Tag Archives: birds

The cost of reproduction in birds

24 Friday Aug 2012

Posted by in Recent Papers

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The concept of trade-off is paradigmatic in life-history theory. an organism can only acquire a finite amount of energy in its lifetime, so it must “choose” how to allocate that energy to growth and survival or reproduction. Reproduction is assumed to be costly so that individuals who spend more on reproduction, for example by laying more eggs, will not survive as well. We suppose that over evolutionary time, natural selection will act on genetic variation for these allocation decisions, so that the sequence of decisions over an individual’s lifetime will represent an optimal allocation of resources.

Unfortunately this intuitively appealing idea has been very hard to find in nature. In fact, many studies have come up with positive correlations: animals that reproduce more tend to survive better. A recent study by Eduardo Santos and S. Nakagawa found that this trade-off was almost impossible to detect in most studies, or non-existent altogether. In a meta-analysis of brood supplementation studies (researchers added eggs to the nests of breeding birds), they found little impact on survival. Their result held across all the major taxonomic groups of birds, the biggest division being between passerines (songbirds, crows, flycatchers, etc) and non-passerines (ducks, loons, parrots, woodpeckers). Regardless of overall “lifestyle” the birds tested in most studies were able to withstand the hypothesized survival cost of additional eggs dumped on them by researchers.

Bird - Seagull enjoying the sunset

Why would this be the case? As always there is the possibility that the studies were poorly designed, or that brood supplementation is not a good way to test for a trade-off. Particularly, brood supplementation only taxes the parents of their ability to defend and feed offspring; it does nothing to the energy that females put into egg production. The other possibility is that adult birds just don’t put that much effort into reproduction in the first place. Perhaps survival is far more important. The trade-off is still there, but it’s just not important for most birds.

The hypothesis that life is just not as Malthusian as we have often supposed in evolutionary biology intrigues me greatly. If evolution acted in the “well-oiled machine” manner that many laypeople and professional scientists find appealing, then we’d expect selection to push annual reproduction right up to the level allowed by the trade-off. What studies have found is birds putting minimal effort into reproduction, parenting or anything that affects their survival. This means that selection is a lot weaker than we expect: this gives genetic drift a lot more room to account for polymorphism. It also makes sexual selection more plausible: if most species have fairly conservative lifestyles and selection for survival is not that strong, then males (or females) can afford costly ornaments.

An unrelated study also appeared this week that is getting a lot of press: researchers in Iceland found a strong relationship between the age of fathers and mutations passed to their offspring. This is the first study to quantify the per-year effect of paternal age on offspring mutations in humans, so it’s a pretty big deal. I will talk more about this in a future posting since it’s related to my dissertation research, but in the meantime, go read the article and enjoy the flurry of debate surrounding it.

E. S. A. Santos, S. Nakagawa (2012). The costs of parental care: a meta-analysis of the trade-off between parental effort and survival in birds Journal of Evolutionary Biology, 25, 1911-1917 DOI: 10.1111/j.1420-9101.2012.02569.x

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The Handicap Principle

20 Friday Jul 2012

Posted by in My Research

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Researchers use sexual selection theory to attempt to explain traits that are exaggerated, seemingly unrelated to survival and seemingly costly. I call these characters “ridiculous.” Almost every sexual selection talk starts with a collage of absolutely ridiculous-looking sexual ornaments and armaments. Most biologists have this “biological diversity slide” near the beginning, but other biologists have theirs filled with perfectly sensible looking animals. Animals that are clearly built for survival. Sexual selection, on the other hand, seeks to explain things like this:

Male Blue Peacock in Melbourne Zoo, Australia.

Male Blue Peacock in Melbourne Zoo, Australia. (Photo credit: Wikipedia)

One way to explain these extravagances is not to use sexual selection at all, but to say that it is, indeed necessary for survival. In fact, the whole idea of sexual selection arises out of a Bob McGuire argument: “Come on, that can’t possibly be necessary for survival! Chuck, you’re crazy.” Bob McGuire was a timeshare salesman who tried to talk Dr. Adamson and I into buying by saying things like “Come on, you’re not gonna take a baby camping, come on!” These appeals to disbelief are rather common in science, not just in anti-science.

