Bioaccumulation is the concentration of toxic substances in animals that are higher up in the food chain. It occurs when chemicals or elements that cannot be excreted in urine or feces are ingested by a small animal and eaten by a larger one. It commonly happens with pesticides, other organic chemicals and mercury. Since predators generally eat about ten times the weight of prey for each unit of weight they gain, each step up the food chain concentrates the toxin by a similar amount. In a complex food chain, this could result in a predator having a ratio of toxins to body weight hundreds or even thousands of times greater of the animals at the bottom of the food chain.