Dung Beetles as Vertebrate Samplers

A biodiversity survey is an important tool that scientists use to gain a baseline understanding of organisms in an ecosystem. These surveys record the abundance (how many) and/or diversity (variety of animals) of species in the environment so scientists can monitor any change over time. The data collected from these surveys are the cornerstone to conservation research because collecting this data tells us which habitats and animals are being negatively affected and require protection efforts.

One challenge with biodiversity surveys is that the traditional sampling methods used to collect data are largely dependent on visual counts that are inherently biased. For example, a survey wanting to track the number of birds in a habitat may be limited by the researcher’s ability to visually spot and correctly identify that particular species. This is especially challenging when the target organism is rare or difficult to spot.

To address this challenge, scientists developed a new technique for large-scale biodiversity monitoring known as environmental DNA surveillance (Figure 1). Environmental DNA (or eDNA) is genetic material collected directly from environmental samples such as soil or water. Using this technique, scientists can identify the DNA of several organisms that have contacted the sample, offering a less-biased surveillance tool that is ideal for counting elusive or rare organisms. While this technique has its obvious advantages over traditional methods, one drawback is that water tends to be better than soil at preserving DNA which means aquatic environments are easier to survey than terrestrial habitats.

Figure 1. The overall workflow for environmental DNA (eDNA) studies with examples of organisms that have been identified from environmental samples. Image Credit: https://doi.org/10.1016/j.biocon.2014.11.019

Previous studies have investigated the use of blood-feeding invertebrates such as flies, leaches, and mosquitoes as alternative sources of eDNA, referred to as invertebrate-derived DNA (iDNA), to survey land-based animals (Beng et al. 2016, Robson et al. 2016, Nguyen et al. 2020). However, these organisms are often difficult to collect, limiting their use in large-scale surveillance programs.

Dung beetles are a widely distributed group of detritivores that feed on the fecal matter of terrestrial animals. Earlier experiments showed that certain cells found in mammal dung and ingested by dung beetles could be used as a source of identifiable DNA (Gómez & Kolokotronis 2016, Kerley et al. 2018). Since dung beetles are easy to collect in large numbers and are widely distributed, and a method for iDNA detection was already identified, this gave dung beetles an advantage over other invertebrates for their potential use in iDNA surveillance.

To further investigate the use of dung beetles as iDNA samplers, Drinkwater et al. (2021) set out to address two questions. How long could mammal DNA remain viable for identification in the gut of one dung beetle species (Catharsius renaudpauliani), and would it be possible to identify the DNA of multiple mammal species from the gut contents of several dung beetles? To address the first question, researchers fed 60 C. renaudpauliani on cow dung, then analyzed the gut contents of selected individuals at set time intervals ranging from 0 to 56 hours. They found that sufficient DNA could be recovered up to 4 hours after feeding, but the amount of DNA recovered dropped to zero at 9 hours post feeding. This meant that mammal DNA could be preserved long enough in wild dung beetles to be identified. And, the animal DNA that was identified from these dung beetles would have been from dung consumed within a short period of time before the beetles were caught. To address the second question, Drinkwater et al. (2021) trapped dung beetles in the field and evaluated their gut contents for mammalian DNA. They were able to confirm DNA of six different mammalian taxa, with three being identified down to species.

Collectively, these findings showed that dung beetles could serve as an easy to collect and widely distributed source of iDNA. This study takes us one step closer to better understanding a new source of invertebrate DNA in the quest for improved sampling methods. It’s the steppingstones of scientific discovery like those published by Drinkwater et al. (2021) that pave the way to developing more advanced surveillance tools that could one day change the world.      

In case you missed it, this publication was also briefly highlighted on this episode of the NPMA BugBytes podcast.

By: Michael Bentley, PhD, BCE

References:

Beng, K. C., K. W. Tomlinson, X. H. Shen, Y. Surget-Groba, A. C. Hughes, R. T. Corlett, And J. W. F. Slik. 2016. The utility of DNA metabarcoding for studying the response of arthropod diversity and composition to land-use change in the tropics. Sci. Rep. 6: 1–13.

Drinkwater, R., Clare, E. L., Chung, A. Y. C., Rossiter S. J., Slade, E. M. 2021. Dung beetles as vertebrate samplers – a test of high throughput analysis of dung beetle iDNA. BioRxiv 2021.02.10.430568.

Robson, H. L. A., T. H. Noble, R. J. Saunders, S. K. A. Robson, D. W. Burrows, And D. R. Jerry. 2016. Fine-tuning for the tropics: application of eDNA technology for invasive fish detection in tropical freshwater ecosystems. Mol. Ecol. Resour. 16: 922– 478 932.

Nguyen, B. N., E. W. Shen, J. Seemann, A. M. S. Correa, J. L. O’donnell, A. H. Altieri, N. Knowlton, K. A. Crandall, S. P. Egan, W. O. Mcmillan, And M. Leray. 2020. Environmental DNA survey captures patterns of fish and invertebrate diversity across a tropical seascape. Sci. Rep. 10: 1–14.

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