Protein-eating butterflies and sex-changing plants

March 14, 2011

Everyone needs protein as part of a healthy diet:  proteins provide the building blocks for cells and are critical for the body’s proper growth and development.  Did you know that butterflies need proteins, too?  Flower nectar contains trace amounts of amino acids (the components of proteins), and that is enough for most butterflies.  You may not know that we lace the artificial nectar (sugar water) in the feeding stations in the Butterfly Center with a dollop of “body builder’s amino fuel”—a dietary supplement touted as containing all 20 amino acids.  This is our way of attempting to provide a “balanced” diet for our butterflies—I like to say that we give them Gatorade rather than Coca-Cola…

But some butterflies need more protein than others.  The longwing butterflies in the genus Heliconius, those elongated, slow-fluttering, brightly colored butterflies so common in the Butterfly Center—we import over a dozen different species from our various suppliers—are unique among Lepidoptera in their habit of collecting and eating pollen, which contains a lot more protein than does nectar alone.  If you look carefully at the butterflies next time you’re in the Center, you may occasionally notice a longwing with a blob of yellowish “stuff” on its proboscis.  This is the pollen that it has collected as it visits various flowers for nectar.  Once it has enough of a “pollen load,” the butterfly will sit and regurgitate a bit of nectar onto the pollen and then slurp up the dissolved amino acids.  As a result of this protein-enhanced diet, longwings can live much longer than many butterflies, and female longwings continue to generate eggs over their entire adult life (shorter-lived butterflies emerge from the pupa with all of their eggs fully formed).  UT Austin’s Dr. Larry Gilbert, who discovered pollen feeding in the longwings, and who has made a career out of studying their behavior, has records of individuals in the wild living up to six months.  Alas, we have not recorded quite such long lifespans in the Center—but in our mark-recapture studies, longwings typically outlive most other species.

Creative Commons License photo credit: Zac Stayton

The accompanying photos show a Melpomene longwing (Heliconius melpomene) probing the flowers of a rainforest cucumber vine, the genus Psiguria, and later sitting to digest the pollen. While longwings will collect pollen from a number of flowers in the Butterfly Center, this is their absolute favorite pollen source, and in their native rainforest habitat, longwings are the main pollinators of this group of plants.  Psiguria and the closely related Gurania, members of the same family as cucumbers and watermelons (Cucurbitaceae), are long-lived lianas that grow up into the rainforest canopy.  In nature longwings cruise from liana to liana, sometimes over several miles, searching for flowering individuals and stocking up on their pollen.  The interdependence between these two groups—longwing butterflies and rainforest cucumber vines—is so strong that it is often cited as a “textbook” example of co-evolution.

Creative Commons License photo credit: Zac Stayton
Hodgsonia heteroclita male plant
of the plant family Cucurbitaceae

Interestingly, these vines can switch sexes, just as some fish do.  Larger vines produce only female flowers—their flowers produce no pollen but must be pollinated in order to produce the cucumber-like fruit.  Female plants depend on the longwings to bring pollen from smaller vines that bear male, pollen-bearing flowers.  Until one of Dr. Gilbert’s students discovered that the same vine makes flowers of different sexes depending on its age and/or size, people thought that Psiguria and Gurania were dioecious plants (i.e., that an individual plant was either male or female—like mulberries and ginkgo trees).  We now know that rainforest cucumbers are monoecious—but produce their male and female flowers at different times, rather than in different places on the same plant (as in corn and pine trees).

When the Psiguria vines in the Butterfly Center get large, they produce only female flowers.  Without pollen, these aren’t as good for the butterflies, thus we are learning to keep the vines pruned back so they will produce male flowers.  Unlike fish, plants can get larger or smaller—so size, rather than age or population structure, determines the sex of the flowers in these rainforest lianas.

Next time you visit the Center, be sure to look for longwings stocking up on pollen.  One of the Psiguria vines grows adjacent to the “frog log” and is currently (spring 2011) producing male flowers.

Authored By Nancy Greig

Dr. Nancy Greig is the founding director of the Cockrell Butterfly Center, which she oversaw from 1994 to 2016. As emeritus director she continues to work with the museum doing outreach and education. Her academic training is in botany and entomology, with a specialty in the interaction between insects, especially butterflies, and plants. In addition to cultivating backyard butterflies, she grows vegetables and bees

One response to “Protein-eating butterflies and sex-changing plants”

  1. Very cool! There are a lot of postman butterflies in the butterfly haven at the museum where I volunteer. This’ll be a new detail I’ll be sharing.

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