New research, published in the journal Behavioral Ecology and Sociobiology, shows that Asian honeybees (Apis cerana japonica) perform an act of incredible self-sacrifice when confronted with an unwelcome guest.

The Japanese giant hornet (Vespa mandarinia japonica) is a particularly nasty predator that can massacre as many as 40 Asian honeybees in a single minute. During the autumn months, when the hornets are at work tending to their larvae, beehives may have to withstand as many as 30 hornet attacks a week. A bee's weapon-of-choice – its stinger – is hopeless against the hornet's rigid exoskeleton, so the Asian honeybee has had to devise a new tactic.

In order to survive the annual barrage of hornet attacks, the honeybees perform an unusual but incredibly altruistic behavior known as “hot defensive bee ball formation”. Hundreds of worker honeybees will swarm the hornet while vibrating their wings to amp up the heat. The entire process can go on for more than 30 minutes, during which time temperatures inside the ball can climb to 46°C (115°F). It is essentially death by heat as far as the hornet is concerned.

These defensive ball formations were originally described in 1995, and since then, researchers have analyzed the neural mechanisms behind the extraordinary behavior. Now, a team of researchers led by Atsushi Ugajin, an entomologist at Tamagawa University, Japan, has shown that all this swarming and vibrating comes at a great personal cost for the honeybees involved.

To find out what effect the ball formation has on the honeybees' lifespan, the researchers compared the survival rates of participating honeybees to those of non-participating honeybees. All honeybees, regardless of what group they fell into, were the same age at the experiment's start (15 to 20 days) to keep things fair.

Following an attack, the honeybees that formed balls were dead within 10 days. In contrast, those that did not participate in the ball formation and were instead kept in the hive at 32°C (90°F) were all dead within 16 days. (For reference, the average lifespan for an Asian worker honeybee is a few weeks.)

The team was also curious to see what would happen when there was more than one attack, as there often is out in the wild. They noticed that the honeybees involved in the ball forming in the first attack were far more likely to join the ball forming in the second attack. And while the researchers can't say why this is for sure, they suspect this self-sacrificing behavior helps reduce the cost that the ball formations have on the colony by limiting the number of individual honeybees involved in the behavior.

To watch the bees in action, check out this video:

[H/T: New Scientist]