Male Zebra Butterfly is pupae-sniffing cradle-snatcher

Male zebra longwing butterflies sit on the pupae of females for up to 10 days before they hatch, ensuring they get first dibs at reproducing with them. But how do they know that they're sitting on a female?

It turns out that the female pupae of Heliconius charithonia emit a pheromone as they approach puberty that betrays their gender before they've hatched. This allows males to perch on them, waiting for them to hatch. As soon as the females emerge, the males fertilise them.

Catalina Estrada at the University of Texas in Austin, and colleagues, extracted the chemicals found in the cuticles of 10 pupae and volatile organic compounds released by 12 other pupae in their laboratory.

The only chemical that was unique to female pupae was the pheromone monoterpene linalool oxide. Males emitted a variant, linalool.

The researchers then painted pupae with both odours. Male butterflies rarely perched on female pupae that had been daubed in male-specific linalool. In contrast, they happily re-perched on pupae daubed in linalool oxide 65 per cent of the time. This was almost as often as they returned to untouched female pupae.

"This paper offers insights into an unusual mating behaviour in Heliconius butterflies," says Chris Jiggins, who studies the species at the University of Cambridge.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2009.1476