Every year from August to October, one of the most wonderous events of the natural world happens: the migration of eastern monarch butterflies (Danaus plexippus). These butterflies migrate south in vast numbers for over 4000 km, from southern Canada to their overwintering grounds in central Mexico using the sun as a compass. As impressive as this sight is, it's even more amazing that these butterflies need to do so under very stable weather conditions, including very little rain, low winds, the right temperature and moderate levels of humidity. In short, they require the very things that climate change is set to drastically alter. This realization led Adam Parlin, Mitchell Kendzel, Theresa Culley, Stephen Matter and Patrick Guerra of the University of Cincinnati, USA, along with Orley Taylor of the University of Kansas, USA, to question whether the warming and less predictable environment will affect this migration.
First, the researchers needed to measure how much energy the butterflies use when they fly versus when they are resting; this would allow them to calculate how much energy the butterflies use per day during their migration. The team used data from previous research in their computer model and discovered that these butterflies are using a shocking 25–31 times more energy when they are flapping their wings than they do when they're resting. While the butterflies only flap their wings ∼15% of the time that they are flying, this is still an enormous amount of energy that is required for them to make it to their final destination. Not only do they need to make it all the way to Mexico but also they need to have the energy reserves to survive once they've arrived. This means that the butterflies must conserve as much energy as possible during their lengthy trip. So how do rising temperatures and inclement weather affect how much energy they use on their migration?
It turns out that how much energy a butterfly burns while resting is determined by the temperature; the warmer the temperature, the more energy it burns. Parlin and the team looked back at historical data from over 50 years ago and found that the metabolic rate of these flying insects was increasing during the autumn migration and so was the temperature. Perhaps more importantly, the night-time temperatures were getting warmer as well. This means that while the butterflies are supposed to be resting and recovering for the next leg of their journey, they are having to expend more energy than they used to, just to relax, causing them to burn through their energy stores even faster.
The team then used a computer to model how long the butterflies could fly for before having to stop. Historically, the butterflies could fly for an estimated 880 h before they had to stop and refuel, but with today's temperatures and extreme weather, the butterflies could only fly for 750 h. This may not seem like a big deal, but it causes the butterflies to spend more days migrating and, therefore, expending huge amounts of energy for longer than they would have in the past. Parlin and colleagues also point out that the butterflies will be more reliant on food sources that may not exist if they need to stop in new areas or could also be decimated by climate change. One thing is for certain, if climate change keeps making the temperature warmer and the weather more unpredictable, we might lose this amazing natural event for good.
