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temperature up to a reasonably comfortable 65oF or so.

One can pull a similar trick on the anterior hypothalamic "thermostat." Instead of a wet paper towel, of course, a small probe, called a thermode, is surgically inserted into the animal's brain. The thermode can raise or lower the temperature of a small patch of brain independent of the surrounding tissue. The consequences of localized tampering of hypothalamic temperature are remarkably similar to those that followed my tampering with the thermostat in my office. When the anterior hypothalamus of a dog, say, is warmed a degree or two, the dog will begin to pant, blood flow to the peripheral regions of the body will increase, and it will adopt postures, like sprawling on a cool floor, all of which increase the loss of heat from the body. All this happens even though the body temperature itself (that is, the rest of the body not including the anterior hypothalamus) is normal. Conversely, cooling the dog's anterior hypothalamus elicits shivering, a reduction of blood flow to the extremities, and postural adjustments, like curling up, that increase heat production and reduce the thermodynamic losses of heat from the body. These elegant experiments show that there is a negative feedback system operating in the thermoregulation of a dog, that its components can be localized and discretely manipulated, and that these component parts will respond in ways that are consistent with the operation of a negative feedback system.

Social Thermoregulation in Honeybee Colonies

Colonies of honeybees also seem to regulate their hive temperatures, and the outward signs of this regulation are very similar to the signs of mammalian thermoregulation (Fig. 11.2). For example, a honeybee colony's total rate of metabolic heat production (which represents the summed contributions of heat from all the individual bees) varies linearly with the outside temperature, and with the same result: the hive temperature is stabilized within a range of 4-5oC.

Keep in mind that we are not talking here about the stability of a bee's body temperature but of the hive temperature, which, from the bee's point of view, represents its external environment. Keep in mind also that the stability of hive temperature is not simply a consequence of large numbers of bees all individually regulating their own body temperatures. Suppose, for example, all the bees in a hive were individually homeothermic, each regulating its own body temperature. Each bee would exhibit the linear increase of individual heat production as external temperatures cool, similar to that seen in Figure 11.1 (top graph). That the colony's production of heat also behaves this way might simply reflect the summed contributions of the thousands of individual bees all behaving the same way. However, organismal homeostasis en masse cannot explain the homeostasis of hive temperature: the effectors for colony thermoregulation have a social dimension that is simply lacking in the individual bees. For example, on cool mornings bees tend to huddle into a compact cluster: the cooler the morning, the more tightly they huddle (Fig. 11.3). Huddling confines heat within the cluster, and huddling more closely restricts heat loss more. Furthermore, not all the bees in a cluster are doing the same things. Some of the bees occupy a circular shell spanning roughly the middle third of the cluster. These bees shiver their

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