Hanley D. & Doucet S.M. (2009). “Egg coloration in reing-billed gulls (Larus delawarensis): a

test of the sexual signaling hypothesis.” Behav Ecol Sociobiol (63): 719-729.

The authors attempt to test whether the ‘sexual signaling hypothesis’ explains the presence of bright blue/green eggs in ring-billed gulls. Despite the increased conspicuousness to predators, the authors predict that the blue color has evolved as a signal of female quality to male conspecifics. The authors investigated the applicability of this hypothesis by testing four of its predictions and assumptions: 1) blue/green pigment is limiting so egg chroma will decrease with laying order 2) clutch chroma is positively related to measures of female quality 3) brightness of blue green chroma should correlate positively with quality of the offspring and 4) males invest more in those clutches of higher blue green chroma.

The study examined a total of 81 eggs. Three reflectance measures were made on each egg to approximate their overall chroma and an average chroma for each clutch was calculated. Also, fresh egg mass, length and breadth of each egg and weight of chicks was calculated to measure offspring quality. Furthermore, adults were captured and several measurements of body size were made. Subsequently, blood was drawn for analysis of heterophil to lymphocyte ratio which was used as a proxy for female immune stress. Finally, paternal investment in eggs was measured in relation to clutch chroma, and entire clutches were swapped for 15 pairs of nests in order to investigate whether paternal investment was influenced by some female signal that co-varies with chroma of clutch. Observations of males assessed paternal investment by measuring feeding rates, brooding bouts, threats to neighbors and long call rate.

The results showed that there was no expected negative association between egg laying order and egg chroma; in fact, they found that the second eggs had higher blue green chroma than the other eggs. Similarly, no relationship was found between female quality (i.e. body size and the proxy for immune stress) and average clutch chroma. Furthermore, it was shown that blue green chroma did not correlate with offspring quality. Finally, similar negative results were found for the association between male investment and clutch chroma.

Although the results did not support their hypothesis, the authors’ methods and clear predictions reflect well the information presented in our discussion of signal costs. We learned that conspicuous signals such as bright egg coloration accrue the large cost of increased predation risk. We also discussed the prominence of honest signaling which explains the authors’ prediction that the possession of the pigments used to color eggs should be costly to the female if it truly reflected female quality. Furthermore, we discussed that sexual selection and natural selection are often at odds with each other, so it makes sense that the authors suspected that sexual selection must have influenced the evolution of such a conspicuous signal. This possible conflict between natural and sexual selection is prevalent as we spoke of several examples including the ormia and tungara frog situations in which this was the case. Furthermore, as mentioned in lecture the accurate assessment of all potential costs and benefits of a signal is more complicated than it would seem thus the inability of the study to find the expected predictions does not necessarily take support away from the ‘sexual signaling hypothesis’; it is difficult to know which selective forces are most important so the creation of proper predictions is sometimes difficult. For example, if predation risks have recently increased, this might explain the lack of male preference for blue eggs; predation risk may influence mate choice as we discussed in class with the guppy example.