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How do koalas and monarch butterflies use chemical cues to identify their specialized foods?

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A pine grosbeak with pink and gray plumage perches on a branch, eating red berries among autumn foliage. — Chemoreception Animals, such as this pine grosbeak, use chemoreception to respond to chemicals that can be tasted and smelled in their environments. Many of these chemicals affect behaviors such as food preference and defense.

Specialized feeders like koalas and monarch butterflies rely on specific chemical cues to identify their host plants, as their survival depends on these unique food sources.1

Monarch Butterflies

Monarchs are highly specialized, with caterpillars feeding exclusively on milkweed (Asclepias). Adult butterflies use their chemosensory systems to locate these plants for egg-laying. They possess specialized receptor cells, often located on their tarsi (feet), that allow them to detect host-specific chemicals upon contact. This process ensures that the butterfly identifies a taxonomically appropriate plant, rather than simply assessing its nutritional value.234

Koalas

Koalas are also specialized feeders, consuming the leaves of certain eucalyptus trees. Because eucalyptus leaves contain complex, toxic substances like tannins, koalas must use their sense of smell and taste to navigate their diet. While they have evolved an intestinal pouch containing symbiotic bacteria to degrade these toxins, their ability to select appropriate leaves is guided by the specific mixture of volatile and nonvolatile chemicals present in the foliage.51

In both cases, these animals are specifically attracted to or stimulated by the distinct chemical profiles of their respective host plants.1