Design of an intention signal in the bluehead wrasse (Thalassoma bifasciatum)

The 'design' of any animal signal is affected by at least three selection pressures: (i) the type of information being conveyed (the message); (ii) the transmission properties of the medium; and (iii) the sensory and psychological properties of the receiver. We describe a field study of the bluehead wrasse (Thalassoma bifasciatum) and examine the 'circling' display of the male in the light of these three pressures. Analysis of videotape records of the display shows that it carries accurate information about when a spawning dash will occur. The speed with which the pectoral fins are moved increases steadily over the 10 s before spawning so that the exact time of spawning can be predicted. Dark spots (not present when the male is chasing other males) appear on the ends of the pectoral fins and may 'amplify' perception of fin movements. The colour change shown by the male when chasing other males and when courting females was measured in situ by using Munsell colour charts, and was subsequently converted to dominant wavelength and excitation purity equivalents. The circling display appears to be designed to convey information about intention to spawn to a female that is relatively close whilst not conveying this information to other males that are further away.