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 is, of course, 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. Large males with large horns are more likely to win battles, 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, horns are expensive, and 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.
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 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. The results revealed the metabolic trade-off between horn development and both body size and testis size, in line with predictions. When compared to a control group of beetles allowed to develop normally, the cauterised individuals not only grew larger in size, but also developed disproportionately large testes.
To clarify: 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 then I ‘accidentally’ clicked on ‘images’ and.. yeah. I need to keep safe-search on in future.
References and further reading:
Simmons LW and Emlen DJ (2006) Evolutionary trade-off between weapons and testes. Proceedings of the National Academy of Sciences of the United States of America 103(44): 16346–16351.
Simmons LW, Emlen DJ and Tomkins JL (2007) Sperm competition games between sneaks and guards: a comparative analysis using dimorphic male beetles. Evolution 61(11): 2684– 2692.
Emlen DJ (2008) The evolution of animal weapons. Annual Review of Ecology Evolution and Systematics 39: 387–413.
Parker GA (1990) Sperm competition games – sneaks and extra-pair copulations. Proceedings of the Royal Society of London Series B – Biological Sciences 242(1304): 127–133.
Blatant plug: I have recently had an article published in Wiley-Blackwell’s Encyclopedia of Life Sciences online journal, which discusses life-history allocation in the context of sexual selection. You can find it at the following link, or drop me a line if you would like a copy:
Houslay TM, Bussiere LF. 2012. Sexual Selection and Life History Allocation. In: eLS 2012, John Wiley & Sons, Ltd: Chichester.
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: