Onthophagus Horn Dimorphism

One of my side-projects at the moment is setting up a project to observe the behaviour and life history of male Onthophagus dung beetles (Scarabaeidae). This post will introduce why this is of interest (to me, at least).

First, a bit about the study species. As far as is currently known, there is only one species of Onthophagus on Cyprus (where I am), O. opacicollis, a typical Mediterranean species distributed in places like Spain, Sardinia and Cyprus. It’s most commonly found living on the ground in shrublands, and less commonly in grasslands. I’ll be collecting as many live beetles as I can (during the day, since they burrow at night), putting them in an aquarium or similar observatory with enough space and soil to allow them to live normally, and will observe how they act.

The ambition behind this is simple. Onthophagus is a popular genus for investigating evo-devo (Emlen, 2000; Moczek, 2009). It is the most species-rich genus in the animal kingdom, with over 2400 species. All males of Onthophagus species are dimorphic: some develop a large horn, while others develop no/a tiny horn. The large-horned males are termed majors, the small-horned are termed minors. That there is true dimorphism and not just a range of horn sizes hints that there are selective pressures that favoured the development of two distinct phenotypes (cf. Gross, 1996).

In beetles, male dimorphism is tightly linked to body size (see diagram above, Emlen et al. (2007)): if the males reach a critical size threshold, they develop the horn. To use an alternative example to illustrate how this dimorphism arises physiologically, let’s take a look at how the male stag beetles get it, as recently discovered by Gotoh et al. (2011).

ResearchBlogging.org Gotoh, H., Cornette, R., Koshikawa, S., Okada, Y., Lavine, L., Emlen, D., & Miura, T. (2011). Juvenile Hormone Regulates Extreme Mandible Growth in Male Stag Beetles PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0021139

Male stag beetles are known for their grossly enlarged mandibles which they use for fighting and impressing females. It’s long been known that the size is related to how much food the beetle eats, but how exactly it works has never been figured out, the main reason being that there are generally two male morphs: a male with huge mandibles, and a male with much smaller (but still enlarged) ones. It turns out that the regulator isn’t food or energy from food exactly – instead it’s the most versatile arthropod hormone, juvenile hormone (JH from now on).

As background, beetles are holometabolous, meaning they have a larva-pupa-adult life cycle (like butterflies, moths, flies, bees, etc.). In stag beetles, there are three larval stages during which the beetle feeds and grows – they last several months before pupating and starting metamorphosis. During metamorphosis, the gut is thrown away, larva-specific tissues get reduced, and adult structures grow extremely rapidly (eyes, wings, legs, genitals, mouthparts, the latter including the mandibles).

It’s during this metamorphosis stage that the differences between the two morphs start popping up. If you slice through the mandibles of males and females, you’ll notice that the male cuticle has a lot of folds – the female mandible’s cuticle is flat. And between the male morphs, those that have the mega-mandibles have a lot more of these folds than the mini-mandibled. Interestingly, this difference in folds between male and female is also seen in horned beetles (where males develop a horn) and in termite soldiers (which develop big mandibles as weapons).

Of course, it is the duty of a scientist to keep digging further: what causes the differences in the number of folds? JH. But it’s rather specific. What the researchers did was inject their lab stag beetles with JH (actually, a JH analog called fenoxycarb, because the actual hormone can’t be synthesised yet) at different life history stages. When applied during the larval stages, the larval stage lasted much longer, and the adult beetles were overall larger (because the larva could eat a lot more). But the mandibles themselves were not extraordinarily large, so there must be some other trick to it – more food does not equal gigantic mandibles.

But when the JH was applied right at the beginning of the pupal stage, the adult beetle got the enormous mandible. So mandible growth regulation is affected only during this critical time period (as would be expected, with hindsight ;)). And it was only the mandibles that were seriously affected by the JH injection, not the entire body. Hence, JH at the beginning of the pupa stage is the controlling factor, not food or size – althought he correlation with size is there due to the longer larval stage duration.

Onthophagus horn development is somewhat different, involving developmental biology (beetle horns are novel structures, not just enlargements of preexisting ones), and is explained thoroughly by Emlen et al. (2007); I won’t write it up fully here, since it will just be mass plagiarism. Here’s a quick-and-dirty summary though. Once the prepupal stage is reached, the larval epidermis beneath the cuticle get separated, and several areas of it start experiencing rapid cell division, as prompted by the hormone methoprene (Emlen & Nijhout, 1999); in these areas the epidermis becomes folded and thickened. One of the areas is the head, where the horn will eventually grow. Eventually, the prepupal stage ends and the cuticle is shed in preparation for pupation. In this stage, the horn grows until pupation is finished and it gets covered with the cuticle for the final time.

