[Commentary in brackets by Jeannine]
In some species, males have exaggerated body parts that can serve as both a weapon and as an ornament.[Because they are all about showing off]
Take the mighty fiddler crab for example! That enormous claw serves two purposes:
- As a weapon, the claw is used to defend burrows (great places to seduce the ladies!) against other males.
- As an ornament, the claw is rhythmically waved in a “come hither” manner to attract those ladies to their well-defended burrow
I wonder what other species that we might discuss on this blog that have large ornaments that serve as both weapons and ornaments.
Hmmm…..
Of course, it’s Jeannine’s favorite subject – antlers!
While attractive to hunters, the antlers as ornament theory has never been proven. We know bucks use antlers to compete with other males. And sure, it makes sense that those large bony protuberances might signal to a female that a male with bigger antlers has better genes.
But proving that females actually select males with larger antlers is not easy. First of all, larger antlers are usually associated with larger body size. Are females selecting antlers or body size?
Second, and more importantly, how do we know females are even choosing males with larger antlers? Maybe they simply end up with a larger male because he wins all the fights with other males? [Of course, they are choosing! Females aren’t wallflowers. They have a LOT invested in reproduction. Lactation anyone?]
After all these years of studying antlers, we still don’t know the answer to whether females prefer males with larger antlers!
Well, that all changed recently with an ingenious study of female mate choice selection. [Finally, does are getting the attention they deserve.] Researchers at Mississippi State University’s Deer Lab conducted an experiment demonstrating that while males fight, females choose.
It wasn’t an easy experiment to design because it had to accomplish the following:
- Eliminate male-male competition as an explanation for the observed results,
- Eliminate other factors, such as body size, that might be the trait females select for, and
- Provide a way to measure whether females preferred one male over another.
How did they do this? First, they gave males replaceable headgear. Hardened antlers are simply bone with no nerves, so they sawed off their antlers but left a projection above the burr of several inches. This became the base for interchangeable antlers of various sizes.
Voila! With this setup the researchers could mix and match antlers at will. So, for example, two males that were 180 lbs could be randomly assigned a large or small rack. Pretty cool.
Second, they set up 3 experimental pens in a row where a female pen was in-between two male pens. They put a female in heat (estrus) in the middle pen and similar body-size males on either side. The difference between males was simply their interchangeable headgear. Then they set up cameras to record how much time a female spent near the pen of either buck.
They found that in 20 of 25 trials females spent more time next to the pen with larger antlered males. Statistically, there is less than 0.2% chance you would observe this outcome if females randomly selected either buck.
So are we finished studying mate selection in white-tailed deer? Hardly. There are other factors that likely influence mate choice, such as age, body size, and pheromones. How do they interact with antlers to influence female mate selection?
The never-ending curse of research: Answers simply beget more questions. [Or the never-ending obsession with bucks and antlers…you decide.]
-Duane Diefenbach
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