Understanding the differences in how living beings grow, reproduce and survive is one of the key goals of evolutionary biologists. For example, animals often have to choose between living longer and voyaging over great distances or producing future generations, which can take its toll. To explain how animals trade off life expectancy against the cost of living, biologists have proposed many theories. One idea is that toxic chemicals, known as reactive oxygen species – produced as a by-product of energy-intensive activities – damage cells, decreasing lifespans. However, an alternative theory suggests that these damaging toxins actually activate cellular mechanisms to enhance survival. To resolve which of the two strategies animals may use in practice to enhance their longevity, Alexander Shephard, Amanda Hund and Emilie Snell-Rood at the University of Minnesota, USA, tested the ideas on monarch butterflies (Danaus plexippus), which undertake an immense autumn migration each year from North America to return to their winter roosts in Mexico. During this extraordinary voyage the insects that live eight times longer than their predecessor generations, which made the outbound journey, endure enormous amounts of stress caused by their extreme exertions, yet must conserve sufficient energy to allow them to reproduce when they reach their destination.
To assess whether migration stress affects monarch lifespan, the scientists conducted two experiments. In the initial set, they compelled the monarchs to fly in a confined space for 5 min each day over a 4-day period. This limited space made the butterflies take off more frequently, inducing stress and causing them to use more energy than they would use normally if flying uninterrupted in an open environment. Then they monitored the insects for the rest of their lives, keeping track of how long they survived and how many offspring they produced, while also checking for evidence of damage caused by stress in the males’ testes.
The team discovered that flying more improved the butterflies’ chances of staying alive. They lived longer, averaging 55 days instead of 46, and when the team looked for evidence of the cellular damage that you would expect as a result the insects’ exertions, they found none. However, the levels of stress-fighting proteins that the team found in the flight muscles was high. The insects were prioritizing cell maintenance to protect themselves from the damaging effects of flight. And when the team looked to find out how the period of intense flight had affected the butterflies’ ability to reproduce, they found that although the males’ testes were unaffected, females produced an average of ∼54 eggs, compared with ∼97 eggs when the females were flying normally. This suggests that flying for long periods stresses the insects, reducing the number of eggs and offspring down the line.
In the next set of experiments, the researchers applied a drug called methoprene to the abdomen of the butterflies to cause the females to produce eggs in early adulthood. Then the team kept track of the insects’ longevity, the number of offspring they produced and looked for evidence of physical stress, to test whether reproduction increased physical damage and impacted the insect's life expectancy. On average, the methoprene-treated butterflies produced ∼180 eggs; however, they had the same lifespan (42 days) as untreated females that produced no eggs when young. So, early reproduction did not seem to affect the butterflies’ longevity. In addition, the methoprene-treated females that flew more in early adulthood had fewer offspring because they had redirected internal resources towards survival.
This study by Shephard and colleagues suggests that investing energy in flight helps migratory monarchs to stay alive for longer by enhancing the cellular mechanisms that protect them from the damaging effects of intense exercise. It seems that switching to a self-care mode is the secret to a long life for daring monarch butterflies when embarking on their epic migration to their southern winter homes.
