Within the thermoneutral zone, an endotherm controls its body temperature by passive means. Above or below the thermoneutral zone, an endotherm must expend considerable metabolic energy to control body temperature. Thermoregulatory responses in mammals are regulated by the hypothalamus.
learning outcomes
You should be able to:
Describe the thermoneutral zone and the upper and lower critical temperatures and how they relate to the basal metabolic rate of an endotherm.
Interpret a plot of metabolic rate versus environmental temperature for an endotherm.
Explain differences in physical features of endotherms of the same or similar species that live in climates of different temperature extremes.
Describe how the mammalian brain thermostat uses feedback and feedforward information to regulate body temperature.
Explain the variable features of the mammalian thermoregulatory system.
What determines the upper and lower boundaries of the thermoneutral zone?
Within the thermoneutral zone, metabolism can remain at basal levels because the regulation of body temperature can be achieved by passive means such as changing blood flow to the skin and changing posture. However, as environmental temperature falls, a point will be reached when active thermoregulation is required, resulting in an increase in metabolism. This point is the lower critical temperature. Conversely, as the environment warms, a point will be reached when increasing blood flow to the skin and changing posture will not prevent a rise in core temperature, and active mechanisms such as sweating or panting are required, resulting in a rise in metabolism. This point is the upper critical temperature.
In the metabolic rate versus ambient temperature curve for mammals shown in Figure 39.13, the segment of the curve below the lower critical temperature is described by the equation MR = K(Tb – Ta). In this equation, what does K stand for? Why does this equation predict that the curve it describes will meet the x axis at a temperature that equals body temperature?
The segment of the metabolic rate curve in Figure 39.16 that falls below the lower critical temperature (low end of the thermoneutral zone) represents the amount of heat production necessary to balance the amount of heat loss to the environment. That segment of the metabolic rate curve can be described by the equation shown. If there were no metabolic functions other than balancing heat loss, the heat loss and therefore the MR would be zero when Tb – Ta equaled zero. Thus this MR line below the basal metabolic rate level projects to a Ta that is equal to Tb.
In the equation, the constant K stands for thermal conductance, which is the inverse of insulation. If the curve represented a desert versus a similar sized arctic animal, the curve would have a higher slope (K). The lower critical temperature would be higher for the desert versus the arctic species.
Explain why arctic species of mammals tend to be bigger and stockier than closely related species in warmer environments.
Heat is lost from the surface of the body, so a bigger, stockier animal will have a lower surface area-
What is the role of feedforward information in the mammalian thermoregulatory system?
Feedforward information serves to change set points in anticipation of an upcoming stressor. For example, exposure to a cold environment can elevate the set point for metabolic heat production, so that the increased heat loss from the body to the cold environment is balanced without requiring a drop in core body temperature to trigger the response.
Explain why the mammalian thermoregulatory system has been described as a proportional thermostat with an adjustable set point.
The mammalian thermostat is described as a proportional thermostat with an adjustable set point because the drive it generates to effector organs is proportional to the difference between hypothalamic temperature and a threshold hypothalamic temperature for activation of that response. The descriptor of adjustable set point refers to the role of feedforward information in altering the hypothalamic temperature thresholds for different thermoregulatory responses.