The production and release of this alarm substance are altruistic because the sender of the signal, who does not directly benefit from the signal released upon its injury, has to pay the cost for the production and release of the chemical. In fact, the alarm substance cells decrease in number when the common minnows are in poor physical condition due to scarce food, indicating that there is metabolic cost for producing and maintaining the specialized cells.[9] The apparent altruistic behavior is not clearly understood, because the likely explanation of kin selection is not supported by the shoal structure of common minnows in which shoalmates are not necessarily closely related.[10]