Science is a cumulative pursuit. That is, the thirst for knowledge is never fully quenched. Rather, we answer one question, only to reveal another puzzle or two more. And thus begins another search for answers. That is certainly how my 24-year-old loon project has gone. But sometimes we reach a point where a vexing question is finally laid to rest, and it feels as though we have made real progress. I am at such a point now.

Let me back up. If you have been following this blog, you know that senescence in old loons is a phenomenon we have recently discovered. Senescence, loss of body condition and decline in survival rate in aging individuals within a species, is all too familiar to me and other humans. During the past twenty years, many studies have reported senescence in birds, mammals, fish, and reptiles. So what? Well, we expect that animals that lose condition as they grow old will change their behavior in response. In other words, scientists have long predicted that senescing individuals should start to behave so as to leave more to their offspring and care less for themselves. To put it another way, old individuals should be willing to take a hit to their survival if it allows them to pour more resources into their young and help their young survive. This makes sense, of course, because old individuals reach a point where they stand little chance of surviving longer, so they would do well to give whatever they can to their offspring, which DO have a bright future. Animals that behave this way should leave more and healthier offspring, and thus this behavior should spread in populations. This very logical idea is termed “terminal investment”. Again we can all probably think of human parallels.

Terminal investment, which I have mentioned before, has become a central theme of the loon project, ever since we published a paper 9 years ago on fatal fighting of males. Terminal investment became interesting to us because it was the most plausible explanation for such lethal contests. Our reasoning was as follows. If males are willing to die to defend their territories, then they must reach an age at which they have little to lose. And if males have little to lose, this must mean that senescence hits males (but not females, which seldom battle to the death) very hard to the point where old males have little future to look forward to. In this case, it might make sense for them to fight like crazy to hold a territory for another year or two, rather than give it up easily and leave themselves nowhere to breed during their last year or two of life. So we have two clear predictions here: 1) males, but not females, must start to die off at a certain age, and 2) males beyond this age must still fight like hell for their territories. It is this clash of body condition and behavior among old males that might cause fatal fighting.

At the time we started to consider the terminal investment hypothesis as a means to explain reckless battling by males, we had almost no solid information on the ages of males in our population. With patience and tireless field work by dozens of us, we have now turned things around. Analysis of loons of varying age has shown us that many males hit the wall at age 15. First, and most important, they start to die at a high rate. You can see from the figure below that males (blue bars) are suffering higher mortality than females (red bars), whether they are on territory (Terr) or without one (floaters: “Float”).

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But males also lose mass at age 15, indicating loss in body condition, as shown here:

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Finally (and predictably), males get evicted from their territories at a high rate at age 15:

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Wow, males are really getting slammed after they pass the age of 15 years!

So all of these data tell us that the first prediction of the terminal investment hypothesis, abrupt senescence at a certain age in males but not females, is clearly met in loons. That age, surprisingly, is only fifteen. Females clearly remain strong, healthy, and vigorous well past age fifteen.

As hard as the first prediction of terminal investment was to test, the second prediction is even harder. You see, fights are common in loons if you take the perspective of a loon’s lifetime, but they are quite uncommon if viewed from the standpoint of human observers in canoes. In other words, most individual loons have engaged in several major battles during their long lives, but territorial battles are not common during day to day observations and often occur so quickly that we are not present to witness them.

Patience pays, however. Since we can draw upon 24 years’ worth of field observations, we now have a trove of observations that we can search for any evidence of aggression and territorial behavior. I made this search, looking for two kinds of evidence: 1) territorial yodels, which serve to communicate a male’s aggressive tendencies and willingness to battle, and 2) out and out aggression, in the form of battling, lunging, chasing and underwater attacks launched by territorial loons on intruders to their territories. I was simply asking “Do old male loons (above age 15) tend to maintain a high level of yodeling and aggressiveness towards intruders?”.  The answer is a resounding “Yes”:

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As you can see from the figure above, old males actually increase their tendency to yodel (yodels per intruder), compared to young males. Similarly, old males step up their aggression (see below) and contrast in this way with females, who show no increase:

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By the way, all of these patterns I have shown are “statistically significant” via tests that I have performed.

