The extra energy it takes a human to produce a baby could be as much as 24 times more than some influential mathematical models had estimated, suggest Marshall and his colleagues in a study published in May1. For animals that rely on external heat sources to regulate their body temperature, called ectotherms, the difference is smaller. For the pampas snake (Tomodon dorsatus), for example, the value is around four times more. And for a species of seawater fish called the capelin (Mallotus villosus), it is twice more.
In mammals, which often grow placentas to provide oxygen and nutrients and to remove waste, and which maintain a stable internal body temperature, the researchers found that the indirect costs made up roughly 90% of the total energy costs of reproduction. Just 10% of the total energy is contained in the offspring. In humans, 96% of the 208,000 kilojoules (or nearly 50,000 kilocalories) required for reproduction is taken up by indirect costs.