The monarch butterfly (Danaus plexippus) is well known for the enormous distances over which it migrates — sometimes in spectacularly large numbers — during its annual breeding and life cycle. The monarch is an important pollinator species. It is found across North America, from southern Canada to Mexico, and in many other parts of the world, sometimes as an accidental visitor.
The monarch life cycle consists of an egg stage lasting about a week; five larval (caterpillar) stages, during which its length and width increase about ten times; and a pupal stage from which the adult butterfly emerges after about two weeks. The duration of the complete process is temperature-dependent, extending from 25 days in warm summer conditions to 2–3 months in cooler spring weather.
The autumnal migration of butterflies from the north-eastern U.S. and southern Canada involves a journey of 3000 km or more to specific mountaintop sites in Mexico. They overwinter there, in a kind of semi-dormant, non-feeding state called diapause, during which body fat stores are mobilised, before returning north to breed and lay eggs. The entire process is actually one of repopulation and recolonization because the return journey, taken in warmer temperatures, may involve the production of two or three generations.
The western North American population exhibits a similar annual cycle: it overwinters along the coast of California, returning via breeding sites in California and other locations. For both populations, the summer period is a time of continued reproduction and population expansion, although its success is heavily influenced by temperature, rainfall, the availability of lush feeding plants, and the presence of predators and parasites.
Although the two populations can be geographically separated, they are not genetically distinct: there is some interchange between them and individuals in both populations do not undertake the migratory activity. Monarch populations in some locations, such as Florida, appear not to migrate at all.
Clearly the climatic, nutritional and other risks will be different for individuals that migrate and those that do not. This variety in life strategy contributes to great differences in mortality rates and underlies the variations in population numbers that have been observed over many years of study.
The mechanism of monarchs’ navigation is poorly understood, but probably involves a combination of geographical, sun-compass, and food availability signals. It has been suggested that an instinctive navigational memory may be genetically inherited, but this has not been convincingly demonstrated and its biochemical basis remains unexplained.