measured in calories can be used as a universal measure of nutritional cost is criticized by a number of scientists on the basis of essential nutrients, nutrients that the body cannot produce regardless of calorie availability and the specific nutrients must be present in the diet. It is argued that since there are different dietary conditions in which different essential nutrients are the most scarce in different regions and the few foods that contain the scarcest nutrients that are needed to avoid deficit diseases are therefore the most expensive (the cost may be paid in the form of other goods and services in societies without money), and different functions in the body primarily consume different essential nutrients, no universal ranking of the costs of different aspects of reproduction can be made. For example, it is possible for the few micronutrients that men consume more of the more sperm they produce but the consumption of which does not increase in women during pregnancy or lactation to be the scarcest nutrients contained in the most expensive food in some societies, making sperm production effectively more expensive than pregnancy and lactation under local food prices in such societies. It is also argued that the variability of what food is the most valuable due to containing the rarest essential nutrients extend their effects to the economical significance ratio between hunting and gathering in the case of hunter-gatherer societies, and therefore that any attempt to circumvent the evolutionary psychology paradox of men not being able to be in two places at the same time to hunt and protect his family by reference to hiring guards by bartering meat would fail to make sex roles universal due to the difference between regions where the rarest essential nutrients were contained in one or more types of meat and regions where the rarest such nutrients were contained in some types of plants. It is cited in this context that humans evolved over relatively large parts of Africa with different food ecologies, making it impossible for humans to have specialized evolutionarily for one specific food cost ratio. This variability of food value ratios within Africa may have prepared humans evolutionarily to be able to leave Africa. Biodemography and human reproduction The researchers involved in human reproductive ecology use the combined approach of demography and evolutionary biology to explain the reproductive phenomenon. Biodemography is the study of demography related to