“Pure, sweet, Canada, Canada, Canada,” the thin high voice sings, and I feel like I am back home in the spruce and pine north woods of Michigan, where I learned to be a biologist. I could be walking from my tin cabin right on Douglas Lake towards the entomology lab at Bug Camp, back in 1972. But I wasn’t. I was not 19 years old any more either. But I still love this bird, and all it means.
On the Cypress Creek – Katy Prairie Christmas Bird Count we spotted several white-throated sparrows, Zonotrichia albicollis, hunched down in a tangle of leafless branches across the creek. They were all fluffed out in the cold air, little round balls with vanilla ice cream chins. Bob Honig told us about the color polymorphism and the behavioral differences that go along with them. It was interesting enough that I decided to dig into the primary literature and see what more I could learn about the details. I like to see the data and interpret it for myself. So I poked around just a little and found some nice papers indexed on Web of Science and also on the more accessible Google Scholar. Unfortunately the Rice library had a 12 month embargo on Journal of Evolutionary Biology, and I hadn’t gotten around to joining their society yet, so I needed to beg a little. I located Vincent Formica easily at University of Virginia, and sent him an email at 7:44 AM requesting his interesting paper as a PDF so I could read it. By 11:24 AM that same day, I had the paper and a nice email from this busy scholar I had never met. I just love science, and scientists! Did I mention that it was a Sunday?
So, what’s the story? I’ll start with the cool behavioral stuff, then move on to the genetical details. Basically, in these sparrows there are two kinds of guys, the party animals that go aggressively for the females with song, and mating with anyone whether she is their mate or someone else’s. These white-lined guys are not so great at guarding their own mates from other like-minded males, and they are slackers when it comes to taking care of the babies. These aggressive, sexually active males establish territories in the best areas where they have to fight to keep them. Then there are the tan males and they are helpful fathers who are focused on monogamy and their off-center territories. Guess which morph has higher testosterone levels? Right, it’s those white males! By the way, there are photos of each kind below.
There is a neat family twist to these two kinds of males. Guess who the white guys choose as their mates? How about the tan guys? Each type mates with females of the opposite type! This makes complete sense for the white guys, because if they mated with a white female, the babies would carry a lethal chromosomal combination, described below. It is interesting that the tan guys also go for opposite type females. I don’t know if this is true or not, but it could be the tan males prefer the white females because the intruding white males won’t go for them. Since the other tan males are also faithful mates, confidence of paternity might be higher for the tan males if they chose a white female. This is just what Elaina Tuttle found in a paper in 2003. In the white male x tan female pairs 31.8 % of nestlings were not sired by the social father, whereas in the tan male x white female pairs only 4.4% of nestlings were not sired by the social father. Apparently the expression of fidelity in females is not dependent on the same genes as the males in this interesting system.
By the way, Elaina Tuttle has some great educational material on her website directed at a variety of ages. Check it out for lots of cool info and photos!
This behavioral variation is just one example of a behavioral trade-off. Males have only so much energy to invest, and they can invest it in one female and in caring for her young, or in hunting and wooing many females, with less promise of care. This latter strategy is most likely to work in rich habitats where females can get enough food for their young without dad’s help, and this may be part of the story here. After all, what good does it do for a male to philander if it means the babies at home starve? Even if he isn’t the father of all of them, he’s likely to be the father of more of them than at any other nest.What the males do, when the males help, why females help more, and what strategies evolve for is a huge topic in animal behavior. One of my favorite papers on the topic was written by my very own husband, David Queller, and it’s called Why do females care more than males? The bottom line is because females are more sure the babies are theirs than the males are, but there are some nice twists. Check it out!
One of the biggest surprises in studies of birds once we got good molecular markers was how much fooling around there is. Species after species has males who diligently tend babies that turn out to not be their own, or at least not all their own. In the case of the white-throated sparrows, Formica and Tuttle report that 35 percent of the nests they sampled in 2000 had at least one chick that was the result of the female copulating with a male other than her mate. This made up 27 percent of all chicks. This philandering varied with years. In a year with average density the tan males had no extra pair chicks in their nests. When density of nests was low, even the tan males had some extra pair chicks. I guess extra-pair is jargon. It just means a chick in the nest that doesn’t belong to dad.
It turns out that these behavioral and appearance differences are based on something we know. The two different forms of the white-throated sparrow have a genetic difference we understand, something that is all too rare in science. The white form is heterozygous for a chromosomal rearrangement on autosome 2, the second longest chromosome. What does that mean? It means that the white birds have big differences in this region of the chromosome called 2 that is not a sex chromosome. One chromosome has an inversion and the other doesn’t. An inversion is when a chunk of a chromosome with several genes on it gets stuck in backwards, so a gene at the end is now at the beginning. This often is the result when a loop of chromosome forms and then a break gets repaired slightly incorrectly. Then there is no crossing over in this area because the genes between the inverted and the normal can’t line up at recombination.So the tan bird has two copies of the normal ancestral chromosome and the white one has one normal and one flipped. The homozygotes (two copies of the same genes – remember birds are diploid and have two copies of every chromosome, just like humans) of the tan form do just fine. But the homozygotes of the white form are very rare, apparently because it is a lethal combination! One could speculate about why – too much of that aggressive, promiscuous behavior – but it is much more likely to be because of some other essential gene function.
By the way, all of this work was conducted in the field at Cranberry Lake Biological Station. I’d love to go there and study just about anything! I bet they have lots of my current favorite organism, Dictyostelium discoideum, even including the newly-discovered farmer form!
Here are some references to this cool story that I used in the narrative above.
Formica, V. A. and Tuttle, E. M. 2009. Examining the social landscapes of alternative reproductive strategies. Journal of Evolutionary Biology 22:2395-2408. doi:10.1111/j.1420-9101.2009.01855.x This paper explains the different places the two morphs nest, and discusses the significance of having two strategies.
Queller, D. C. 1997. Why do females care more than males? Proceedings of the Royal Society, B, Biological Sciences 264: 1555-1557. doi: 10.1098/rspb.1997.0216. This is a theoretical treatment of why females are more likely to provide parental care than males.
Romanov, M. N. et al. 2009. The value of avian genomics to the conservation of life.BMC Genomics 10. doi:10.1186/1471-2164-10-S2-S10 This paper talks mainly about the genetics of the chromosomes and genomes, but gives a summary of the behaviors and it is open access.
The sparrows were in this thicket.
White-throated sparrow, Zonotrichia albicollis, white form, taken by Ken Thomas (public domain).
This one is the tan form.
The figure above is from Tuttle 2003. It shows that the white males (white bars) spend less time guarding their mate than the tan males (dark bars). The white males are also less likely to follow their mate.
This figure above is taken from the Formica and Tuttle paper from 2009 in Journal of Evolutionary Biology. The brown shapes are the tan male’s territories, and you can see they are more on the edge, where there are fewer other males. The yellow territories of the white males are more in the center, where there are more other birds. You can see there is also overlap in the areas. That’s biology! We just don’t find perfect patterns, and there is always overlap and other factors we haven’t considered.