Colorful Tits Produce Speedier Sperm

tags: evolution, evolutionary biology, evolutionary ecology, plumage color,carotenoid-based colour, carotenoids, lipid peroxidation, oxidative stress, sperm motility, sperm quality, sperm velocity, birds, ornithology, Great Tit, Parus major, researchblogging.org,peer-reviewed research, peer-reviewed paper

In some species of birds, males are more brightly colored than females. This phenomenon is due to female choice: females choose to mate with males that have the brightest plumage colors and most elaborate ornaments. But these characters are more than mere glitzy advertising, they are an example of honest signals because the brightest plumage and most elaborate ornaments are worn only by those males who have managed to procure enough resources necessary to grow them. But what is the link between plumage brightness and male quality? According to a newly published study of Great Tits, Parus major, males with more intensely colored breast plumage produce faster and more motile sperm.

Earlier work found a correlation between plumage color and male quality, but this new research delved further into this relationship and found that free radicals are the key to the puzzle.

Free radicals are produced by living cells as a part of normal metabolic processes and also when cells are exposed to pollution and other stressors. Free radicals are molecules, ions and even atoms with unpaired electrons, which makes them highly chemically reactive. Free radicals damage living cells by oxidizing DNA, proteins and lipids, thus causing a variety of negative effects from cancer to aging. Because they have sparse molecular repair machinery, sperm cells are particularly sensitive to free radical damages, which weakens their swimming ability, leading to decreased fertility in both animals and humans.

Free radicals are absorbed by a group of molecules known as carotenoids. These molecules not only defend cells against free radicals but carotenoids are a group of organic pigments that provide vibrant colors to all sorts of familiar items, from tomatoes and oranges to autumn leaves.

One hypothesis proposes that when a bird consumes more carotenoids than it needs to neutralize free radicals, those extra molecules are deposited into newly-growing feathers that are replaced during moult; the more carotenoids deposited into the feathers, the brighter those feathers will be. (Humans who eat excessive amounts of carotenoid-containing foods, such as carrots, likewise deposit carotenoids into their integument, rendering their skin a delightful shade of orange). According to another hypothesis, carotenoid-based plumage colors may be an honest signal of an individual male's capacity to acquire, absorb and metabolize carotenoids or other more potent, colorless dietary antioxidants, which may in turn protect carotenoid pigments from oxidation and make them accessible to signaling.

To test the relationship between carotenoid-based plumage color and the quality of individual male birds, an international team of scientists from Switzerland, France, Norway and the UK studied free-ranging Great Tits, a common European bird with carotenoid-based yellow breast plumage. First, the brightness of the birds' breast plumage was measured using a spectrophotometer and rated with a positive ("bright") or negative ("pale") number.

After the plumage brightness was determined, the team increased the stress levels of adult birds by giving them two extra nestlings to raise. At 7 and 15 days after the chicks hatched, the researchers trapped both the stressed and non-stressed males and massaged their cloacas to make the birds ejaculate. Speed and motility of the sperm were observed microscopically. The scientists found that sperm quality was roughly the same in non-stressed males, but among day 7 stressed males, birds with paler plumage suffered a decline in sperm quality (although this difference had nearly disappeared for all day 15 stressed birds).

According to these data, stressed males with paler plumage were able to compensate for the additional stress of having to raise more nestlings, given enough time. To test this observation, the researchers trapped 60 stressed male Great Tits seven and eleven days after their chicks hatched and fed them insect larva. The control group was fed unadulterated "placebo" insect larva while experimental group males were fed carotenoid-laced larva (provided in the relative proportions found in insects that Great Tits consume in the wild: 80% lutein, 3% zeaxanthin -- the two pigments found in great tit feathers -- and 17% β-β carotene). The experimental group were provided four times the daily amount of carotenoids that males obtain from their natural diet. (Figure 3)

Again, the team found that this "vitamin supplement" improved the paler males' sperm quality -- adding support to the link between plumage color, carotenoids and sperm quality.

This study begins to unravel the century-long mystery of why males often have brilliant plumage colors, even though this makes the birds more conspicuous to predators, says the team leader Fabrice Helfenstein, an evolutionary ecologist at the University of Bern, Switzerland.

Variations in individual male's resistance to oxidative stress may underlie variations in survival. Therefore, if a male's ability to resist oxidative stress is heritable, this study suggests a mechanism by which sperm quality and viability would be associated.

Not only does this study support a direct link between plumage color and male quality, but it also provides insight into why female Great Tits cheat, adds Helfenstein.

Females that choose to copulate with more brightly colored males gain direct fitness benefits by fertilizing their eggs with sperm with less damage caused by free radicals, hence avoiding the risk of infertility associated with sperm carrying more free radical damages. Additionally, oxidative damages to sperm DNA translate into deleterious mutations in the zygote. Thus, unfaithful females paired to males with pale plumage not only avoid the increased risk of infertility but also avoid producing low quality offspring that carry heritable deleterious mutations.

Because "females cannot always get the [colorful] males they want," Helfenstein explains, females will often settle for a less flashy mate. "But they will still sneak off for a rendezvous with a better-looking male -- and better sperm."

"It adds a bit of understanding to this puzzle."

Source:

Helfenstein, F., Losdat, S., Møller, A., Blount, J., & Richner, H. (2010). Sperm of colourful males are better protected against oxidative stress. Ecology Letters, 13 (2), 213-222 DOI: 10.1111/j.1461-0248.2009.01419.x