Dung beetle moms protect their offspring from a warming world by digging deeper

 

If the TV series Dirty Jobs covered animals as well as humans, it would probably start with dung beetles. These hardworking critters are among the insect world’s most important recyclers. They eat and bury manure from many other species, recycling nutrients and improving soil as they go.

 

Dung beetles are found on every continent except Antarctica, in forests, grasslands, prairies, and deserts. And now, like many other species, they are coping with the effects of climate change.

 

I am an ecologist who has spent nearly 20 years studying dung beetles. My research spans tropical and temperate ecosystems and focuses on how these beneficial animals respond to temperature changes.

 

Insects don’t use internally generated heat to maintain their body temperature. Adults can take actions such as moving to warmer or colder areas. However, earlier life stages such as larvae are often less mobile, so they can be strongly affected by changing temperatures.

 

But dung beetles appear to have a defense: I have found that adult dung beetles modify their nesting behaviors in response to temperature changes by burying their brood balls deeper in the soil, which protects their developing offspring.

 

Champion recyclers

It’s easy to joke about these busy insects, but by collecting and burying manure, dung beetles provide many ecological benefits. They recycle nutrients, aerate soil, lessen greenhouse gas emissions from cattle farming, and reduce pest and parasite populations that harm livestock.

 

Dung beetles are also important secondary seed dispersers. Dung from other animals, such as bears and monkeys, contains seeds that the beetles bury underground. This protects the seeds from being eaten, makes them more likely to germinate, and improves plant growth.

 

There are roughly 6,000 species of dung beetles around the world. Most feed exclusively on dung, though some will feed on dead animals, decaying fruit, and fungi.

 

Some species use stars and even the Milky Way to navigate along straight paths. One species, the bull-headed dung beetle (Onthophagus taurus), is the world’s strongest insect, able to pull over 1,000 times its own body weight.

 

That strength comes in handy for dung beetles’ best-known behavior: gathering manure.

Rolling and tunneling

Most popular images of dung beetles show them collecting manure and rolling it into balls to spirit away. In fact, some species are rollers and others are tunnelers that dig into the ground under a dung pat, bring dung down into the tunnel and pack it into a clump or sphere, called a brood ball. The female then lays an egg in each brood ball and backfills the tunnel with soil. Rollers do the same once they get their dung ball safely away from the competition.

 

When the egg hatches, the larva feeds on dung from the brood ball, pupates, and emerges as an adult. It thus goes through complete metamorphosis—from egg to larva to pupa to adult—inside the brood ball.

Warmer temperatures, smaller beetles

Dung beetle parents don’t provide care for their offspring, but their nesting behaviors affect the next generation. If a female places a brood ball deeper underground, the larva in the brood ball experiences cooler, less variable temperatures than it would nearer the surface.

 

This matters because temperatures during development can affect offspring survival and other traits, such as adult body size. If temperatures are too hot, offspring perish. Below that point, warmer, more variable temperatures lead to smaller-bodied beetles, which can affect the next generation’s reproductive success.

 

 

Smaller males can’t compete as well as larger males, and smaller females have lower reproductive output than larger females. In addition, smaller-bodied beetles remove less dung, so they provide fewer benefits to humans and ecosystems, such as nutrient cycling.