The Heliconidae are a small group of Neotropical butterflies with some remarkable features that make them unique amongst the 30,000 or so species of butterfly found worldwide.
Almost all adult butterflies are nectar feeders. This is an excellent source of carbohydrate, and flowers are a convenient and accessible food source. However without protein to repair tissue damage all butterflies are condemned to a short life span. By developing the biochemistry to digest pollen, the Heliconidae have been able to access an ubiquitous source of protein. They have a life span measured in months rather than days.
As a consequence of greater longevity, the females can spend much longer searching for suitable food plant on which to lay their eggs. The larvae of all the Heliconidae feed on Passiflora species. Some species of Heliconius are generalists and will oviposit on several species of Passiflora, others are specialists and their larvae will eat the leaves of only one species. Passiflora contain chemical compounds toxic to a variety of vertebrate and invertebrate herbivores. The Heliconidae are able to detoxify these poisons and to sequester them in their own body tissue. The adult butterfly is thus toxic to potential butterfly predators including birds, reptiles and amphibians.
The poisonous adult butterfly does not need to be able to fly rapidly to avoid avian predators. On the contrary it is able to fly slowly around the forest, looking for a mate, or carefully checking out nectar sources and oviposition sites. This slow flight pattern demands less energy and has two consequences. Firstly it means the adult butterfly needs to spend a smaller proportion of its active life searching for food. Secondly it means that the Heliconidae do not need bulky thoracic muscles to power their wings. This in turn means that the larvae do not have to spend so long feeding and growing large. The time spent as a caterpillar can afford to be short. Caterpillars are vulnerable to parasitism, disease and predation and a short larval stage is an advantage in a hostile environment.
Being poisonous is no use without your potential predators knowing it. The wings of the Heliconidae are brightly coloured, often using primary colours in simple easily recognisable designs. Although some Heliconidae have more complicated colour patterns, no species are cryptic. The colour patterns advertise clearly to predators that they are poisonous. The colours are not, however, random. In fact the patterns are strongly related to patterns commonly found amongst other poisonous species of butterfly. This is called mimicry. Although the patterns used to advertise toxicity vary throughout South America, in any small region different species share the same mimetic pattern. Almost all the 50 or so species of Heliconidae are involved in mimicry. There are strong mimetic relationships within the Heliconidae themselves and extensive mimetic relationships with other poisonous butterflies (particularly the Ithomiidae) and day-flying moths. Some mimicry rings involve over a dozen different species.
Observations of Heliconidae in captivity suggest that they have good colour vision and are able to recognise artificial colour patterns to locate food sources on a repeated basis. In the forest individuals have been observed visiting the same wild nectar sources on a systematic daily basis. This suggests that their longevity is associated with good memory and complex behaviour patterns.
Some species of Heliconidae have been observed roosting in large groups at night. It is unclear how this happens but the same behaviour can be seen in captivity. Presumably these assemblies offer some additional protection against predation perhaps from amphibians or reptiles high in the forest canopy.
Some species of Heliconidae exhibit the unusual behaviour pattern of pupal mating. Males will start to assemble near a female pupa that is close to emerging. In some species the males will wait to mate with the female as soon as she has emerged, but in others the male will insert his abdomen into the pupa before the female has fully emerged. Some of the mimicry between species of Heliconidae is so good that it is almost impossible to tell them apart. It has been suggested that pupal mating has evolved with the need to minimise the possibility of mating errors between pairs of very similar mimetic Heliconidae. If one species of a mimetic pair uses pupal mating, the other can mate normally and both species can then inhabit the same environment. It is unclear whether the mimetic pairs or the pupal mating came first.
Heliconidae have proved relatively easy to breed in captivity. For almost 40 years they have been used in research into genetics. The dramatic colour patterns enable scientists to ask questions about how rapidly species are evolving and about how new species might form. Recent evidence has, for instance, shown that hybridization between species occurs in the wild on a regular but infrequent basis. This may allow distinctive mimetic patterns to spread very much more quickly than if each species were left to develop its own patterns in isolation. Far from being just pretty things to admire, Heliconius butterflies are helping scientists to answer some of the most difficult questions about evolution and the formation of new species.
With kind permission of the author, Dr Yealand Kalfayan