Horny beetles

So you’re a dung beetle. You have got yourself a nice ball of poo and a female who lives in a tunnel below. You spend your days at the top of that tunnel using your massive horn to fight off other males who want to steal your empire from you.

That sounds all fine and dandy – unless your horn is somewhat less impressive and you’re constantly losing your home to other bigger males.

Sheesh – you would feel pretty hard done by right? Maybe not.

Let someone else do the hard work

Why not skip all the hard work of maintaining your empire and sneak into someone’s tunnel, get the matings you need, and then go on your merry way. Why bother fight huge macho males if you don’t need to?  Especially if you will probably lose.

Male dung beetles (Onthophagus nigriventris) come in two distinct forms: macho males with huge horns, or small males with next to no horn.

Macho males spend their days fighting off other macho males… yet little do they know small males are literally burrowing their way into the tunnel below to steal cheeky visits with their female mates.

Look at that horn?!

Is that horn big enough for ya? (CSIRO via Google Image [CC-BY-3.0])

Same species but two forms? Whaaat?

Yep. There’s two very different versions of the male in one species. “How does such a thing evolve?!” I hear you ask. Basically this happens when there’s no benefit from being in between.

If big males can fight off others to maintain a home, and little males are discreet enough to go completely unnoticed by the macho males then what would happen to those in-between. Well not a whole lot (in a mating sense).

Males with medium size horns would never be able to defend an empire for long. Having a medium sized horn would also prevent you from burrowing. Your hole would need to be a lot bigger to fit in that horn, and you would probably get found out before you got anywhere near the female.

As a result only the males with the biggest horns are successful at guarding a female and having babies, and only the smallest sneakiest males could too mate often enough to leave it’s genetics behind. The medium males are very unsuccessful resulting in only the extremes to be selected for.

Male with medium horns do not occur (or very rarely) in the wild populations.

Animals of extremes

These dung beetles are not the only ‘extreme is best’ animals, where completely different male forms have evolved.

A personal favourite of mine are the rock-paper-scissor lizards. These dudes play a real life game of rock-paper-scissors. Males have either a blue, orange, or yellow throat with each winning or losing to the others.

Monogamous blue males fight off polygamous orange males, who in turn can beat off yellow males for the unguarded females. The yellow males can however take advantage of the blue males by sneaking matings from their females while they’re distracted with the orange males.

I’d love to hear of more examples if anyone knows some! Please share below.