[Explainer] The beetles that roll dung and follow the stars

What are dung beetles?

Dung beetles use the dung of warm-blooded herbivores for… everything. They are attracted to fresh dung via volatiles (meaning, the smell). Adults feed on the soup of the dung. “The incisor lobe of the adult is flattened and fringed for handling soft food, and the particulate components of the food are filtered out before being ingested,” write authors of a book chapter on dung beetles.

For breeding, the beetles roll a perfectly round ball of dung and take it in a special nesting place underground and lay their eggs on it so that the larvae can feed on the dung.  The dung of an herbivore can differ in terms of size and moisture consistency, depending on the plant species consumed, rainfall, etc. This in turn influences the number of eggs the beetles produce on the dung. They produce many eggs on dung from animals grazing on green fresh plants and produce fewer eggs on dung from animals grazing on dry senescing pasture.

Dung beetles on the tree of life

The dung beetles belong to the order Coleoptera. The name of the order derives from the Greek word “koleos” which means sheath-wings. All insects belonging to this order have forewings that are hardened into hard sheaths called elytra. This protects the softer hind wings and the abdomen and gives them an advantage to survive in various habitats that would otherwise break their delicate wings. During flight, the elytra opens enough for the beetle to fly with the hind wings. The family Scarabaeidae in the order Coleoptera consists of more than 30,000 species, often called scarabs or scarab beetles. Not all members of this family use dung; the “true dung beetles” belong to the subfamily Scarabaeinae.

Yes, India has them

A 2015 publication in the Indian Journal of Entomology lists 420 species from 38 genera in the country. New species discoveries continue to be made, with one of the latest being the stunning metallic blue Enoplotrupes tawangensis added in 2019 from Tawang district of Arunachal Pradesh.

How do dung beetles build nests?

The dung beetles make balls of dung from the dung pat to make a “brood mass”. Eggs are laid inside the brood masses. Dung beetles are divided into three groups based on the way they collect and store the brood mass.

• Teleocoprids or rollers: The beetle makes a sphere from the dung pat or the dung pellets and rolls it away, usually with its hind legs and buries it in the soil and lays one egg.
• Paracoprids or tunnellers: The beetle digs a tunnel at the bottom of the dung pat and breaks bits of the dung pat with its head, front legs and body and takes it through the tunnel where it stocks it up end to end to form a compacted brood mass. One egg is laid inside the brood mass. The beetles then place a layer of soil on the brood mass before starting the next one.
• Endocorpids or dwellers: They construct brood masses in cavities of the dung pat itself.

Dung beetles make good parents!

Dung beetles are known for their biparental care. Both males and females can roll dung from the pat and take it through the tunnels. In some species, the male and the female take dung balls and store them in chambers to make a large brood mass of about 100 grams. The female cuts this into four brood balls and lays one egg in each. The mother stays back to take care of the brood ball by cleaning any fungal growth from the outside and repairing any damages in the brood ball for the larvae. Some males may also assist, but they are only active if they are sure that the offspring is theirs.

Some males (major males) have enlarged horns and compete for mating and assist the females to build broods and guard them while nesting. Other males are small, (minor males) without horns who do not guard nesting and instead sneak into breeding burrows to copulate with females guarded by major males.

For example, in the case of Onthophagus, it is only the major males that provide paternal care, and males will not provide care when they encounter the minor males sneaking copulations in their breeding tunnel.  This intensive parental care doubles the survivor rate of the offspring, but at the same time reduces the life span of the female. Due to this intensity, the fecundity (reproductive rate) of the dung beetle is usually quite low.

The life cycle of a dung beetle

The larva hatches from the egg after a week or two and feeds on the brood mass. The larva has a biting mouthpart that enables it to eat even the fiber content of the dung (unlike the adults). Larvae usually complete three instars in a period of 12 weeks and then shift to the pupal stage for 1-4 weeks and then emerge as adults, dig out of the soil and fly to find fresh dung to eat. Some species, depending on their biology, may also have periods of diapause or quiescence (phases of dormancy with interrupted metabolic activity) in either their larvae, pupae or adult life during dry summers or long winters in order to enhance their survival rate.

Weight lifters

Dung beetle is one of the strongest insects in the world, with the capacity to pull 1141 times its own body weight – the equivalent of an average man lifting two fully loaded 18-wheeler trucks. This makes the dung beetle way stronger than the ant, the most commonly cited example of a strong insect, which can lift 10-50 times its body weight.

The Egyptian connection

Ancient Egyptians were highly appreciative of this fantastic creature. Dung beetles were represented in amulets and worn as charms or as emblems sacred to Khepri, a major god associated with the dung beetle itself. The beetle coming out of the soil in autumn represented rebirth and the pushing of the perfect ball of dung across the ground represented the pushing of the sun across the sky.

Stargazing beetles

A 2013 study found that dung beetles use the night sky, specifically the Milky Way, to navigate and orient themselves. It was known before that the beetles and other insects and birds use the symmetrical pattern of the polarized light that appears around the sun. These patterns are visible to the insects because they have special photoreceptors in their eyes. For night-time navigation, it was assumed that they use the faint light of the moon, but these dung beetles rolled the ball in a straight line even on moonless nights.

The study was conducted by placing a table with dung beetles on it, in the Johannesburg Planetarium. The results showed that the beetles could orient themselves under a full starlit sky as well as when only the Milky Way was present.

When the scientists put little cardboard hats on the study beetles’ heads, thereby blocking their view of the sky, the beetles just rolled around aimlessly.

Ecological significance

The dung beetles deliver important ecosystem services such as nutrient cycling, waste removal, seed dispersal, and by removing dung from the surface, the flies and parasites that breed in dung are not prominent. Burrowing, aerating and mixing the soil increases its nutrient content as well as improves the water holding capacity of the soil. This may be beneficial for agriculture too.

Read more: Insects are disappearing in India, and we don’t even have data

CITATION:

Coleoptera: beetles and weevils. (n.d.). Retrieved October 31, 2019, from http://www.ento.csiro.au/education/insects/coleoptera.html.

Dacke, M., Baird, E., Byrne, M., Scholtz, C. H., & Warrant, E. J. (2013). Dung Beetles Use the Milky Way for Orientation. Current Biology, 23(4), 298–300. doi: 10.1016/j.cub.2012.12.034

Ridsdill-Smith , J., & Simmons, L. W. (2009). Dung Beetles. In Encyclopedia of insects. Amsterdam: Elsevier/Academic Press.

Dell’Amore, C. (2017, December 14). Dung Beetles Navigate Via the Milky Way, First Known in Animal Kingdom. Retrieved November 3, 2019, from https://blog.nationalgeographic.org/2013/01/24/dung-beetles-navigate-via-the-milky-way-first-known-in-animal-kingdom/.

Resh, V. H., Cardé Ring T., & Ridsdill-Smith, J. (2003). Dung Beetles . In Encyclopedia of insects. San Diego: Academic Press.

The Importance of Dung Beetle. (n.d.). Retrieved October 30, 2019, from https://keys.lucidcentral.org/keys/v3/aus_museum/dung_beetles/key/euonlast/Media/Html/generalinfo.html.

Banner image: A dung beetle finds subsistence in the deserts of Rajasthan. Photo by Peter Davis/Flickr.