Interactions between species do not necessarily pair a gain for one participant with a loss for the other. As the example of cacao and midges shows, some interactions benefit both participants, though usually there are still some costs associated with each participant’s benefits. Typical benefits include access to nutrients, shelter from enemies or weather, and direct help in reproduction, but all benefits are ultimately measured by natural selection in terms of reproductive output. Some typical costs include the proteins, fats, and carbohydrates that are invested in building structures such as flowers that attract pollinators, or specialized tissues that house bacteria or algae. Costs can also be energy-consuming activities such as the transport of pollen or seeds, or the loss of food resources used by a partner. When the benefits for each participant outweigh their costs, the interaction is a mutualism.