MYS­TE­RI­OUS MON­ARCHS

Mi­gra­tion cy­cle of the frag­ile monarch but­ter­fly ex­tends beyond its av­er­age life­span

Down to Earth - - CONTENTS - RAM KUMAR

Mi­gra­tion cy­cle of the frag­ile monarch but­ter­fly ex­tends beyond its av­er­age life­span

THE MONARCH but­ter­fly ( Danaus plex­ip­pus) holds the awe of sci­en­tists as it mi­grates across the North Amer­i­can con­ti­nent de­spite its diminu­tive size and frag­ile na­ture. The time taken to cover the stag­ger­ing dis­tance is well beyond the av­er­age life­span of but­ter­flies— while one gen­er­a­tion mi­grates dur­ing win­ter, the re­turn­ing but­ter­flies are of the next gen­er­a­tion.Yet,they find their mi­gra­tory path and des­ti­na­tion un­err­ingly, a fact that has been ob­served dili­gently by sev­eral re­searchers for the past few decades.

It is widely ac­cepted that the time-com­pen­sated sun com­pass helps but­ter­flies ori­ent them­selves.In sim­ple words,the sun’s rel­a­tive po­si­tion in the sky and its time vari­a­tion in relation to lat­i­tude and lon­gi­tude helps guide the in­sects.But this does not ex­plain how the but­ter­flies man­age to fly de­spite cloudy skies. Even in the ab­sence of di­rec­tional day­light cues, the mi­grants have been ob­served fly­ing in the ex­pected south­ern mi­gra­tory di­rec­tion.

Steven Rep­pert, Pa­trick Guerra and Robert Gegear from the Univer­sity of Mas­sachusetts Med­i­cal School, and the Worces­ter Poly­tech­nic In­sti­tute, Mas­sachusetts, worked on a hy­poth­e­sis that the mon­archs could use a “weather-proof ” mech­a­nism to guide them­selves dur­ing cross con­ti­nen­tal mi­gra­tion from the cold North East US and Canada to Mex­ico to spend their win­ters. “We hy­poth­e­sised that one such back-up mech­a­nism would be the use of a mag­netic com­pass, as found in other longdis­tance mi­gra­tory an­i­mals such as sea tur­tles and birds,”says Guerra.

The re­searchers stud­ied but­ter­flies which were made to fly through a wind tun­nel sur­rounded by a mag­netic coil sys­tem to see the im­pact of this ex­tra mag­netism on their in­ter­nal mag­netic com­pass. The re­searchers found that the an­ten­nae play an im­por­tant role as they con­tain light-sen­si­tive magne-

tosen­sors.The team also re­alised that a mag­netic com­pass is in play in the nav­i­ga­tion and also that this com­pass is light-de­pen­dent.The find­ings were pub­lished on June 24 in Na­ture Com­mu­ni­ca­tions.

Ut­tarak­hand-based lep­i­dopter­ist Peter Smetacek, an au­thor­ity on In­dian but­ter­flies and moths, says the find­ings are re­mark­able and fill the gap in our un­der­stand­ing of the world of but­ter­flies. “Within the south In­dian penin­sula, we have but­ter­fly mi­gra­tions fol­low­ing a pat­tern.We have the Hi­malayan but­ter­flies who de­scend to the plains dur­ing the win­ter,” says Smetacek. “Some species cross into Europe reg­u­larly from North Africa,some like the Drag­on­fly but­ter­fly fly from the Hi­malayas to the Mald Mal­dives, and the next gen­er­a­tion goes to Africa, and then the sub­se­quent gen­er­a­tions re­turn to the Hi­malayas via the Mal­dives,”he adds.

Krush­namegh Kunte, reader er, Na­tional Cen­tre for Biological Sci­ence Sciences, Tata In­sti­tute of Fun­da­men­tal Re­sea Re­search, says,“The mi­gra­tion could only be pos­si­ble p by a com­plex ori­en­ta­tion tec tech­nique which in­cludes the mi­gra­tio tion path be­ing in­grained in their dna. Thi This helps them with­stand the on­slaugh slaught of weather vari­a­tions and hu­man in­ter­fer­ences.”

But a spate of re­cent stud­ies has in­di­cated that hu­man in­ter­fer­ence does af­fect mi­gra­tion. A study on young steel­head trout ( On­corhynchus mykiss) shows that ex­po­sure to iron and steel, ma­te­ri­als used to con­struct hatch­eries, af­fects its nav­i­ga­tion abil­ity. The abil­ity of the fish to nav­i­gate has di­rect ef­fect on its sur­vival, says Nathan F Putman from the depart­ment of fish­eries and wildlife, Ore­gon State Univer­sity, US, who car­ried out this study pub­lished in Bi­ol­ogy Let­ter in June (see “We are caus­ing mag­netic noise”). The study in­di­cates that small dif­fer­ences in the mag­netic en­vi­ron­ment of hatch­eries could help ex­plain why some hatch­ery fish do bet­ter than oth­ers when they are re­leased into the wild.

Smetacek, the au­thor of the book But­ter­flies on the Roof of the World, says, “When we get to know the amaz­ing com­plex­i­ties of na­ture,we learn to ap­pre­ci­ate the need to con­serve what is left.”

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