So, to summarise, all males are horned. But in some males – the minor – the horn gets remodelled and reduced during pupation. Whether this happens or not depends on the environment: with optimal larval feeding conditions, males become majors; in non-optimal cases, the males become minors (Moczek & Emlen, 1999). What I want to investigate is whether, as in the stag beetle, we can predict the eventual male phenotype from the duration of the larval stages, since they depend on the amount of food received.

But the larger aspect of the project is to see how this dimorphism affects the male’s behaviour. It’s well-known that horns in majors are used for fighting to gain access to females – this is direct competition (Eberhard, 1981). The minors obviously can’t participate in this, so they compete indirectly with the other males (Emlen, 1997). That’s a fancy way of saying that they sneak up on females in the burrows (having a horn would make this infinitely harder) without anybody detecting them and mate. Majors help in raising the brood by prociding food; minors do nothing of the sort (it is apt to call them rapists).

So intuitively at least, there seems to be sexual pressure leading to diversifying selection – both majors and minors have reproductive success, so both phenotypes are selected for. An intermediate horn size would be no good for sneaking, nor would it be good for fighting, hence it’s not selected for, leading to the two distinct phenotypes.

But still, while this may seem like an open-and-shut case of sexual selection, we need to quantify all the differences between majors and minors. The horn affects their ethology, mostly their sexual behaviour, as well as their locomotion. All of these must be investigated in order to get a complete picture of allt he factors that might affect the selective forces. For example, how do horns affect mobility? Intuitively, minors should move faster than majors, especially in the burrows, allowing the sneaky mating. But is this actually true? In other species, it is (e.g. Madewell & Moczek, 2006). But we don’t know if it’s applicable to all species, hence it’s useful to test in O. opacicollis.

What about aggression? Are hornless males aggressive towards other males, or do they run away and hide? Admittedly, setting up such an experiment would require several bioethics checks, but it is also an important factor: these are all in the same population, so it is possible that aggressive majors (and they’re always aggressive) will meet with minors, and one has to see what happens in these cases. Does the presumed higher mobility of the minors allow them to escape a horned male attack? Or do the minors get forced into a fight (and inevitably lose)? Only by observing these scenarios and the male behaviours can we get an idea of the whole picture.

In other Onthophagus species, e.g. O. taurus, there is also evidence of differences in the genitals between the two morphs (House & Simmons, 2003) – this would also be worthy of study in O. opacicollis, to see if this is unique to O. taurus or whether the genital evolution within two morphs of the same species might actually be a general rule.

I mentioned earlier that Onthophagus is the most speciose genus in the animal kingdom. I hope that the data I generate here will be useful to someone else studying whether this sexual selection played any role in this extreme diversification, and if so, how. (Personally, I doubt that they have much to do with each other, but I could be wrong, as is usually the case.)

References:

Eberhard WG. 1981. The Natural History of Doryphora sp. (Coleoptera, Chrysomelidae) and the Function of its Sternal Horn. Annals of the Entomological Society of America 74, 445-448.

Emlen DJ. 1997. Alternative reproductive tactics and male-dimorphism in the horned beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae). Behavioral Ecology and Sociobiology 41, 335-341.

Emlen DJ. 2000. Integrating Development with Evolution: A Case Study with Beetle Horns. BioScience 50, 403-418.

Emlen DJ & Nijhout HF. 1999. Hormonal control of male horn length dimorphism in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae). Journal of Insect Physiology 45, 45-53.

Emlen DJ, Lavine LC & Ewen-Campen B. 2007. On the origin and evolutionary diversification of beetle horns. PNAS 104 Supp. 1, 8661-8668.

Gotoh H, Cornette R, Koshikawa S, Okada Y, Lavine LC, Emien DJ & Miura T. 2011. Juvenile Hormone Regulates Extreme Mandible Growth in Male Stag Beetles. PloS One 6, e21139.

Gross MR. 1996. Alternative reproductive strategies and tactics: diversity within sexes. Trends in Ecology & Evolution 11, 92-98.

House CM & Simmons LW. 2003. Genital morphology and fertilization success in the dung beetle Onthophagus taurus: an example of sexually selected male genitalia. Proc. R. Soc. B 270, 447-455.

Madwell R & Moczek AP. 2006. Horn possession reduces maneuverability in the horn-polyphenic beetle, Onthophagus nigriventris. Journal of Insect Science 6, 21.

Moczek AP. 2009. The Origin and Diversification of Complex Traits Through Micro‐ and Macroevolution of Development: Insights from Horned Beetles. Current Topics in Developmental Biology 86, 135-162.

Moczek AP & Emlen DJ. 1999. Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae). Journal of Evolutionary Biology 12, 27-37.

Reseach Blogging necessities :)
Emlen, D., Corley Lavine, L., & Ewen-Campen, B. (2007). Colloquium Papers: On the origin and evolutionary diversification of beetle horns Proceedings of the National Academy of Sciences, 104 (suppl_1), 8661-8668 DOI: 10.1073/pnas.0701209104

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