Recently, my friend Griffin saw this refrigerator magnet and said "That red bird has more carotenoid pigments. That's an indicator of better immune function and fitness, so he will attract more females."
|I don't know what he is so cranky about…he should have better luck finding a mate than that smug Bluebird!|
|Beta-carotene, one of the more well-known carotenoids|
|Carrots are packed with beta-carrotene (Ha!)|
Carotenoids are known to have significant physiological functions, especially in aiding the immune system [1-5]; specifically, carotenoids seem to stimulate T-lymphocyte response efficiency  and act as potent antioxidants, cleaning up cellular waste from the immune system kicking pathogen butt. It has also been suggested that carotenoid compounds support the production of steroid hormones that regulate reproduction such as testosterone and estrogen . In other words, carotenoids play a huge role in keeping birds (and other animals) healthy and happy!
In any case, male birds with the right machinery can put on an irresistible display for the females. Birds of Paradise (shown in the video below), not unlike hip-thrusting rockstars, can use a combination of flamboyant colors and dramatic movements to woo their mates:
So, how many of you think: "WOW! Carotenoids function in immunity AND colorful plumage for mate selection?? Sounds like a win-win situation to me!" Show of hands…NOW!
Well you're all wrong! There are trade-offs at play here, which bodes well for female birds looking for a mate with the best genetic and phenotypic background. Males that do incorporate carotenoid pigments into their plumage must do so at the cost of not having available carotenoids circulating in the blood to aid the immune system. Birds that are battling an infection or just have a weaker immune system than others will allocate more carotenoids to the body's defenses, resulting in a duller color in the feathers or bill [1-5]. This acts as an "honest" display of fitness and immune function in males; the brighter the male, the fewer carotenoids being shunted for immune function and the more carotenoids making him the bold and beautiful macho-bird he is. Females prefer bright, sexy males, so this sexual selection leads to brighter and sexier males (and females hard-wired to be attracted to them) in subsequent generations. The video below explains the phenomenon of sexual selection well:
So how exactly do we know there is a trade-off between secondary sex characteristics like bright plumage and immune response? By using…drumroll please... SCIENCE!!! Several research groups studied avians such as blackbirds  and finches [1, 2, 4] by observing key characteristics before and after an immune challenge. These parameters included: bill/plumage color (determined by sight and by high-performance liquid chromatography), serum carotenoid levels (determined by HPLC), and antibody production. The challenge was administered in the form of sheep red blood cells (SRBC) or control injected into the birds, injection with lipopolysaccharide from E. coli, and/or a phytohemagglutinin (PHA) skin test. They predicted that if there is a significant evolutionary trade-off, then visual and chemical decreases in carotenoid-based pigments will occur in the treated groups.
|Figure 1 from Faivre et al. (2003). Blackbirds challenged with sheep red blood cells (SRBC) had a significant decrease in bill color relative to birds injected with PBS alone. |
Birds that were in the experimental groups exhibited marked differences in bill/plumage color, lower serum carotenoid levels, and high antibody production due to the movement of pigments out of the bloodstream and into affected tissues as part of the immune response [1-4]. Males that had a significant drop in bill/plumage color had an inversely correlated immune response, meaning these colorful sex characteristics are dynamic traits that reflect the current health status of the male . Additionally, an excess of dietary carotenoids supplemented in the feed or water of birds did not significantly change the allocation of pigments. Regardless of what is available, most carotenoids will be shunted to the immune system when challenged. However, supplementation of extra carotenoids does boost the immune response significantly compared to birds fed a regular diet .
|Figure 3 from McGraw et al. (2003). (a) Male finches that had higher levels of plasma carotenoids had a stronger cell-mediated immune response. (b) Male finches that had more red beaks were able to mount a better response to SRBC injection. |
In summary, healthy males use their intake of dietary carotenoids to brighten their plumage and attract a female mate. These heritable traits are passed down to their offspring and the trend continues. If the males fall ill, their colors become dull as they fight disease and they become less attractive to prospective females. Just like when we get the flu, we become pale and sickly looking…not exactly a handsome appearance. It looks like the colorful birds must find a balance between looking good and feeling good, and effectively apportioning carotenoids can pay off big time!
Stay sexy, my feathered friends!
1. Alonso-Alvarez, C., S. Bertrand, G. Devevey, M. Gaillard, J. Prost, B. Faivre, and G. Sorci. 2004. An experimental test of the dose-dependent effect of carotenoids and immune activation on sexual signals and antioxidant activity. The American Naturalist 164:651-659.
2. Brush, A. 1990. Metabolism of carotenoid pigments in birds. The FASEB Journal 4:2969-2977.
3. Faivre, B., A. Gregoire, M. Preault, F. Cezilly, and G. Sorci. 2003. Immune activation rapidly mirrored in a secondary sexual trait. Science 300:103.
4. McGraw, K. and D. R. Ardia. 2003. Carotenoids, immunocompetence, and the information content of sexual colors: An experimental test. The American Naturalist 162:704-712.
5. Olson, V. and I.P.F. Owens. 1998. Costly sexual signals: Are carotenoids rare, risky, or required? Tree 13:510-514.