After I explained that breeding loons must learn about their neighbors’ chicks through yodeling of neighboring males, my mother-in-law shot back:
Wouldn’t that behavior be counterproductive for the yodelling male and hence, be modified in his progeny……….unless it’s paired with positive results. What would that be? What’s the point of the yodel once he has a mate?
There are a couple of points to address here. First, a small one. Joanne implies that male loons — like most songbirds — use the yodel to attract a mate. The concept that bird song is a signal to potential mates is so firmly entrenched in our brains that we almost take it for granted. We know that most males that yodel are already paired with a female (like the Manson Lake male in Linda’s awesome photo, above). So, at best, mate advertisement could only be one of multiple functions of the call. Alas, though, we have no data on this question. I know — the yodel as a signal to potential mates seems a simple idea — but no one has yet played yodels from a territory in April and early May and counted the ratio of males and females that hear the yodels and visit the territory. If yodels function as advertisement for a mate, of course, we would expect a high proportion of all visitors to a territory from which yodels are emanating to be unpaired females. Sounds like a good Masters’ project!
Now on to Joanne’s main point. She is absolutely correct that behavior, like other biological traits of animals, should not occur unless it increases fitness — that is, unless it increases the number of offspring produced by the individual showing the behavior — and, hence, the number of individuals that possess that trait in the next generation. So yodels by males should not have evolved if all they do is bring the neighbors in from next door to spotlight the chicks! However, as Joanne suggests, we might sometimes expect that a behavior could have two (or more) impacts on evolutionary fitness. In other words, behavior might affect fitness negatively in one way, yet provide a benefit in another way that more than offsets the negative impact.
Now let’s apply this concept of “multiple impacts” to the loon case. Based on published work, we know that yodels are useful to loons in that they: 1) prevent landings of intruders that might evict a territory owner, and 2) keep intruders that have entered a territory from approaching the chicks closely. Therefore, I surmise that these two fitness-enhancing consequences of yodels are so beneficial to yodelers that they more than offset the negative impact of increased spotlighting of chicks by neighbors. In the interest of full disclosure, I must admit that it is very difficult to measure the impact of behaviors on fitness precisely, so we must be content, at this point, to know that yodels are beneficial in some ways and costly in others.
In thinking about Joanne’s question some more, let me provide a bit of context for the idea of spotlighting. While it is an exciting advance for loon biology to learn that loons are probably spotlighting each other’s chicks, “eavesdropping” on each other’s yodels is not surprising to behavioral ecologists. Eavesdropping, defined as intercepting of signals directed at a specific receiver by a third party that is not the intended target of the signal, turns out to be widespread in animals and makes a lot of sense. If you are a young loon floater trying to obtain a territory — especially a male floater — you would benefit immensely from listening to the territorial defense signals of established males in your neighborhood, since yodels are known to convey information about the identity, body size, body condition, age, and motivation to attack of the yodeler. If you hear from his yodel that a certain male is large and likely to be aggressive to intruders, you would do well to avoid landing on his territory!
Eavesdropping is one of several phenomena that behavioral ecologists have discovered by taking a second look at animal behaviors that seemed not to square completely with known behavior patterns. Another such phenomenon is “prospecting” by young animals looking to settle in a region. Young adults of many species that are in search of breeding territories explore their habitat extensively in one year and then settle the next year in specific areas where they had detected cues indicating successful reproduction (such as the sight or sound of offspring) the year before. Loons prospect for territories too.
Prospecting?? Eavesdropping?? Wow…..loons sound more like us every day!