Bigger Arms Race

by Adam Kane

He stands 189cm tall and weighs 107kg. Who is this giant? He is none other than an average player in the Irish Rugby Union team’s starting 15 this weekend. By contrast, your bog standard Irish man typically stands 177 cm tall. Clearly, rugby is a strong selector of body size.

Image by Eoin Gardiner (Creative Commons License)

Image by Eoin Gardiner (Creative Commons License)

This is no secret, we know that rugby selects for big players but some researchers have shown how dramatic this increase in size has been over the past 100 years or so. If we look at body weight, from 1905-1974 an average player weighed 87.8kg but from 1975-1999 this had risen to 95.1kg. The onset of professionalism in rugby was a big driver of this growth spurt.

And why are we seeing such a trend? We can argue about the benefits of tactics, French Flair and the Haka all we want, but there is an obvious advantage in being bigger than your opponents. Taller backs can reclaim the ball more easily, taller forwards are better at taking lineouts and bulkier props can shunt the opposing scrum. Success in the Rugby World Cup backs this up, teams with the taller backs and heavier forwards get the job done more often than not.

What we have here is a type of Arms Race in the sport such that no team can afford to fall behind in the size game. Biologists are familiar with this kind of dynamic in evolution where we have the Red Queen effect, typically realised as a game of one-upmanship between a host (Ireland) and its parasite (England) or a predator (Ireland) and its prey (England). To paraphrase the Red Queen herself, when it comes to rugby it takes all the growing you can do, to keep in the same place.

Image: RWC 2007 (Creative Commons License)

Image: RWC 2007 (Creative Commons License)

But as the rugby environment selects for ever greater size the pool of potential players shrinks. All animals have a limit to the how big they can become and humans are no different. The loading stress on our bones mean we’ll never see a team of King Kong sized players duke it out. Even over the past decade player size is starting to plateau.

To take the natural selection analogy further, changes in the rules of the game can change the selection pressure on the preferred morphology of the players. In the future, for example, if scrums are banned or rarely occur then we would see a move away from the gigantic front rows populating the sport today.

What about the game on Saturday? The Irish team outweigh the English starting 15 by 2kg and are about 3cm taller. So an Irish win then. But wait! England are playing at home and this confers big advantages. For instance, the home team produces more testosterone before a game than the away team. This can be explained in evolutionary terms as well, whereby the home team view their opponents as territorial intruders so they need to get ready to defend themselves. So it’s England to win then? Well do remember the aphorism, prediction is difficult, especially about the future.

Adam Kane is a postdoctoral researcher at UCC who is studying the ecology of seabirds.

Email: adam.kane@ucc.ie

Twitter: @P1zPalu

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