While the benefits gained by female birds paired with males exhibiting bright, sexually dimorphic colors may include territorial resources, parental care, and more attractive offspring with better viability, the functions of colorful traits that continue to be exhibited after pair formation in monogamous birds are unclear. One possibility is that the colorful traits evolved to stimulate and motivate female birds, and thus influence reproductive choices such as number of offspring produced, and investment in individual offspring in a particular breeding season. Torres and Velando (2003) experimentally modified the foot color of courting male Blue-footed Boobies Sula nebouxii, such that it resembled the dull, dark blue foot color of males in a poor nutritional state, and then examined the responses of females to these birds. Male behavior was unaffected, but female birds courted less (e.g. decreased sky-pointing rates), and were less likely to copulate, when paired with males that had dull blue feet. Female Blue-footed Boobies also have colorful feet, and the brightness of their foot color in turn affects the frequency of intrapair courtship, as well as the likelihood of extrapair courtship (Torres and Velando, 2005).
Blue-footed Booby, Sula nebouxii
Photo: Dr. Jim Smith, © 2008
Long-lived animals that reproduce repeatedly (iteroparous) should continuously assess the value of their current reproduction, and in monogamous species, this value may be influenced by the immediate condition and health of the mate. To determine whether foot color may reflect the condition and immunological status of male Blue-footed Boobies, Velando and colleagues (2006) first determined the influence of diet on integumentary color. Foot color during courtship varied from dull blue to bright green, and derived from both structural and carotenoid pigment contributions. Both food supply and dietary carotenoids affected foot color, with a significant change observed after 48 hours. Carotenoids in the diet also improved cell-mediated immunity, measured as the T lymphocyte response to intradermal phytohemagglutinin injection in the wing web.
Changes in foot color after manipulation of nutrition and dietary carotenoids in male Blue-footed Boobies (Velando et al., 2006). Panel b shows the carotenoid-deprived color, and panel c shows the carotenoid supplemented (more attractive) color.
Next, the investigators experimentally manipulated foot color in males, by using a water-resistant makeup that mimicked the dull blue color of low condition birds; control males had their feet “sham-colored” with a plastic-wrapped crayon. Blue-footed Boobies normally lay two eggs each season, with the second egg being heavier in times of plenty, to compensate for asymmetries in the siblings. When food supplies are poor, the second egg laid is lighter in weight, such that brood size reduction is more likely. The researchers found that females, mated to males with experimentally duller feet, laid significantly smaller second eggs, supporting the hypothesis that rapid changes in male foot color cause corresponding changes in female reproductive investment. The reduced egg size for females mated with duller-footed males is likely to be due to decreased albumen mass, a factor that affects hatchling size and hatching success. Thus, male foot color in this species is a dynamic, condition-dependent sexual trait that allows mate evaluation and modification of breeding decisions after pairing.
* I apologize for the title of this post, but the alternative was “Healthy Boobies = Better Boobies”.
^’^ Wednesday Wings series, blogging about bird biology
Torres, R. and Velando, A. (2003). A dynamic trait affects continuous pair assessment in the blue-footed booby, Sula nebouxii. Behav. Ecol. Sociobiol. 55, 65-72.
Torres, R. and Velando, A. (2005). Male preference for female foot colour in the socially monogamous blue-footed booby, Sula nebouxii. Animal Behavior 69, 59-65.
Velando, A., Beamonte-Barrientos, R., Torres, R. (2006). Pigment-based skin colour in the blue-footed booby: an honest signal of current condition used by females to adjust reproductive investment. Oecologia, 149(3), 535-542. DOI: 10.1007/s00442-006-0457-5