Sunday, April 14, 2013

Northern Pintails: From Punk to Hunk to Studly Duck

In the animal world, competition for a mate is a struggle with a thousand different strategies. Let's follow the twisting path taken by just one species. Male Northern Pintails (Anus acuta) start courtship as young punks who must grow into handsome hunks before they even have a chance to change into studly ducks. But only the chosen ones will make the final, hormonal transformation.

Northern Pintail courtship flock (c) John Ashley
Northern Pintail courtship flock, six determined males following one female (upper right)

In much of the bird world, it's the female who picks her mate from among the mass of males who strut, sing and/or otherwise trying to get her attention (while simultaneously knocking down rival males). We call this, "female choice," and it's a big part of what drives the evolution of many male bird traits, from gaudy feathers to complex song repertoires.

Even slight genetic variations can potentially lead to exaggerated traits. Females who choose a mate for, say, a slightly longer tail feather, will tend to produce daughters with the same (genetically-driven) taste in tail feathers, and sons who tend to inherit the longer-tail-feather genes from their father. Run this through enough generations and you get, for example, the peahen and the peacock. The plain brown chicken and long-plumed rooster.

Over the past month, tens of thousands of Pintails migrated through Montana on their way to dispersed nesting sites. Early in the migration we often saw small courtship flocks that consisted of one female followed by a cohort of males. Some of the males were doggedly determined to stay as close as possible to the lone female, while other males would soon give up. Whenever she took to flight, the group turned into an amazing aerial ballet of high-speed, precision maneuvers that would leave our own Blue Angel studs dizzy.

As these male Pintails aged and began courting females, there have been a number of invisible, chemical changes taking place in the bloodstream. Careful hormonal and behavioral research has partially teased apart these chemical maneuvers.

When one- and two-year-old male Pintails were experimentally grouped together, some were more aggressive and some were less aggressive, and they established a dominance ranking or "pecking order" that was unrelated to age. Also, the before and after levels of four plasma hormones (testosterone, dihydrotestosterone, luteinizing hormone and corticosterone) were unchanged by the male-male competition for rank.

And when female Pintails were placed together with different males, one at a time so that male's dominance rank was eliminated as a factor, the females showed a significant preference for males with pure-white breast feathers and colorful scapular (shoulder area) feathers. They paired only with the more handsome males, the "hunks." So the males' dominance rankings were unrelated to their pairing success.

But when a female Pintail was placed in with a group of males, things got real interesting. Male behavior returned as a factor in pairing success. Compared to the younger "punks," the two-year-old males tended to be more attentive to the female, and courted them much more aggressively. The females chose a mate from among these attentive males. From this sub-group, the females again chose the male with the brightest and most colorful feathers.

Northern Pintails (c) John Ashley
Aggressive paired male (center) picks a fight
Surprisingly, the winning hunk wasn't always the highest ranking member of the all-male group. However, after a male was chosen by a female, he quickly became more aggressive and initiated fights with the more-dominant males. And the newly-paired male soon fought his way to the top of the pecking order.

Why? Because getting chosen by a female changed the male's blood chemistry, his hormones, in a surprising way. Chosen males saw a decrease in testosterone and an increase in corticosterone. Increasing corticosterone levels meant that the male could quickly mobilize the energy he had stored as fat, which increased his energy levels and helped him drive away other males. The male suitors who were not chosen by the female did not experience a significant change in hormone levels.

Again, there was no correlation between a male's pre-pairing dominance rank and his age, or between his pre-pairing dominance rank and his pairing success. Male dominance behaviors did not constrain female choice. Overall, mate selection was driven by a sequence of different strategies.

First, the male had to be old enough to aggressively court females. This age- and hormone-induced behavior moved him into the second round. Then he had to be healthy enough to produce handsome feathers. This was the genetic evidence that the female actually based her decision on. And finally, after being chosen by a female, male hormones shifted in a way that raised his rank among males so that he could more easily protect her from unpaired males.

In Northern Pintails, it's not a straight line between courtship and pairing. Finding a mate is seldom a simple matter, and it gets even more complex when you start looking beneath the colorful feathers.

Female Northern Pintail following her chosen mate (c) John Ashley
Female Northern Pintail (right) following her studly mate
Behind the hormones: The dip in testosterone seems counter-intuitive and still has me a bit puzzled, but none of the studies I've read addressed it, so that one's still a delightful mystery to me.

Of course, adult human males are not immune to the effects of females, either...