Dung beetles are industrious creatures that are among the most important recyclers in the insect world. They eat and bury manure from many other species, recycling nutrients and improving the soil at the same time.
Dung beetles are found on every continent (except Antarctica) in forests, grasslands, prairies, and deserts. And now, like many other species, they are struggling with the effects of climate change.
Kimberly Sheldon is an ecologist who has spent nearly 20 years of her life studying these beetles. Her research spans both tropical and temperate climates and focuses on the responses of these insects to changes in temperature, Sheldon wrote for Popular Science.
How do dung beetles defend themselves against temperature changes?
Insects do not use internally generated heat to maintain their body temperature. Adults can take steps such as moving to warmer or cooler areas. However, earlier life stages, such as larvae, are often less mobile, so they can be strongly affected by temperature changes.
But these beetles seem to have a defense: Sheldon found that adult dung beetles alter their nesting behavior in response to temperature changes by burying the brood balls deeper in the soil, which protects the developing offspring.
It’s easy to joke about these insects, but by collecting and burying manure, these beetles provide many ecological benefits. They recycle nutrients, aerate the soil, reduce greenhouse gas emissions from raising cattle, and reduce populations of pests and parasites that harm livestock.
These beetles are also important secondary seed distributors. Dung from other animals, such as bears and monkeys, contains seeds that the beetles bury in the soil. This protects the seeds from being eaten, makes them more likely to germinate and improves plant growth.
Dung beetles are surprising insects
There are about 6,000 species of dung beetles worldwide. Most feed exclusively on dung, although some feed on dead animals, decaying fruit and fungi.
Some species use the stars and even the Milky Way to navigate straight paths. One particular species, the bull-headed dung beetle (Onthophagus taurus), is the strongest insect in the world, capable of pulling over 1,000 times its own body weight.
This strength is useful for dung beetles’ most well-known behavior: scooping up manure.
Life cycle of dung beetles
The most popular images of dung beetles show them collecting manure and rolling it into balls. In fact, some species roll and others dig tunnels in the ground under a piece of dung, bring it into the tunnel and pack it into a ball, called a brood ball. The female then lays an egg in each brood ball and fills the tunnel with soil. The “rollers” do the same once they get their dung ball away from the competitors.
When the egg hatches, the larva feeds on the dung in the ball, pupates, and then emerges as an adult. It thus goes through a complete metamorphosis, from egg to larva and from pupa to adult, inside the brood ball.
Dung beetle parents do not provide care for their offspring, but their nesting behaviors influence the next generation. If a female places a brood ball deeper underground, the larva within it experiences cooler, less variable temperatures than if it were closer to the surface.
Why is temperature important to these bugs?
This matters because temperatures during development can affect hatchling survival and other traits such as adult body size. If the temperatures are too high, the offspring die. Below this point, higher and more variable temperatures result in smaller-bodied beetles, which can affect the reproductive success of the next generation.
Smaller males cannot compete as well as larger males, and smaller females have a lower reproductive output than larger females. Additionally, smaller-bodied beetles excrete less dung, so they provide fewer benefits to humans and ecosystems.
Climate change is making temperatures more variable in many parts of the world. This means that insects and other species have to cope not only with higher temperatures, but also with larger changes in temperature from day to day.
How was the dung beetle study conducted?
To examine how adult dung beetles responded to the kinds of temperature changes associated with climate change, Kimberly Sheldon designed cone-shaped mini-greenhouses to fit over 26-liter buckets buried in the ground up to the brim . Will Kirkpatrick, an undergraduate student in her lab, conducted the field tests.
The two randomly placed a rainbow scarab (Phanaeus vindex) fertilized female 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, they checked whether soil temperatures in the “greenhouse” buckets were higher and more variable than soil temperatures in the uncovered buckets.
Sheldon and Kirkpatrick gave the beetles fresh cow dung every other day for 10 days and let them make brood balls. They then carefully dug through the buckets and recorded the number, depth, and size of the chicken balls in each bucket.
What did the researchers discover?
The researchers found that the mother beetles in the greenhouse environments created more brood balls in total, that these brood balls were smaller, and that these females buried their brood balls deeper in the soil than the mother beetles in the buckets Control. The brood balls in the greenhouses still ended up in areas that were slightly warmer than those in the control buckets, but not as warm as if the cockroach mothers had not altered their nesting behavior.
However, digging deeper, the adults fully compensated for the temperature variation. There was no difference in the temperature variation experienced by the seedling balls in the greenhouse buckets compared to those in the control buckets. This reflects the fact that soil temperatures become more stable with depth as the soil becomes more isolated from changes in the temperature of the air above it.
The findings also suggest a possible trade-off between burial depth and brood ball size. Cockroach mothers who dug deeper protected their young from temperature changes, but provided less dung in their brood balls. This meant less nutrition for developing offspring.
Studies will continue
Climate change could still affect adult dung beetles in ways that haven’t been tested, with consequences for the next generation.
In future work, the researchers plan to investigate how the resulting seedlings develop and survive under the conditions now being tested.
So far, Sheldon and her colleagues are encouraged to find that these hardworking beetles can modify their behavior in ways that can help them survive in a changing world.