Mysteries of a golden beetle
STRI/DICYT As the sun set over the cloud forest in western Panama, Lynette Strickland still hadn’t found what she was looking for. Strickland spent all day exploring Fortuna, a mountainous area spanning the Continental Divide near the border of Panama and Costa Rica, in search of a glimmering golden beetle. But each time she flipped over a leaf or asked Smithsonian beetle expert Don Windsor ‘Is that it?’ the answer was ‘No.’
With time running down on her field trip to one of Panama's most picturesque forest reserves, Strickland strayed a few meters from the group. "I finally bent down, turned over a leaf and there was this amazing drop of gold," recalls Strickland.
The tortoise beetle species, Chelymorpha alternans, is a multicolored mystery. Individuals come in five distinct color morphs, including the common metallic-striped gold form, a brick-red version and three different black and red patterns. They coexist in different proportions in Panama’s tropical forests and no one knows why this species sports such a variety of paint jobs.
“When I first came to Panama years ago, I found two of these very differently colored beetles mating, and thought it was probably a fluke—sometimes animals mate with another species by mistake—although their offspring don’t usually survive,” said staff scientist Annette Aiello. “Then Don told me that these were color morphs of the same species.”
“Yes, a number of scientists who’ve seen my Panama collection also couldn’t believe they all belong to the same species,” Strickland said.
" Color is super fascinating,” Lynette said. “In this tortoise beetle species, it's about discovering how color differences might be beneficial by giving them some sort of advantage when it comes to predation, selection, and/or living in different climates."
Color figures in the three main rules of survival: eat, avoid getting eaten, make babies. This tortoise beetle nibbles on vines in the morning glory family. But Strickland's first suite of experiments with the beetle morphs tackles the other two topics and has led to some fascinating results.
First, to look at predation, Strickland took the five beetle morphs and fed them to common predators — or at least she tried to. The taste-testers delivered the whole gamut of results from total predation to total abstinence.
Golden orb-weaving spiders ate (or wrapped in silk and saved for later) all beetles of all color morphs that Strickland threw into their webs. Picky praying mantises, on the other hand, only stabbed at the beetles with their forelegs (which act as taste buds) before rejecting them outright.
The most curious results came from colonies of the ant Azteca chartifex, known for their enormous, teardrop-shaped nests. Strickland built platforms at the bottom of nests where the defensive ants swarm over and attack any beetle invader. Since the ants are social creatures that make group decisions, they would initially take up to a couple of hours to decide what to do with the beetles. Ultimately, the golden ones would be thrown off the platform, the red-and-black phenotypes would be consumed, and the ones that were entirely red would be eaten the first three times they were given to the nest, but thrown off the next seven times they were presented with the beetles.
"This is pretty indicative of a learned aversion to something," says Strickland. "The point of having bright or conspicuous coloration is to, in theory, send a signal other predators: 'I'm distasteful, don't eat me."
Strickland and Windsor’s experiments showed that females prefer to mate with other beetles of the same color, which appears to be counterintuitive when it comes to maintaining color variation. They can, however, mate with any color morph (called phenotypes), confirming that the beetles are, in fact, a single species — at least for now.
Strickland's genetic research shows that color in these beetles is a very simply inherited trait. The color-determining loci—specific pieces of DNA within the chromosome—require only minor differences to produce the drastic variation in color.
Her future genetic work will delve deeper into the differences between each color morph to determine if they are on paths to becoming individual species. And chemical analyses will determine if the morphs have different toxins, which may explain the selective eating behavior of the A. chartifex ants.
"Understanding how different factors come together to maintain diversity is beautiful," says Strickland. "Saying that a species does better the more diverse it is, is a really cool idea."