The dung beetle microbiome

The gazelle scarab, Onthophagus gazella, with a dung ball in Victoria, Australia.

Dung beetles are an extraordinarily diverse group of insects, with more than 5,000 species in the subfamily Scarabaeinae. They eat mostly or exclusively dung, and thereby are important ecosystem members. Dung, however, especially the dung of large grazing mammals (which is what dung beetles mostly eat), is not an easy food. It is composed predominantly of the tough cellulose grass bits that have remained undigested even after passing through the multichambered stomachs of grazers. In order to digest this dung, it has long been thought that dung beetles are dependent on the bacteria in their own gut (their microbiota) to break the dung down into the sugars, amino acids, and vitamins that they need. Like many animals, humans included, dung beetles are believed to derive an important component of their microbiota from their mothers. This week's paper, in American Naturalist, examines the importance of maternally derived gut microbiota for dung beetle larval development. 

The gazelle scarab, Onthophagus gazella, is widespread African and Asian dung beetle. It was actually introduced to the US for management of livestock dung in pasture. Female O. gazella, (perhaps named because of their gazelle-like little horns?) tunnel into the ground underneath dung patties where they digs out brood cells, provision each brood cell with a ball of dung, poop, and lay on egg on top of their poop (called a "pedestal"). For clarification here, I am using "poop" to refer to the mother beetle's own feces, and "dung" to refer to the grazing mammal feces. 

Diagram of the brood cells of Onthophagous beetles. "Females lay several brood balls in each tunnel that would all be at the same developmental stage. However, for illustrative purposes all life stages are represented in one tunnel. These stages include the: (A) egg, (B) 1st larval instar, (C) 2nd larval instar, (D) 3rd larval instar, (E) pupa, and (F) an eclosing adult beetle that is tunneling toward the surface. The brood ball chamber is larger with each successive life stage as the larva feeds on the chamber walls within the brood ball. The top inset shows (G) the fecal pedestal the egg is positioned upon in brood ball. The bottom inset shows (H) the larval instar feeding on the walls of the brood ball chamber." From Estes et al. 2013.

In this week's paper, Daniel Schwab and colleagues from the University of Indiana show that the bacteria in the mother's poop enhances the total growth and the growth rate of developing dung beetles, and that these effects are amplified when the developing beetles are exposed to temperature or water stress. Interestingly, when they compared treatments in which developing beetles acquired microbiota either from the mother's poop or from the herbivore dung ball that the mother provided, there was no difference in the average total growth, or growth rate, of these beetles. But, beetles that acquired their microbiota from the herbivore dung were more variable in size and growth rate than beetles that acquired their microbiota from the mother's poop. This result highlights the complex interaction between microbiota, genetic variation, and environmental effects.