Organisms maintain homeostasis in a variety of ways. In the first part of this book, mammals are shown to regulate their body temperatures through homeostatic mechanisms. The data from thermoregulation experiments that demonstrated the role of neurons in body temperature homeostasis are examined. The second part of this book discusses how organisms allocate the limited energy that is available to them for survival, growth, or reproduction. Excess energy in individuals can translate to growth of populations: if enough remains after survival and growth, it can be allocated to reproduction. However, even closely related organisms may have different strategies for allocating resources that are dependent upon the environmental conditions in which they exist.