That’s correct, friends – the two beetles you see in this image are both adult males of the same species of dung beetle, Onthophagus nigriventis. The chap on the right is clearly larger, and has a rather ostentatious horn extending from his thorax. This horn is a sexually-selected trait: horned males can use their armaments in battles over females, driving rivals away from mating sites, and even prying other males off a female whilst in flagrante. Sexual selection is all about the struggle to reproduce, and so traits are ‘sexually selected’ if their expression confers some benefit to the holder in terms of reproduction. In this case, large males with large horns are more likely to win battles with rivals, enabling them greater access to females, so there is a clear advantage to investing resources into weapons development.
Given that big, horned males fight rivals and guard their female partners (they may engage in the rather ungentlemanly pursuit of trapping lady beetles in mating burrows in order to have their way with them), then what the crap is going on with the guy on the left? Well, these horns are likely expensive in terms of resources, and any energy ploughed into growing horns is not available for investing in other traits – indeed, horns are known to trade off against morphological structures including eyes, antennae, and wings. Species of Onthophagus are well known for the size and diversity of their horns, but often these are only expressed by the largest ‘major’ males. What happens, then, if you’re a down-on-your-luck, resource-starved ‘minor’ male? Is there really any point in cashing in your precious metabolic chips for a gamble on a crappy little horn that’s never going to help you win any contests anyway? Surely there’s another strategy to be taken?
Indeed there is, and it’s called being a ‘sneaky fucker’*. While some males guard their mates, others will try to ‘sneak’ copulations with females. We now enter the realm of sperm competition: females may mate with multiple partners, so there is a battle amongst the sperm within her reproductive tract to fertilise eggs. If ejaculates are costly, males have to trade off resource investment on gaining fertilisation with investment on gaining additional matings. The more sperm ejaculated in a mating, the more eggs are likely to be fertilised – but, again, this requires resource investment. Furthermore, an increased risk of sperm competition should favour the evolution of increased expenditure on the ejaculate (i.e., the more likely that your little swimmers are going to be racing against some other dude’s, the more investment you should be making in ensuring your ejaculate is the biggest and best it can be).
In plain English (or, at least, an approximation thereof): if you’re a big horned dude protecting a little beetle harem, then you shouldn’t be all that worried about the fertilisation aspect – after all, you should be the only one for your ladies. You want to invest in lots of mating, not lots of ejaculate. Meanwhile, as a sneak, you’ve got to make those precious moments count, and ploughing your resources into the ejaculation makes sense – it’s in the female’s interests to have a few flings behind the dung-balls, so the greater the ejaculate, the better your chances of gaining fertilisations. Of course, the best way to produce larger amounts of ejaculate is to invest more resources into testis development.
All of which leads us nicely to what I think is one of the most ingenious (albeit slightly harrowing, once you really think about it) experiments I’ve read about while studying up for my PhD. Leigh Simmons and Doug Emlen (yes, this is another Doug Emlen-related post) cauterised those cells on beetle larvae which produce the thoracic horns in O. nigriventis, manipulating investment by ensuring that they could not grow these weapons. When compared to a control group comprising beetles allowed to develop normally, the cauterised individuals not only grew larger in size, but also developed disproportionately large testes. These results revealed the metabolic trade-off between horn development and both body size and testis size, in line with predictions from evolutionary models of ejaculate expenditure.
But what does this mean for the two beetles at the top of the page? Well, there’s a general tip here: if you’re going to sneak around, you’d better have gigantic balls.
*I’ve been told that Geoff Parker coined this phrase, but have been unable to find a reference for this, and during googling I accidentally clicked on ‘images’ and.. yeah. I need to keep safe-search on in future.
This post is a slightly modified version of an earlier entry on my ‘Nature!Sex!TopTips!‘ website.
Research blogging reference:
Simmons, L., & Emlen, D. (2006). From the Cover: Evolutionary trade-off between weapons and testes Proceedings of the National Academy of Sciences, 103 (44), 16346-16351 DOI: 10.1073/pnas.0603474103
Other references and further reading:
Blatant plug: I am really interested in the intersection between sexual selection and life-history allocation – the way that individuals invest their resources – and (along with my long-suffering supervisor) have written an article on this topic for Wiley-Blackwell’s Encyclopedia of Life Sciences online journal. You can find it at the following link, or drop me a line if you would like a copy:
The original image is the copyright of Alexander Wild, an entomologist, photographer, and all-round great guy. You can find the original, and more of Alex’s work, at the links below:
http://blogs.scientificamerican.com/compound-eye/
https://twitter.com/#!/myrmecos
Tom Houslay 
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