You cannot be as excited as I am about this set of results. No one is. But, as I mentioned, this is one of those rare cases where we have finally managed to answer a burning question to our satisfaction. Even better, the question is one that had been the foundation of my research funding from the National Science Foundation. So I can now report to them that I have found the holy grail! What makes this clear finding even more significant is that terminal investment is quite rare in vertebrates. Of the hundreds of species studied thus far, the only other one to show such a clear pattern of terminal investment is the California Gull. Appropriate, don’t you think?

(Photo by Woody Hagge.)

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Two mornings ago, I saw my very first chick of the year on Woodcock Lake. My visit to Woodcock was memorable — not just for that reason, nor because of the rarity with which the loon pair there hatches a chick. Rather, I chanced to witness with great clarity one strategy that adult loons employ to defend their offspring. It is moments of  this kind, when loons’ behaviors and motivations become visible suddenly and starkly,  that fuel much of my thinking and writing about the species.

If you have read my recent posts, you know that we have learned a good deal about how loons defend their young from opportunistic intruders, which on occasion find and kill chicks less than two weeks old. We know that males possess a acoustic tool that females do not — the yodel — which conveys aggressive motivation and therefore can be used to discourage intruders within earshot of the territory from landing there and imperiling the chicks. What was less clear was whether males yodeled at a high rate as a generalized strategy to inform would-be intruders that visits to the yodeler’s territory would likely provoke an attack or whether, instead, yodels were targeted at specific intruders as they passed over a territory or began to land there.

On Saturday morning I arrived at Woodcock to find the male with a tiny chick riding on his back. The female was on the nest 200 meters away, incubating the second egg, which by now might have produced a second chick. As the sun rose above the horizon that day, intruders criss-crossed the airspace above the territory. Two such “flyovers” were especially enlightening. At 535, two intruders began to descend as they crossed the lake, preparing to land next to the male, near the lake’s center. The male crouched down, dumping the chick into the water, and uncorked a deafening yodel at just this moment. In response, the two flyers checked their glide down towards the lake, flapped rapidly to regain lost altitude, and flew off to try their luck elsewhere. Seventeen minutes later, this pattern was repeated. This time a lone flyer crossed just above my canoe, descended to within three meters of the lake surface — the whistling of wind across its wings easily audible in the morning stillness — before hearing the male’s acoustic objection and beating its wings desperately to abort the landing and ascend.

Though we had statistical evidence to suggest that male loons used yodels to repel specific intruders, I had never observed as clearly the effective targeting of flying intruders by a yodeling male. While songbirds sing incessantly during the spring in nonspecific fashion — that is, they sing repeatedly over hours, days and weeks to communicate their readiness to mate to any female in the vicinity and/or their willingness to defend their territory to any male that happens to be nearby — male loons seem to use yodels with surgical precision. In other words, loon males yodel rarely, and when they do, they aim their yodel at a specific target with a specific goal in mind.

Why are male loons so stingy with their yodels? Two possibilities come to mind. First, yodels appear to have a relatively high metabolic cost compared to songs of other animals. Perhaps, then, mere energy conservation places a limit on the frequency of this call. Second, Jay Mager’s work has shown that males reveal both their physical size and their physical condition when they yodel. By yodeling, therefore, a male might convey information about himself that he would prefer to keep private. Of course, a large male in good physical condition should be more apt to yodel, one might argue, whereas a small, ailing male should keep his bill shut so as to avoid an eviction attempt by a rival passing overhead.

I look forward to testing the prediction that male loons yodel rarely — and vary systematically from one to another in their tendency to emit the vocalization — as a means to avoid sharing information about themselves. That is a clear, well-grounded prediction that might produce an important finding. But my morning on Woodcock reminded me of a great benefit that I earn from spending time in the field observing loons. One can spend countless hours entering data and churning through statistical analyses to reach a rock-solid conclusion about animal behavior (and I do). But the occasional “Eureka” moment spent with animals in the field is invaluable.