There are three main reasons why I do field research. First, I love the outdoors. While it is unpleasant and inconvenient to many, the field is my briar patch. Second, I love observing and being with animals and gain new insights by watching loons up close and not just looking at data in my office. And third — well, I enjoy looking at data in my office. Complex puzzles and contradictions concerning loon behavioral ecology often become comprehensible when I look hard the numbers and run statistical tests.
Yesterday I had one of those Eureka! moments during data analysis. For the past several months, I have been looking at factors that: cause loons to: 1) be evicted from their territories and 2) disappear from their territories (that is, die). Some months ago I shared with folks that having your mate get evicted puts you at risk for eviction yourself. That is an interesting pattern and one that I continue to explore. But yesterday, I conducted the most sensitive and powerful analysis to date on the potential impact of age on rate of eviction and death in territorial residents. The results were clear: age has a substantial impact on both eviction and death rates. Specifically, old loons are at risk for being booted off of their territories and they are even more strongly at risk for dying. The pattern was a bit tricky to detect, because very young adults are also at risk for eviction and death. So loons — like humans — have a “prime of life”. Loons of 4 to 6 years of age are still developing and improving in condition; at this age, they have a rather high rate of eviction and death. They reach prime condition about age 7 or 8 and remain in good condition until about 20, then condition falls off again, exposing them anew to a high rate of eviction and death.
The figure above shows the shape of the pattern with respect to age and death rate, when other factors are controlled. Sorry for the arcane title of the Y axis, but here is how to interpret the pattern. The curve shows how often territorial loons die, higher values indicating a higher death rate. The values are higher for young ages and old ages. Thus, the “U” shape shows that young and old loons die at a higher rate than do loons of moderate age. If you look closely, you can see that old age has a much stronger, more consistent impact on mortality than does young age. So senescence is very clear from the graph.
If you are a loon aficionado, you know that male loons often die in the course of defending their territories, while females rarely do so. How does the new finding of senescence fit in with lethal contests among males? On the one hand, it fits, because senescence might make male loons “desperadoes”, who fight hard for their territories because they have no future to lose. On the other hand, the pattern of senescence cannot by itself explain male-only fatal battles, because females and males both senesce. Oh well….I am always looking for another excuse to look at data in my office.