Estimating the Thermo-Neutral Zone of Dromedary Camels Using Surface Heat Transfer Modelling

Thermal homeokinesis is a term used to describe a steady state, where core body temperature of any homeotherm is relatively maintained constant with little or no additional energy expenditure. However, to maintain such state there has to be a thermal balance. The simplest form of the thermal balance implies that the total rate of heat production (Ptotal) must be met by an equal rate of total heat dissipation (Qtotal). Dromedary camels, as a homeotherm, demand to maintain a state of thermal homeokinesis within a specific range of ambient temperatures (Ta); the thermo-neutral zone (TNZ). Nevertheless, there is no study we are aware of reporting any information about the TNZ for any kind of camelidae species. Instead of measuring the absolute heat production, the current study offers an alternative method to estimate the TNZ using the theories and principles of heat transfer modelling. This study was carried out on 10 dromedary camels (5 calves and 5 bulls) individually fed at maintenance, and was divided into two periods; preliminary and experimental periods. During the preliminary period, several measurements were predetermined for each camel encompassing the following; the biophysiological parameters (body weight and daily feed intake) as well as body biomorphometry (length, circumference, surface area) and surface thermal properties (skin emissivity and coat distribution). Additionally, all camels were surgically implanted with highly-calibrated data loggers to record their core (Tb) and tympanic (Ttym) temperatures. Furthermore, camels were accustomed to the measuring equipment and trained to be inside a climatic-controlled chamber. During the experimental period, on the other hand, each camel at first was placed inside a chamber under a stable Ta (24°C; presumed to occur inside the TNZ) for at least 48 hours to be acclimatized, and then randomly exposed to one of 5 different levels of Ta; 0, 10, 20, 30, 40, and 50°C. In each experimental Ta set, each camel was kept for 2 hours to be stabilized to the selected Ta, and then several measurements -mentioned below- were quantified twice in the next 60 min. Thereafter, the camel was transferred to 24°C for at least 2 hours to be stabilized, and then one more set of Ta was randomly chosen and measurements were recorded again. Accordingly, two Ta were tested per day for each camel. Each camel was given at least 7 days of rest between experiments. Experimental measurements were include; the ambient and radiant temperatures as well as relative humidity inside the chamber, in addition to camel's body temperatures (core, tympanic, rectal, skin, coat, and surface temperatures) and thermophysiological responses (respiratory, heart, and sweating rates). For analysis, heat exchange from camels was modelled on the basis of a simple geometric shape. Camel's body was divided into five regions. The head and body regions were modelled as a horizontal circular cylinder, the neck and appendage regions as vertical circular cylinders, while the feet were modelled as flat plate. Using the appropriate heat transfer modelles, the rate (W•m-2) of radiative, convective, conductive, and evaporative heat dissipation mechanisms from each region were calculated. Ultimately, the rate (W•m-2) of Qtotal from whole body was then estimated by adding the rates of heat dissipation from each region at the intended Ta. On the other hand, total body thermal insulance (Itotal, m2•K•W-1) was estimated using the combined and calculated values of the three body thermal insulative layers (i.e. body tissue, coat, and air boundary) from whole body at the intended Ta. To determine the TNZ range, it is widely useful to use two limit temperatures (i.e. the lower and upper limit temperatures). When Ptotal was considered to follow Qtotal, the starting point of increasing Ptotal at a low set of Ta, while maintaining a relatively constant Tb and simultaneously expressing maximum Itotal, was found to be at 10oC and selected to be the lower limit temperature of TNZ in camels. Meanwhile, the starting point of increasing evaporative heat dissipation mechanisms at a high set of Ta, while maintaining a relatively constant Tb and simultaneously expressing minimum Itotal, was coincided with 40oC and selected as the upper limit temperature of TNZ in camels. This study was designed to give us a better understanding and to improve the knowledges in camel's basic thermophysiology. Due to the morphophysiological adaptabilities of camels compared to other species, current study clearly substantiates that both the heat exchange profile and body thermal insulance have manifested particular differences in camels, which reflected on possessing a wider TNZ that occurs in a higher range of Ta. We hope that the knowledge gained from this study will enhance our understanding of the bioenergetic requirements of camels under different environmental conditions, and will improve the housing management of camels in regard to their thermal comfort, which subsequently may increase their productivity (e.g. growth, milk, meat, et al.) as well as their welfare (e.g. neonatal post-parturition care, during transportation, or post-racing) under the harsh environmental conditions.