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Caroline Chaboo’s eyes light up when she talks about tortoise beetles. Like gems, they exist in myriad bright colors: shiny blue, red, orange, leaf green and transparent flecked with gold. They’re members of a group of 40,000 species of leaf beetles, the Chrysomelidae, one of the most species-rich branches of the vast beetle order, Coleoptera. “You have your weevils, longhorns and leaf beetles,” she says. “That’s really the trio that dominates beetle diversity.”
An entomologist at the University of Nebraska-Lincoln, Chaboo has long wondered why the kingdom of life is so skewed toward beetles: The tough-bodied creatures make up about a quarter of all animal species. Many biologists have wondered the same thing, for a long time. “Darwin was a beetle collector,” Chaboo notes.
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Of the roughly 1 million named insect species on Earth, about 400,000 are beetles. And that’s just the beetles described so far. Scientists typically describe thousands of new species each year. So—why so many beetle species? “We don’t know the precise answer,” says Chaboo. But clues are emerging.
One hypothesis is that there are lots of them because they’ve been around so long. “Beetles are 350 million years old,” says evolutionary biologist and entomologist Duane McKenna of the University of Memphis. That’s a great deal of time in which existing species can speciate, or split into new, distinct genetic lineages. By way of comparison, modern humans have existed for only about 300,000 years.
Yet just because a group of animals is old doesn’t necessarily mean it will have more species. Some very old groups have very few species. Coelacanth fish, for example, have been swimming the ocean for approximately 360 million years, reaching a maximum of around 90 species and then declining to the two species known to be living today. Similarly, the lizard-like reptile the tuatara is the only living member of a once globally diverse ancient order of reptiles that originated about 250 million years ago.
Another possible explanation for why beetles are so rich in species is that, in addition to being old, they have unusual staying power. “They have survived at least two mass extinctions,” says Cristian Beza-Beza, a University of Minnesota postdoctoral fellow. Indeed, a 2015 study using fossil beetles to explore extinctions as far back as the Permian 284 million years ago concluded that lack of extinction may be at least as important as diversification for explaining beetle species abundance. In past eras, at least, beetles have demonstrated a striking ability to shift their ranges in response to climate change, and this may explain their extinction resilience, the authors hypothesize.
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Complicating the mystery of beetle diversity is the fact that some branches of the beetle family tree have many more species than others. For example, dung beetles, which spend their lives rolling deftly crafted balls of excrement, are only modestly diverse. “This family is around 8,000 species, so it’s not a huge group,” says community ecologist Jorge Ari Noriega at El Bosque University in Bogotá, Colombia.
By contrast, Chrysomeloidea—a superfamily containing longhorn and leaf beetles—includes 63,000 species, while Buprestidae, a group of metallic wood- and leaf-boring beetles also known as jewel beetles for their glitzy iridescent colors, includes about 15,000 species.
This large variation in species richness among beetle lineages means that “no one explanation holds very well for any one group,” says McKenna. Still, among plant-eating beetles—which make up roughly a quarter of all beetle species—a clear pattern is emerging. Based on genetic analyses of different beetle lineages, McKenna and his colleagues have found evidence that a major factor spurring beetle diversity was the diversification of flowering plants during the Cretaceous period.
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During the Cretaceous period, which started around 145 million years ago, an explosion of new flowering plant species spread across the Earth’s surface, colonizing many different habitats. Today, plants make up about 80 percent of the mass of Earth’s life. Making the most of plants as food is an ecological strategy that has helped fuel the radiation of not only beetles but also herbivorous species including ants, bees, birds and mammals.
In the case of herbivorous beetles, their most species-rich lineages carry a fascinating assortment of genes that permit the digestion of plants, McKenna has found. Many of these genes code for enzymes that help to break down plant cell walls, allowing access to sugars stored in hard-to-digest compounds like cellulose, hemicellulose and pectin. “The lineages that have these genes were the ones that are so incredibly successful,” McKenna says.
These genes were ingenious adaptations that turned indigestible plant parts into food. They allowed herbivorous beetles to eat more and different kinds of plants, which in turn enabled the insects to move into new habitats and occupy new ecological niches. As plant-eating beetles spread out geographically and adopted different diets and lifestyles, the genetic differences between them grew, resulting in new species.
