Pigment-based skin colour in the blue-footed booby: an honest signal of current condition used by females to adjust reproductive investment
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In monogamous species, the value of present reproduction is affected by the current condition of the mate, and females may use male ornaments to evaluate his condition and adjust their level of investment according. Many animals display colour in fleshy structures which may be accurate indicators of quality due to their potentially rapid response to changes in condition. Here we show that in the blue-footed booby, Sula nebouxii, male foot colour is structurally (collagen arrays) and pigment based. In 48 h foot colour became duller when males were food deprived and brighter when they were re-fed with fresh fish. Variation of dietary carotenoids induced comparable changes in cell-mediated immune function and foot colour, suggesting that carotenoid-pigmentation reveals the immunological state of individuals. These results suggest that pigment-based foot colour is a rapid honest signal of current condition. In a second experiment, we found that rapid variation in male foot colour caused parallel variation in female reproductive investment. One day after the first egg was laid we captured the males and modified the foot colour of experimental males with a non-toxic and water resistant duller blue intensive make-up, mimicking males in low condition. Females decreased the size of their second eggs, relative to the second egg of control females, when the feet of their mates were experimentally duller. Since brood reduction in this species is related to size differences between brood mates at hatching, by laying lighter second eggs females are facilitating brood reduction. Our data indicate that blue-footed booby females are continuously evaluating their mates and can perform rapid adjustments of reproductive investment by using dynamic sexual traits. We suggest that this fine-tuned adjustment may be widespread in socially monogamous animals.
KeywordsDynamic trait Differential allocation Sexual signal Carotenoids Brood reduction
We thank Emma Cunningham, Diego Gil, Juan Freire, Dave Anderson, Kevin McGraw and Carol Vleck for valuable comments on the manuscript, Ana María Estrada Sánchez for helping during field work, Pablo Lago Velando for donating carotenoids and Glenn Furnier for improving the English. The study was supported by the Universidad Nacional Autónoma de México (IN230603). Logistic support was provided by the Armada de Mexico, the staff from the Parque Nacional Isla Isabel and the fisherman from San Blas and Camichin. During the study, AV was supported with a grant from Universidade de Vigo and “Ramón y Cajal” fellowship from the Spanish Ministerio de Ciencia y Tecnología. The experiments performed comply with the current laws of Mexico, where the work was performed (permissions from SEMARNAT 01907 and 03687).
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