the metabolic rate of muskrats for a period of at least 5 h. The peak postprandial rate of oxygen consumption averaged 1.42 times the level established for fasted animals, and the heat increment of feeding accounted for about 40% of the metabolizable energy intake of muskrats. There was no evidence of a postprandial rise in oxygen consumption of muskrats that entered water at 18–19°C after feeding. In aquatic trials, average and minimum steady-state oxygen consumption rates of fed muskrats were similar to, or even lower than values recorded from fasted animals, implying substitution of heat increment of feeding for thermoregulatory heat production. Our data did not support the hypothesis that heat increment of feeding retards body cooling in water. Net body temperature decline in water was actually higher in fed animals than in fasted controls. However, since previously fed muskrats also entered water at an elevated body temperature, the final body temperature (at 30 min immersion) was similar in all groups. These findings suggest that metabolic heat generated incidental to preimmersion feeding could provide a thermoregulatory benefit to muskrats by reducing the need for active thermogenesis in water.     

Using infrared thermography to assess seasonal trends in dorsal fin surface temperatures of free-swimming bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida

The temperature differential (Δ T ) between a body surface and the environment influences an organism's heat balance. In Sarasota Bay, FL, where ambient water temperature ( T _w) ranges annually from 11° to 33°C, Δ T was investigated in a resident community of bottlenose dolphins ( Tursiops truncatus ). Dorsal fin surface temperatures ( T _dfin) were measured on wild, free-swimming dolphins using infrared thermography. Field and laboratory calibration studies were also undertaken to assess the efficacy of this non-invasive technology in the marine environment. The portability of infrared thermography permitted measurements of T _dfin across the entire range of environmental temperatures experienced by animals in this region. Results indicated a positive, linear relationship between T _dfin and T _w ( r ²= 0.978, P < 0.001). On average, T _dfin was 0.9°C warmer than T _w across seasons, despite the 22°C annual range in T _w. Changes in integumentary and vascular insulation likely account for the stability of Δ T _{dfin − w} and the protection of core temperature ( T _core) across seasons. The high thermal conductivity of water may also influence this Δ T . The use of infrared thermography is an effective, non-invasive method of assessing dorsal fin skin surface temperatures (±1°C) across large numbers of wild, free-swimming dolphins throughout their thermally dynamic aquatic environment.     

An integrated physical and biological model for anaerobic lagoons

A computational fluid dynamics (CFD) model that integrates physical and biological processes for anaerobic lagoons is presented. In the model development, turbulence is represented using a transition k-ω model, heat conduction and solar radiation are included in the thermal model, biological oxygen demand (BOD) reduction is characterized by first-order kinetics, and methane yield rate is expressed as a linear function of temperature. A test of the model applicability is conducted in a covered lagoon digester operated under tropical climate conditions. The commercial CFD software, ANSYS-Fluent, is employed to solve the integrated model. The simulation procedures include solving fluid flow and heat transfer, predicting local resident time based on the converged flow fields, and calculating the BOD reduction and methane production. The simulated results show that monthly methane production varies insignificantly, but the time to achieve a 99% BOD reduction in January is much longer than that in July.     

Dietary effects on pelage emissivity in mammals: Implications for infrared thermography

Infrared thermography is becoming popular to measure animal surface temperature non-invasively. However, its application in quantitative mammal research is restricted by a paucity of pelage emissivity measurements, which are necessary to acquire accurate temperature readings. Furthermore, the factors influencing pelage emissivity remain largely unknown. We therefore examined the putative links between diet (fat content), hair length, hair diameter, and pelage emissivity in laboratory mice. Individuals maintained on high-fat diets had higher pelage emissivity values than those on standard diets, which may be due to fur being oily and/or the fact that the fur clumped together, exposing the skin underneath. Alternatively, the chemical composition of the fur of individuals on a high-fat diet may vary from those on a standard diet. We found no significant relationships between various hair metrics and emissivity. This study highlights that aspects of an animal's life history (e.g. age, sex, diet) may contribute to the emissivity of its pelage. As such, a single emissivity value may be inappropriate for use in infrared thermography across all species or individuals; other aspects of an animal's biology, which may affect emissivity, should also be considered. Best practice should involve measuring emissivity for every individual animal used in thermography studies.