Basically everything in ecology comes down to one basic equation:
Energy In = Energy out
Plus or minus a bit of storage (just ask my thighs, they know all about that).
In animals energy in refers to food, and energy out is anything from tissue repair and growth to movement and reproduction. Importantly energy out can also be heat. Historically ecologists have thought of the "energy in" side of this equation as the limiting step. However, more recently there is growing evidence that "energy out" may sometimes be to blame. Animals are not always in food limited environments, so why not just eat more, do more, and have a higher reproductive output? Most likely animals are constantly shuttling between input and output limited states.
Speakman and colleagues (2010)* discuss the evidence for this idea. They thought, "hey let's try and force lab rodents to eat more by putting them in the most energetically demanding situations possible." Lactation is one of the most energetically challenging things a mammal will do in its life. I heard on the grapevine that human mothers expend an equal amount of energy breastfeeding as they would if they ran a marathon everyday. EVERYDAY. Now I can't say I have fact checked that one, but instead of relying on lactation alone to challenge the rats, these researchers also tried artificially increasing litter size, making them pregnant at the same time, and even forcing them to exercise (ok now I'm imagining human breastfeeding mothers actually running daily marathons!).
Figure 1.
But in none of these situations did the rats increase food intake (Figure 1). The only situations that made the rats increase how much food they ate were putting them in the cold, or shaving them (Figure 2)!
Figure 2.
What do these two treatments have in common? They allow excess body heat to be dissipated into the environment more readily! They called this idea "Heat Dissipation Limitation". That sometimes, it is the rate at which excess heat can be dissipated into the environment that limits the rate at which the process generating the heat can take place, and this idea could potentially apply to any heat generating process in the body!
*Speakman JR, Król E. Maximal heat dissipation capacity and hyperthermia risk: Neglected key factors in the ecology of endotherms. J Anim Ecol 2010.(doi:10.1111/j.1365-2656.2010.01689.x).