When a fig tree falls in the Panamanian rainforest, it calls an army of strangely shaped insects to war.
Known as jousting weevils, these beetles have heads shaped like a knight’s lance, which males use to flip each other off logs as they fight over females.
“The tree releases volatiles and beetles from the forest literally emerge out of the woodwork and fly onto that tree, because this is a place where they can lay their eggs,” says Ummat Somjee, an evolutionary biologist at the University of Texas at Austin and a research associate at the Smithsonian Tropical Research Institute.
Once a male wins over a female, they mate, and then he lays his face across her body to guard her against the pursuit of other males while she drills a hole in the tree and lays her fertilized eggs inside.
Jousting weevils are impressive enough, but it turns out they have another whopping distinction: Possessing one of the biggest ranges of body sizes on Earth. Only the closely related giraffe weevil has a greater disparity. (Read more: “Choose your weapon: How animals got their tusks, horns, and antlers.”)
When Somjee and his colleagues collected jousting weevils from the wild and measured them, they found “the smallest individual can be about 26 times smaller in mass than the largest individual,” he says.
For reference, when the scientists looked across 11 orders of other insects, a more typical size variation between adults was about three times. Even when they expanded their search to vertebrates such as mammals and birds, no variation within a single species came anywhere close, according to a study published June 20 in the journal Evolution.
Two large males fight over a female, who is using her lance to drill into the wood to lay her eggs. A small male is hiding underneath her.
Photograph By Ummat Somjee
A ‘war zone’ writ small
Most of this variation comes from the size of their lances: The largest males sport weapons that can account for up to 69 percent of their body length.
“You see these large males walking around, it’s almost awkward,” says Somjee, who led the research. “They’re having a hard time stepping over things.” (See “These stunning insect close-ups reveal dazzling bug complexity.”)
This would be like a boxer having a fist more than half as big as their body, he says.
And in a world full of miniature giants, you might think that only the largest males successfully breed with the females. Not so.
Surprisingly, tiny males sometimes mate with giant females, and large males sometimes mate with tiny females, Somjee says.
In fact, Somjee described a scene in which a very large male laid its head across the body of a female he had just mated with. But then a much smaller male crept in and appeared to mate with the female before the larger male knew what was going on. Once the larger male clued in, though, he sent the little guy flying with a flick of his lance.
Intrigued, the scientists discovered smaller males pack a different sort of weapon—much larger testes, which the scientist suspect allows them to transfer more sperm than larger males.
This hyper-competitiveness likely stems from the scarcity of fallen trees.
“A tree doesn’t fall very often, and when it does, there are beetles in the surrounding area that are all looking for a chance to exploit that resource,” says Somjee, who notes that there can be something like 400 beetles per square meter.
“They can reach these very, very extreme densities. And it’s a war zone.”
A graphic shows size differences between males and females. Unlike other insect species, females also sport headgear, which they use to fight other females.
Photograph By Peter Marting
Females fight, too
While males in many species sport the craziest headgear, in jousting weevils at least, the females are also armed.
Even as males are fighting over access to females, females actively duel with each other over the most prized egg-laying spots, says Somjee. And just as with the males, females can vary in size up to 22 times between the most tiny and most gargantuan.
Once a female weevil finds a nice location, she then uses her head weapon to drill a hole in the bark—meaning its useful in two scenarios.
“We talk about male weapons and how crazy and big they are, and that’s definitely something more well understood when it comes to sexual selection,” says Nicole Lopez, an evolutionary biologist and Ph.D. candidate at the University of Montana. “So I’m really happy that they included the females in it.” (Related: “As males evolve to have better weapons, females develop bigger brains.”)
Lopez, who was not involved in the research, says the new study offers a new glimpse into how evolution can push animal weapons to extremes.
“There’s been a lot of papers that look at developmental trade-offs and constraints,” says Lopez. “But being able to look at it within a species, naturally, is pretty unique.”
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