New Paper: Sex specific signals help explain aggregations of dung beetles

Last summer, a keen student (Jack Ford) got in touch wanting to learn more about what being a scientist/entomologist is all about.  Jack helped with one of my research projects by helping process soil samples.  Between handfuls of worms and cold, wet earth – we discussed designing and running a small experiment together. We managed to pull it all together, discovered an interesting result, and wrote it up. Excitingly, we have recently had the results published as a short communication in Ecological Entomology (links at the bottom of the post).

The basis of the experiment was simple, when looking for dung beetles in cattle dung, you’ll quickly find individuals are  aggregated amongst dung pats. Some pats have plenty, some have a few, and some have none. There have been considerable efforts made to understand what factors influence the ‘clumping’ of these beetles within pats. A big part of this involves how attractive the dung pat is to colonising insects: what makes ‘Dung Pat X’ so much more attractive than ‘Dung Pat Y’?

beetle

The answer to that question is complex.  A number of factors influence the attractiveness of dung. For instance: whether the animal produced the dung during day or night, the size of the dung pat, the species of the animal, the exposure of the dung, the weather during the beetle’s dispersal flight, etc.

One hypothesis is that dung beetles themselves enhance the attractiveness of a dung pat. We thought this could very well be the case with the species Aphodius fossor – as it never seems to be alone. This species always hanging out with a friend (usually a mate), but often you’ll find a number of pairs within the same dung pat.  It’s a burly beast (as far as the dwelling Aphodiinae go) – measuring a little over a centimetre in length, and having a very convex (almost jelly bean-like size/shape). We figured the commonness, large size, and behaviour of this species would make it a good contender for our experiments.

We suspected the aggregation of these beetles could be due to a few reasons. First of all, when dung beetles colonise a dung pat – they tunnel through it. This aerates the dung environment, encouraging different bacterial communities thus altering the composition of gases fluxing from the dung pat. Beetles might be using this cue to search out new dung pats (possibly an area with friends already enjoying a dung-feast themselves).

An other interesting question involves the pairing of this species. Individuals are almost always with a mate; How does this happen? The world is a big place, even more so for a small insect. One strategy that insects use to find mates are pheromones – chemical signals produced and received by the same species. There are many different types of pheromones that serve a variety of purposes. One such type, attraction/aggregation pheromones are fairly well documented in related scarabaeidae.

We set out  test whether beetles colonising a dung pat made it more or less attractive to additional colonising beetles, and whether differences were sex specific by creating a simple choice experiment. We collected dung from a single (adorable welsh black) cow at Dr Beynon’s Bug Farm, homogenising, then splitting the dung into nine small pats. We then assigned each dung pat to one of three conditions: add four female beetles, add four male beetles, add no beetles. We then gave them a little time in their new home. From herein, these dung pats are considered the ‘experimental pats’.

In the meantime, we constructed arenas using large buckets. We filled them with sand, and drilled holes in the side to fit small lengths of tubing. These tubes led into the smaller containers that would eventually connect to the experimental pats.

F12
Depiction of our experimental set-up

After giving the beetles some time to enjoy their new homes in the treatment pats, we began our experiment. We introduced ninety individuals of each sex into the larger arena in groups of ten. The beetles ambled (slowly) around the arena. Most often, beetles walked cautiously into the tubes – pausing at the tube end with rapid antennae movement – eventually heading back into the main arena, or dropping into the smaller buckets. We promptly removed the beetle when it dropped in, and recorded which bucket it landed in.

When we tallied up the results, and analysed the data – we found something quite interesting. Male beetles had no preference for any type of dung. Females on the other hand preferred dung colonised by males, and consistently avoided dung colonised by other females.

We think that females might avoid dung pats colonised by other females to reduce likelihood of encountering density-dependent mortality (too many mouths, and not enough food/space). This is something that we see relatively often from other insects, and is often achieved through production of ‘host marking’ or ‘anti-aggregation’ pheromones.

The attraction of females to dung pats colonised by males suggests that there could also be signals produced by males, that draw the females in. Male produced pheromones have been identified in a few species of related scarabaeidae. We observed beetles burying part-way into the dung after being added to the dung pats. In the dung beetle, Typhaeus typhoeus (Geotrupidae), males have been observed digging a short tunnel under dung – and defecating with their abdomens in the air (this is thought to represent pheromone release).

What’s sort of curious about this experiment, is that with the two mechanisms we identified – (female-female avoidance) and (female-male attraction), it’s unclear how this would lead to the aggregated distributions we see in nature. Partly, we think this could be due to one signal (attraction) being stronger than the other (avoidance). But there is so much about these beetles we just don’t know, that could be the key to understanding their distributions amongst habitat patches? For instance:

  1. Who moves first? Do males and females arrive/leave at the same time?
  2. Are the signals that we’ve identified from the beetles themselves, or as a result their interaction with the physical/microbial environment of the dung pat?
  3. Do signals depend on beetle density?
  4. As these beetles primarily disperse through flight, how representative of attractiveness are ‘walking’ experiments like this one?
  5. How do the presence of other invertebrate groups act to affect subsequent invertebrate colonisation?

So many questions, so much to discover, and so little time to do it all. I’ve found the deeper I venture into ecology, the more apparent these opportunities and limits become. On that note, I should probably get working on my plans for the final field season of my PhD, time is ticking.

Thanks for reading, and here are links to the paper:

Early View at Ecological Entomology: Manning and Ford (2016)
Pre-Review Draft: Manning_Ford_2016_Draft

 

 

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