A friend of the monarch might soon become a foe. Cardenolides, a chemical in milkweed, are toxic to many predators of monarch butterflies, but monarchs themselves have evolved to tolerate the toxins. Monarchs are able to feed on milkweed and lay their eggs on the plant, giving the larvae more protection against predators. Tropical milkweed in particular has a higher level of cardenolides, which is actually good for these butterflies. Scientists have found monarchs that feed on the tropical variety tend to be heavier and are more likely to survive. However, researchers are also finding that increasing temperatures due to climate change can increase the amount of cardenolides in tropical milkweed. This excess amount may become fatal to feeding monarchs. Higher rates of the chemical could also poison larvae, delay growth, and stunt adults.