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Female monarchs on the decline

23 September 2009

It's one of nature's most spectacular phenomena. Each autumn, millions of bright orange, Monarch butterflies migrate from Canada and the US to Mexico to sit out the cold winter months. But new research, published this week in the Royal Society Journal Biology Letters, has revealed how this much studied species could now be in jeopardy.

Ever since scientists discovered the migration sites in central Mexico where the monarchs travel to over 30 years ago, the butterflies have been studied for all sorts of research from the effects of bird predation to mating behaviour. This research often involves comparing numbers of males and females, and now Dr Andrew Davis, from the University of Georgia, and Dr Eduardo Rendon-Salinas from the WWF in Mexico, have taken a closer look at all this data to reveal some alarming results.

By analysing almost 70,000 butterfly records, gathered over 30 years, the pair of scientists discovered that the percentage of females is steadily declining, and has dropped by ten percent in that time. "This discovery was certainly unexpected and is not readily explainable," say the authors.

This decline in females is likely to be the work of the parasite Ophryosistis elekreoscirrha which tends to affect females more than males, and has been prevalent in overwintering sites in the last 5 years. The alarming trend needs further research, say the authors but it also highlights the many questions that remain to be answered about the biology of this unique phenomenon, which is at risk of disappearing in the near future.