Timing is everything, especially when you live for just three months and have to undertake a 350 km journey: female Milkweed butterflies wait till their gruelling migrations — from one side of the Indian peninsula to the other — are over, to invest in reproductive tissues for birth. The Indian monsoon, which prompts these species to migrate to drier areas, shapes the physiology of the female butterflies more than they do to males, reveals a study published in Oikos.

India's Milkweed butterflies undertake a fascinating yearly migration. In April-June, just before the onset of India's intensive southwest monsoon, millions of these butterflies migrate from the wet Western Ghats to the relatively drier eastern plains and hills, across distances of 350-500 km. After migration, they 'swarm' in large numbers: hanging around each other and roosting on plants. They then mate, lay eggs and die. The next generation of butterflies flee from the northeast monsoon that now hits the eastern plains and they migrate to the Western Ghats just as the southwest monsoon retreats from there.

While non-migratory butterflies do not have to worry about timing, how do these Milkweed butterflies invest in body tissues: do they invest in flight tissues in the thorax to fly better, or reproductive tissues in the abdomen for birth? Scientist Dr. Krushnamegh Kunte and his student Vaishali Bhaumik of the National Centre for Biological Sciences in Bengaluru measured the thorax and abdomens of 934 individuals of ten Milkweed migratory and non-migratory butterfly species and examined 3,734 individuals across Kerala, Karnataka and Andhra Pradesh to see how they invest physiologically in flight versus reproduction, during and after their migration.

They found that male butterflies across species – whether migratory or not — did not differ in their physiological investments. But the bodies of female migratory butterflies changed drastically: they invested significantly in abdominal tissue in the reproductive phase after migration, without decreasing investment in flight muscles.

“Our results suggest that female butterflies have more to lose if they do not invest optimally in flight over reproduction during migration. A hiker wouldn’t carry an unnecessarily heavy burden during her trek, and neither would a butterfly,” says the lead author Bhaumik.

“We hope to concentrate on climatic, behavioural, and genetic aspects of their migration,” says Dr. Kunte. “If there are genetic differences between the migratory and non-migratory populations, studying them will help us understand this phenomenon better.”