The main factor affecting metabolic rates in ectothermic organisms is temperature. Ectotherms, or cold-blooded animals, rely on environmental temperatures to regulate their metabolic processes. As external temperatures rise, their metabolic rates typically increase, allowing for greater activity but also higher energy demands. Conversely, lower temperatures can lead to reduced metabolic rates, affecting growth and reproduction. Thus, temperature fluctuations can profoundly impact the survival and behavior of these organisms in various ecosystems.