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

If the “Dirty Jobs” TV series covered animals as well as humans, it would probably start with dung beetles. These hard-working critters are among the most important recyclers in the insect world. They eat and bury the manure of many other species, recycling nutrients and improving the 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 dealing 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 do not use internally generated heat to maintain their body temperature. Adults can take action such as moving to warmer or cooler areas. However, early life stages such as larvae are often less mobile, so they can be greatly affected by temperature changes.

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

Without dung beetles, the world would be dirtier and smellier.

Recycling champions

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 the soil, reduce greenhouse gas emissions from cattle farming, and reduce populations of pests and parasites that harm livestock.

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

There are approximately 6,000 species of dung beetles in the world. Most feed exclusively on manure, although some feed on dead animals, rotting fruit and fungi.

Some species use the stars and even the Milky Way to navigate straight paths. One species, the bull-headed dung beetle (Onthophagous bull), is the most powerful insect in the world, capable of pulling more than 1,000 times its own weight.

This strength is useful for the most well-known behavior of dung beetles: picking up manure.

Rolling and tunneling

The most popular images of dung beetles show them scooping up dung and rolling it into balls to get away. In fact, some species are rollers and some are tunnelers which dig into the ground under a bale of manure, roll the manure down the tunnel and pack it into a clump or sphere, called a brood ball. The female then lays an egg in each ball of brood and fills the tunnel with dirt. Rollers do the same once they have retrieved their ball of dung safely away from the competition.

Two human fingers grasp a ball of dung the size of a ping-pong ball with a fingernail-sized egg embedded in the surface
An egg is visible in the center of a ball of brood from a female rainbow beetle (Vindex Phanaeus).
Kimberly SheldonCC BY-ND

When the egg hatches, the larva feeds on the droppings of the brood ball, pupates and emerges as an adult. It thus undergoes a complete metamorphosis – from egg to larva, from nymph to adult – inside the ball of brood.

Warmer temperatures produce smaller beetles

Dung beetle parents do not 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 and less variable temperatures than they would nearer the surface.

This is important because temperatures during development can affect offspring survival and other traits, such as adult body size. If the temperatures are too hot, the offspring perish. Below this point, warmer and more variable temperatures lead to smaller beetles, which can affect the breeding success of the next generation.

Smaller males cannot compete as well as larger males, and smaller females have lower reproductive output than larger females. Additionally, smaller-sized beetles eliminate less droppings, so they provide fewer benefits to humans and ecosystems, such as nutrient cycling.

Beetles in the greenhouse

Climate change is making temperatures more variable in many parts of the world. This means that insects and other species must manage not only warmer temperatures, but also greater day-to-day temperature changes.

To examine how adult dung beetles reacted to the kinds of temperature changes associated with climate change, I designed mini cone-shaped greenhouses that could hold more than 7-gallon buckets buried in the ground to the brim. Will Kirkpatrick, an undergraduate student in my lab, led the field trials.

We randomly placed a fertilized female rainbow beetle, Vindex Phanaeus, in each greenhouse bucket and in the same number of uncovered buckets to serve as controls. Using temperature data loggers placed four depths into the buckets, we verified that the soil temperatures in the “greenhouse” buckets were warmer and more variable than the soil temperatures in the buckets. not covered.

A large round beetle with shades of red, green and gold
A male rainbow dung beetle (Vindex Phanaeus).
Dan MeleCC BY-ND

We fed the beetles fresh cow dung every other day for 10 days and allowed them to make brood balls. Then we carefully dug into the buckets and recorded the number, depth and size of brood balls in each bucket.

dig deeper

We found that beetle mothers in the greenhouses created more brood balls overall, that these brood balls were smaller, and that these females buried their brood balls deeper in the soil than the beetle mothers in control buckets. . The brood balls in the greenhouses still ended up in slightly warmer areas than those in the control buckets — but not as warm as if the beetle mothers hadn’t altered their nesting behaviors.

A cone-shaped lid placed in a patch of dirt
A dung beetle greenhouse placed over a buried bucket of soil in the author’s field trial.
Kimberly SheldonCC BY-ND

However, by digging deeper, the adults fully compensated for the temperature variations. There was no difference in the temperature variation experienced by the brood balls in the greenhouse buckets and the control buckets. This reflects the fact that ground temperatures become increasingly stable with depth as the ground becomes more and more isolated from the temperature variations of the air above it.

Our results also suggest a possible trade-off between burial depth and brood ball size. Beetle mothers that burrowed deeper protected their offspring from temperature changes but provided less manure in their brood balls. This meant less nutrition for the developing offspring.

Climate change could still affect adult dung beetles in ways we haven’t tested, with consequences for the next generation. In future work, we plan to place balls of brood from Vindex Phanaeus and other species of dung beetles in the greenhouse and control the buckets to the depths at which they were buried so that we can see how the beetle offspring develop and survive.

So far, however, my colleagues and are encouraged to discover that these industrious beetles can modify their behavior in ways that help them survive in a changing world.

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