Meet the Giraffe Weevil. It’s Got a Secret Up Its Long Snout.
The biggest jousting competitions in the world take place on some rotten trees of the New Zealand bush, where hordes of males vie for the opportunity to mate. Knights are not humans but New Zealand giraffe weevils, a species of spear-snouted beetle. Larger males tap each other on the snout until the other withdraws or is unceremoniously knocked down by the bark.
The competition is particularly fierce as male giraffe weevils come in an astonishing range of sizes: the larger male weevil is 30 times the size of the smaller one. In human terms, it would be like having a friend the combined size of two adult giraffes.
As male weevils grow larger, their snouts become disproportionate, which would seem to suggest that larger males use relatively more energy to wield their huge heads. Some biologists have speculated that these exaggerated traits are what they call honest indicators of the animal’s suitability as a potential mate or competitor; according to this logic, a weak momentum would not have the energy to maintain huge horns.
But an article published Friday in the journal Functional Ecology reports that New Zealand’s largest giraffe weevils actually use relatively less energy than their smaller-nosed counterparts, thanks to an energy-efficient anatomical hack.
Ummat Somjee, an evolutionary biologist at the Smithsonian Tropical Research Institute in Panama and author of the article, originally studied leaf-footed bugs in an attempt to understand how much energy the bugs expended on their huge hind legs.
One day, Dr Somjee came across a rotting log in a Panamanian jungle teeming with jousting weevils. “It was like a little war zone on the newspaper,” Dr Somjee said. (The Panamanian species is represented in the tweets embedded below.)
Dr Somjee has learned that these weevils have an even stranger cousin, the New Zealand giraffe weevil. The huge size gap between adult male New Zealand giraffe weevils made the species an ideal candidate for studying the energy costs of a range of muzzle sizes.
He contacted Chrissie Painting, a behavioral ecologist at the University of Waikato in New Zealand and author of the study, who began studying weevils in 2009. In 2019, Dr Painting drove Dr Somjee to his place of secret collection, which she calls “the city of weevils”.
If surprised, a New Zealand giraffe weevil will jump out of a tree. To collect the weevils en masse, Dr. Painting held a container of ice cream under the tree, waved his hand, and watched the weevils simultaneously turn over in the container.
Back in Dr. Painting’s lab, the researchers measured the metabolic rates of their weevil medley. They placed the weevils in sealed glass syringes – their long bodies seemed tailor-made for the tubes – and measured their oxygen depletion while at rest. They also placed the weevils on tiny treadmills to measure the energy expended during a fast trot. Both results revealed that the larger weevils used less energy per gram of tissue than the smaller weevils. In other words, 30 small weevils would use considerably more energy than the larger weevils, which are the size of 30 small weevils.
These results may seem confusing: Wouldn’t a male polar-headed weevil use a lot more energy? The researchers beheaded their weevils, dried the buds in an oven, and placed them in a solution to digest living tissue but preserve the cuticle, a hard, inert tissue.
They found that larger male weevils have snouts and legs that contain more and more cuticles and less metabolically active tissue than smaller males. “It’s kind of a really cheap way to get fat,” Dr Somjee said. In fact, the larger males, smaller males, and females all spent roughly the same percentage of their snouts on soft, energy-consuming tissue.
The researchers compared their own metabolic data to a range of other insect species, as there were no other insects of this size. The pattern they found in their own data – that larger weevils used relatively less energy per gram of tissue – was consistent across species.
Douglas Emlen, a University of Montana biologist who was not involved in the research, says the new paper validates the long-held theory that exaggerated guns are honest traits; for example, a bigger snout means a stronger weevil.
But the new research also shows that guns are disproportionately expensive for little men, Dr Emlen said.
“We all focus on the really big bucks,” he said. “But what matters is whether the little guys can pretend.” They can’t, he adds, and that makes a big muzzle an honest signal of a weevil’s muscular physique.
Adult weevils do not grow. “If you emerge as a little buck, there is no turning back,” said Dr Painting. So, the smaller weevils have developed an alternative mating strategy to compensate for their stocky sniffers: they squeeze under the larger males to mate with the females. But they still don’t do as well as their bigger rivals and also die younger, Dr Painting said.
Medium-sized weevils also struggle, too small to joust and too big to squeeze in.
“There is nothing right in nature,” said Dr Emlen. “The rich get richer and the poor get poorer. “
Alas, the beetles that have the short end of the snout really seem to be the lesser of two weevils.
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