For unclear reasons, some species of plant-eating beetles lost their digestion-aiding genes as they evolved, including a gene coding for pectinase, an enzyme that enables the breakdown of pectin. Evolutionary ecologist Hassan Salem at the Max Planck Institute for Biology in Tübingen, Germany, explains that to compensate, some beetles evolved a different strategy for eating plants: They forged relationships with bacterial partners—called symbionts—that also aid plant digestion.
For some beetles, these special symbiotic microbes became an alternate tool for keeping plants on the menu, expanding the number of habitats where new species could evolve and thrive. For example, in the vast majority of tortoise leaf beetle species, the group Salem studies, it’s not a genetically encoded enzyme that breaks down pectin, but a bacterial symbiont. The beetles get the bacteria from their mothers: Every time a female deposits an egg, she also leaves behind a capsule containing the microbes. The tortoise beetle embryo develops inside the egg, then burrows into the capsule to digest the symbiont about a day before it emerges.
“It's the first thing it encounters in life … so it’s a very intimate association,” says Salem. When Salem and his team have experimentally removed the microbe caplets from developing larvae, the adult, germ-free beetles that emerge have a high mortality rate because they can’t access pectin in the plant cell.
In addition to making plants easier to digest, some plant-associated microbes may have paved the way for beetle diversification because they provide beetles with predator protection. In the tortoise leaf beetle Chelymorpha alternans, for example, a fungus called Fusarium—often found in crops like bananas and sweet potatoes—grows on the surface of beetle pupae during metamorphosis. “We’ve demonstrated that if you remove the fungus, then ants readily find them and feed on them,” says Aileen Berasategui, an evolutionary biologist at the Amsterdam Institute for Life and Environment in the Netherlands. Fusarium, in other words, may be shielding the beetles from harmful predators, further expanding beetle territory and enabling diversification.
Berasategui adds that plenty of bark beetles, like ambrosia beetles, also benefit from Fusarium fungi, but in a different way. The beetles carry the fungi from tree to tree in specialized pockets called mycangia. Once the tree’s fungal infection is underway, the beetles indulge in a fungi feast.
Adapting to conduct this kind of agriculture—sowing spores that will grow into food—has also helped beetle species to exploit new habitats. “From their own nest, they take a little piece, and then … fly to a new tree where they start their own nest, they sow the new fungus, they generate this new garden,” says Berasategui. Called fungiculture, the approach has independently evolved in ambrosia beetles seven times. The evolution of new beetle species is thought to have been shaped by mutually beneficial relationships with these fungi—part of a 50-million-year history in which insects such as ants, termites and ambrosia beetles have independently evolved to farm fungi, according to a 2005 article published in the Annual Review of Ecology, Evolution and Systematics.
Plant-eating beetles have evolved other innovations that may have allowed them to speciate more than other beetle groups. In the leaf beetles that Chaboo studies, for example, the emergence in the fossil record of defensive fecal shields—structures built from a beetle’s own excretions and sloughed-off skin—“coincide with massive species radiations,” she says. Most beetle shield-users are solitary species, but some live in groups, arranging themselves in formations that protect them from predators. Fecal shield protection may have helped the beetles move into more open habitats, Chaboo says.
Whether they eat plants or dine on other fare such as carrion, beetles from all groups have evolved an impressive array of tools to solve many different problems. In that sense, beetles are a microcosm of the tree of life, McKenna says.
Resilient as beetles are, however, we can’t take their survival for granted. Insect populations are in decline in many places—“and, yes, beetles are part of that,” says Beza-Beza. How they’ll survive the impacts of humans is “one of the core questions right now,” he adds, though he’s betting there will be beetles on Earth “longer than there will be humans.”
Beetling away on scientific puzzles in the Central American cloud forest sky islands where he works, Beza-Beza has a special affinity for Ogyges politus, a beetle species that lives and feeds on rotting logs. “It only occurs in the mountains next to my hometown,” he says. “So it reminds me where I’m from … and that there are these jewels everywhere.”
