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Butterflies “treat” sick young

Oct. 11, 2010
Courtesy of Emory University
and World Science staff

Mon­arch but­ter­flies seem to “treat” their un­born offs­pring for a parasitic in­fect­ion by lay­ing the eggs on a plant that helps fight the ill­ness, a new study sug­gests.

In­fected but­ter­flies “pre­fer to lay their eggs on plants that will make their off­spring less sick, sug­gest­ing that monar­chs have evolved the abil­ity to med­i­cate their off­spring,” said Jaap de Roode, an ev­o­lu­tion­ary bi­ol­o­gist with Em­o­ry Uni­vers­ity in At­lan­ta, Ga., who led the re­search.

Mon­arch but­ter­flies seem to use plants to treat their off­spring for a type of in­fec­tion, bi­ol­o­gists are claim­ing. (Photo by Ran­dy Lof­tus, USFWS)

Var­i­ous stud­ies have point­ed to ev­i­dence of apes or oth­er mam­mals “self-medicating,” but the dis­cov­ery of such be­hav­ior in in­sects would be new. De Roode and col­leagues pub­lished a re­port on the find­ings Oct. 6 in the the re­search jour­nal Ecol­o­gy Let­ters.

Mon­arch but­ter­flies are known for their spec­tac­u­lar migra­t­ion from the Un­ited States to Mex­i­co each year, and for the strik­ing pat­tern of or­ange, black and white on their wings. The bright col­ors are a warn­ing to birds and oth­er preda­tors that the but­terfly may be poi­son­ous.

In their young, cat­er­pil­lar stage, monar­chs feed on any of doz­ens of spe­cies of milk­weed plants, in­clud­ing some spe­cies that con­tain high lev­els of car­de­no­lides. These chem­i­cals don’t harm the cat­er­pil­lars, but make them tox­ic to preda­tors even af­ter they emerge as adults.

A monarch but­ter­fly ca­t­er­pil­lar crawls over a milk­weed plant. Mon­ar­chs feed on any of doz­ens of spe­cies of milk­weed plants, in­clud­ing some spe­cies that con­tain high lev­els of chemi­cals that make them tox­ic to pred­a­tors. (Image cour­tesy USDA)

Pre­vi­ous re­search has fo­cused on wheth­er the but­ter­flies choose more tox­ic spe­cies of milk­weed to ward off preda­tors. De Roode won­dered if the choice could be re­lat­ed to the Ophry­ocys­tis elek­troscir­rha, par­a­sites that in­vade the gut of monar­chs when they are in the young cat­er­pil­lar stage and stay in­to the in­sects’ adult­hood. The par­a­sites sap the but­ter­flies’ en­er­gy, short­en their lives, and some­times kill them, their flu­ids ooz­ing out of their bod­ies. 

An in­fected fe­male that lays eggs passes on the par­a­sites to her off­spring. But ex­pe­ri­ments in de Rood­e’s lab, he said, re­vealed that in­fected fe­males, un­like oth­ers, show a spe­cif­ic pref­er­ence for lay­ing eggs on tox­ic milk­weed.

Few stud­ies have been done on self-medica­t­ion by an­i­mals, he added, but some sci­en­tists have the­o­rized that the prac­tice may be more wide­spread than we real­ize.

“The re­sults are al­so ex­cit­ing be­cause the be­hav­ior is trans-genera­t­ion­al,” said Thierry Le­fevre, a post-doctoral fel­low in de Rood­e’s lab. “While the moth­er is ex­press­ing the be­hav­ior, only her off­spring ben­e­fit.”

The find­ings al­so may have im­plica­t­ions for hu­man health, said Uni­vers­ity of Mich­i­gan chem­i­cal ecol­o­gist Mark Hunt­er, who col­la­bo­rat­ed with de Rood­e’s group. “S­tudy­ing or­gan­isms en­gaged in self-medica­t­ion gives us a clue as to what com­pounds might be worth in­ves­ti­gat­ing for their po­ten­tial as hu­man medicines.”

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Monarch butterflies seem to use plants to treat their offspring for a type of infection, biologists are claiming. Butterflies infected with a parasite “prefer to lay their eggs on plants that will make their offspring less sick, suggesting that monarchs have evolved the ability to medicate their offspring,” said Jaap de Roode, an evolutionary biologist with Emory University in Atlanta, Ga., who led the study. Various studies have pointed to evidence of apes or other mammals “self-medicating,” but the discovery of such behavior in insects would be new. De Roode and colleagues published a report on the findings Oct. 6 in the the research journal Ecology Letters. Monarch butterflies are known for their spectacular migration from the United States to Mexico each year, and for the striking pattern of orange, black and white on their wings. The bright colors are a warning to birds and other predators that the butterfly may be poisonous. In their young, caterpillar stage, monarchs feed on any of dozens of species of milkweed plants, including some species that contain high levels of cardenolides. These chemicals don’t harm the caterpillars, but make them toxic to predators even after they emerge as adults. Previous research has focused on whether the butterflies choose more toxic species of milkweed to ward off predators. De Roode wondered if the choice could be related to the Ophryocystis elektroscirrha, parasites that invade the gut of monarchs when they are in the young caterpillar stage and stay into the insects’ adulthood. The parasites sap the butterflies’ energy, shorten their lives, and sometimes kill them, their fluids oozing out of their bodies. An infected female that lays eggs passes on the parasites to her offspring. But experiments in de Roode’s lab, he said, revealed that infected females, unlike others, show a specific preference for laying eggs on toxic milkweed. Few studies have been done on self-medication by animals, he added, but some scientists have theorized that the practice may be more widespread than we realize. “The results are also exciting because the behavior is trans-generational,” said Thierry Lefevre, a post-doctoral fellow in de Roode’s lab. “While the mother is expressing the behavior, only her offspring benefit.” The findings also may have implications for human health, said University of Michigan chemical ecologist Mark Hunter, who collaborated with de Roode’s group. “Studying organisms engaged in self-medication gives us a clue as to what compounds might be worth investigating for their potential as human medicines.”