Featured paper: Thanksgiving edition. And it’s doubly relevant – it’s about turkeys and family!

AH Krakauer. 2005. Kin selection and cooperative courtship in wild turkeys. Nature vol. 434, pp. 69 – 72.

Wild turkeys males show off in front of females in the hopes of being impressive enough to get to mate. While some males show off alone, others form “coalitions” of two to four males and all display for females together. However only one male in each coalition – the dominant male – ever gets to mate. So why in the world do the other male turkeys help him, if they never get to mate? Why don’t they display alone, where they’d at least have a chance at mating?

One key is to remember that evolutionary fitness – the “points” in the Great Video Game of Evolution, basically – isn’t just having babies yourself: it’s reproducing your genes in any form. Having babies is, of course, a great way to do this. But if your cousin has babies, you get a little bit of fitness from that, because you and your cousin share some genes, so your cousin’s babies will have some of the same genes as you. This is called indirect fitness, because you get the fitness indirectly, through your relatives.

This means that it may be worth helping relatives reproduce even if it costs you. To calculate whether you will gain fitness by doing something, we use Hamilton’s rule: for the action to benefit your total fitness, the gain in your relative’s fitness (B, for Benefit) times the relatedness of your relative to you (r, for relatedness) has to be greater than the fitness you lose by that action (C, for Cost). In equation form: is rB > C?

Say you’re a hungry lizard, and you come across four of your sister’s babies. If you eat them, you’ll have enough energy to have three lizard babies of your own, but then your sister will have lost the fitness of her babies. What do you do?

The gain in your sister’s fitness, if you don’t eat the babies, will be 4 (for 4 babies). The relatedness of your sister to you is 1/2 (if you don’t follow that, just trust me; I’ll go over relatednesses in a later post). The cost, to you, of not eating the babies is 3 (for 3 of your own babies). So Hamilton’s equation looks like this: (1/2) * 4 > 3?

That simplifies to 2 > 3, which is untrue – two is not greater than three – so to maximize your fitness, you should not leave those babies alone, but instead you should gobble your four nieces and nephews up!

Anyway, back to those turkeys. Dr. Krakauer thought that maybe the subordinate males were helping the dominant male get matings because they were getting indirect fitness from it. He used microsatellites (just like I’m using) to find out that the males were related to each other by about 1/2, which is what you would expect if they were brothers. He calculated the fitness benefit to the dominant male of having those subordinate males helping him by comparing how many chicks the dominant males had to how many chicks were had by males who displayed alone. It turned out that being dominant was much better than being solo: the dominant males had, on average, 6.1 more chicks than the solo males.

Now we have r = 1/2 and B = 6.1. C, the cost of helping, was the average number of chicks that solo males had, since if they hadn’t been helping out Brother Dominant, the subordinates could have displayed solo and had some chicks that way. This number turned out to be 0.9: solo males don’t get many chicks.

So: rB > C?

(1/2) * 6.1 > 0.9 simplifies to 3.05 > 0.9 which is true! So the subordinate males were indeed gaining fitness by helping their brother get matings, even though they themselves never had chicks.

Evolution is not as simple as “nature red in tooth and claw.” Sometimes selection promotes helping your brother land some ladies, even if it means you don’t get any.

Happy Thanksgiving!