The alternative looks at how ridiculous-looking traits do impact survival: if they are really costly, perhaps that cost relates to their value as signals. Perhaps they tell females something. Perhaps the cost itself is really important, and females should pay attention to that cost. How would a signal convey all that?

These important things to consider are fitness components. We talk a lot about fitness, but there’s no one measurable quantity that actually is fitness. Survival forms one fitness component, attractiveness forms another. When females want to have attractive offspring, one way to get them is just to mate with an attractive male: “You mate with a good-lookin’ bird like me, you’ll have good-looking babies, and good-looking grandbabies and so on. It’s a win!” The trait doesn’t need to be particularly costly to tell females that their offspring will be attractive. But what if the offspring don’t survive to mate?

Handicap to the rescue!

A really costly signal, on the other hand, could tell females that a male not only is attractive, but he’s able to survive well. The signal tells females that males carrying them are able to survive well because their trait does not impact their health as much as it would someone who was less healthy. This is called the handicap principle: if a male shows a handicap, he must be well-adapted and healthy, or else he wouldn’t be able to handle the cost. This idea was first proposed independently by Bob Trivers and Amotz Zahavi, and roundly rejected as completely preposterous. The 1989 edition of The Selfish Gene contains the typical argument: if it’s costly, then it’s costly and it will be selected against.

However, in 1990 Alan Grafen showed that handicap signals are evolutionarily stable: if everyone in the population is using a costly signal, then someone using a non-costly signal to convey the same information can’t make a living. If a signal is truly costly, then you can’t fake it. Grafen showed with his characteristic style that this applied to communication across the board, not just in sexual signals. Some researchers go so far as to say that every signal is a handicap, although I think “the finger” is enough of a counterexample. Incidentally, “the finger” did originally have meaning: it meant you had never been captured in war and were still able to shoot an arrow. I think this meaning has been lost since the Battle of Agincourt.

Kinds of handicap

There are a few different ways to have a handicap. One is called “pure epistasis” or “Zahavi’s Handicap.” In this case, every male grows the same trait, but it kills less-healthy males more often than it kills more healthy males. This is the version that Zahavi came up with originally, and it was widely ridiculed. It turns out the ridicule was partially correct, and this sort of handicap doesn’t really work. Zahavi’s handicap doesn’t work because after selection for survival, all males will start to look pretty much the same, and then females have no incentive to choose among males (a female who mates randomly will get the same good genes as one who goes to the trouble of choosing). Then the trait is useless, and costly, and will be lost from the population.

The second way is the “revealing handicap.” In this case all males grow the trait, but the signal itself has much better quality in more healthy males. The caricature is of a peacock’s tail: all males grow similarly-sized tails, but less healthy males let theirs drag on the ground and get infested with mites. It doesn’t kill them, but it’s not as shiny and dazzling and sexy as it would be for a healthy male. In this case the signal itself tells females how healthy a male is. This kind of handicap can lead to an exaggerated male trait, as long as there is sufficient genetic or environmental variation for the tail dragging on the ground.

The third way is condition-dependent signaling: males who are of good health grow larger traits and are less impacted by the costs of carrying the trait. The nice thing about this theory is that everything seems to work as far as genetic variation. Females benefit by mating with showy males, from having more healthy male and female offspring. Also, if the trait is condition-dependent, there is plenty of mutation in “condition,” which is basically the sum of all the selective forces across the genome. There should always be plenty of genetic variation in condition, so females should always benefit from being choosy.

I study how condition-dependent signaling plays out over the lifespan. If a male is really healthy, he can expect to live a long time, and he could potentially conserve his resources until he gets older. By growing his trait over a long period of time, he would have more opportunities to mate, and be just as sexy in middle- or old-age when selection is less intense. This does present a few problems in how the traits would actually change over time. If selection is intense enough, a male might be killed even for having a small trait, and once he gets older, his sperm will be harboring more deleterious mutations. I’ll go into more detail about these in future posts.

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