age

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I have pointed out many times how science proceeds not in simple linear fashion but haphazardly. A finding may appear rock solid but subsequent findings modify our understanding, even forcing us to discard earlier conclusions in some cases. Viewed from space — and over large stretches of time — we might seem to advance steadily in understanding a phenomenon. From ground level, progress in comprehending a topic is herky-jerky; we make clear progress for awhile, then reach a dead end, back up, and find a new path forward.

So it has been with loon conservation. Thirty years ago, we feared that methylmercury was a major health hazard for breeding loons and their offspring. At the time, our fears seemed well grounded. Now, having spent millions of dollars measuring mercury levels in loons and looking for its impacts, we must reassess. We now see that, at worst, mercury has the potential to harm loons in the eastern 1/3 of the breeding range — and even there only in acidic lakes. Consequently, where loon populations appear to be declining, we can cross mercury off the list of threats and turn our attention to other potential causes. By the way, it was no mean feat to cross mercury off the list of dangers to loons. Thoughtful, rigorous work by dozens of biologists across North America has made this conclusion possible.

Our study of a much narrower topic — the loon yodel — also seems to be moving forward by fits and starts. (Linda’s gorgeous photo above shows a male yodelling in typical crouched position.) A really cool paper by Jay Mager — a collaborator with the Loon Project for several years — showed fifteen years ago that the frequency of the yodel was strongly dependent upon body mass. Large males, Jay found, had low-pitched yodels. This was fascinating to us, because it meant that male loons were revealing their size to territorial competitors! Now it is well and good to advertise your size if you are a big loon. In that case, the fact that you are yodeling tells opponents that you are motivated to defend your territory, and the low frequency of your yodel informs them that you can back up that determination with beef! But consider the mixed message that a small male sends by yodeling: “I am small but feisty”. Hmmmm. Not such a deterrent to a territorial challenger, it would seem. We are still coming to grips with this so-called “honest-signalling” of body size and trying to learn how small yodelers might benefit from yodeling.

Among males whose ages are known exactly (blue) and males whose ages are estimated (red), older birds have higher-pitched yodels.

But wait! Lately we have had cause to wonder whether body size really does have a strong impact on yodel pitch. Now that a good many males that we marked as chicks have settled to breed, we know the ages of many of our breeding males precisely. Brian Hoover, a postdoc here at Chapman, took these new age data — data unavailable to Jay Mager — and looked to see whether age is correlated with yodel frequency. As you can see from the figure, age is strongly correlated with yodel frequency: old males have high-pitched yodels. Reassuringly, the better our age information is, the stronger the correlation. That is, the blue points show a tighter age/frequency relationship than the red points.

Where does this leave us? I just explained that small males have high-pitched yodels. Now I am telling you that old males have high-pitched yodels. Can it be both? It is possible that both mass and age influence yodel frequency and could exert additive effects. (Imagine the extreme falsetto of a male that was both old and small!) However, Brian examined mass in his more complete analysis and found no evidence that it affects yodel frequency after all. In other words, we might have found a mass/frequency pattern back in 2006 simply because we did not have age data that would have shown us what was really an age/frequency pattern. Statisticians are quite familiar with this frustrating phenomenon; conclusions can change dramatically when a variable that was left out of a previous analysis (age, in this case) is added.

The finding that it might be age, not mass, that affects yodel pitch would be more palatable if high-pitched yodels by old males made more sense than high-pitched yodels by small males. But, intuitively, the new finding makes less sense! This is the delightful pickle we now find ourselves in. “Delightful” because the age pattern is robust and clear, as the graph shows. A “pickle” because we have no idea, at present, why or how old males have high-pitched yodels.

Well…so it goes in science!

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I try not to steal a glance through the lab window each time I pass. But I usually fail. You see, Marco Bisoffi, a molecular biologist and colleague of mine at Chapman, has restarted our study of telomeres* in loons as a possible tool to measure age and the effect of stress. Each week Marco churns out telomere measurements on a new set of loons, as he tries to troubleshoot the PCR** procedure. So when I walk by his lab and see him bent over his laptop, I wonder whether his promising early finding that telomeres indicate age in loons has held up.

It has. Now that Marco has run twelve males and ten females of known age, the trend is stronger than before. If you study the plot above, in fact, two patterns are evident. First, old males and females have shorter telomeres than young males and females. Second, males as a group have shorter telomeres than females. (This latter finding repeats what Jeremy Spool had found a few years ago.) There is some scatter in the data, especially among females, but both patterns show high statistical significance. Of course, we will have an even better fix on these patterns when we have run the other 83 DNA samples we have collected from adults of known age.

It is hard to exaggerate the value of these findings for loon biology and our own research in Wisconsin and Minnesota. There are countless benefits to studying loons, but one drawback has always been our inability to “age” individuals effectively. To our enormous frustration, we cannot even distinguish a 5-year-old from a 30-year-old. If this telomere pattern holds up, however, that source of vexation will be considerably diminished. In the future, we will be able to take a DNA sample from an unknown adult, measure its telomeres, and assign it to an age-class. Indeed, if the unknown bird is a male and we record both its yodel and its tendency to yodel at intruders, we shall be able to narrow its estimated age range still further — probably to within a few years.

Why does it matter that we are on the brink of being able to age adult loons accurately? First, age has a strong effect on a great range of behaviors, including aggressiveness, ability to hold a territory — which increases in young loons and then declines later in life — and even willingness to incubate eggs when black flies are abundant. Second, age impacts survival rate, especially in males. So knowing the ages of loons helps us refine our estimates of survival and improves our models of population dynamics.

Speaking of age and decline, the featured photo for this post is Linda’s Grenzer’s pic of “Clune”, the male on her lake. Despite the inevitable shortening of his telomeres, this 23-year-old still looks pretty fit in his winter attire!

FOOTNOTES

*telomeres — protective DNA sequences (“end caps”) on chromosomes that permit DNA to be replicated many times but become shorter with age and stress

**polymerase chain reaction — a common molecular technique that permits efficient study of specific regions of DNA