It is hard being successful. I don’t know this from personal experience, of course, but I have studied the topic in some detail. In fact, I spend a disproportionate amount of my time analyzing the behavior and ecology of highly accomplished loons.

You see, the individuals that I talk about in my blog and focus on in my scientific papers are the crème de la crème among loons. These animals have passed myriad biological milestones. They have hatched from eggs, survived the cold and dangerous first few weeks of life, dodged eagles and loon intruders to reach adult size at four months of age, completed energetically-costly molts, and navigated through thousand-mile migrations to and from the ocean. Above all, though, the subjects of my research effort have won battles to claim and defend breeding lakes, found safe nesting locations, and reared healthy chicks. Those chicks — their life’s work — permit my team to capture and mark them for study.

Don’t get me wrong; I did not set out 24 years ago to study this elite class of loons. In fact, I have always been most interested to learn how young adults without territories collect information about potential breeding sites and decide where to settle. But it is difficult to study such “floaters”, because, being floaters, they bounce around. Moreover, floaters cannot be captured easily, because they do not have chicks to protect. Only two years ago did we accumulate enough information to assemble a scientific paper that describes the goals and strategies of this itinerant cohort of individuals.

We are mostly stuck investigating the lives of life’s winners, like “Honey”, the breeding female on Muskellunge Lake whom Linda Grenzer has immortalized in countless photos (see the recent one above). I am not really complaining. We have learned a good deal about the lives of loons in general, despite focusing on the loon elite. Even winners face adversity and evolve interesting strategies to cope with it.

Jeremy Spool, a Ph.D. student at U.W.-Madison who works with Lauren Riters, developed an interesting research question aimed at the coping mechanisms of winners in our study population. Jeremy asked an intuitive, reasonable question about territorial breeders. Territorial pairs, Jeremy thought, should defend their lakes in a way that reflected their recent success. Pairs that had produced chicks in the past few years should be aggressive in territorial defense, because they were defending a resource whose value was clear and which would be costly to lose. Pairs that had not been successful rearing chicks recently might be expected to be a bit more lackadaisical about territory defense. Jeremy tested his hypothesis by exposing some of our territorial pairs to a loon decoy, which simulated a territorial intruder, and measuring their behavioral responses.

Jeremy’s results were unexpected. Pairs with recent breeding success did not behave more aggressively toward the decoy than unsuccessful pairs. In fact, they showed less aggressiveness towards intruders than did failed pairs. But successful pairs were clever about their defense; they became aggressive towards intruders in the few days leading up to egg-laying, a period when territories become vulnerable to intruders owing to incubation. In contrast, pairs without chicks the previous year showed no change in level of aggressiveness during the season. Jeremy concluded that successful pairs save energy by becoming aggressive only when they need to.

What Jeremy’s findings appear to show is that long-term pairs get into a groove with respect to territorial defense, targeting their defense towards times when it is most crucial. As with all good research findings, his raise a number of new questions. One obvious one is “Why should failed pairs be so inefficient about their territorial defense?”.    Another is “Must loons learn to defend their territories efficiently instead of doing so instinctively?”. These are exciting questions for the future that we look forward to tackling. For now, we are celebrating that Jeremy has just had his findings accepted for publication in the Journal of Avian Biology, a flagship scientific journal for avian research.



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”).


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


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


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”:


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:


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.)