1. “Piep Piep Piep – Ich hab’ dich lieb” Rhythm as an indicator of mate quality Eva van den Broek and Peter M. Todd evdbroek@phil.uu.nl, ptodd@mpib-berlin.mpg.de Utrecht University, Department of Philosophy MPI for Human Development, Center for Adaptive Behavior and Cognition, Berlin I. Individuals The male tries to sing his inborn template. Inversely related to his quality, he makes mistakes. Mistakes can take the form of a note insertion or a deletion. Both the male and the female have an inborn template. The template is a temporal sequence of 10 notes. Males have a randomly- assigned quality. The female compares the male song against her template. For every note, the male gets a positive or negative score. The final male score [0-10] is the summed score modified by a small perceptual error. I. Individuals Mate Quality: 0.7 II. Singing insertion deletion A “0” is inserted at random, the remaining template is shifted to the right, the last note is deleted A random note is deleted, the remaining template is shifted to the left, the last note becomes a “0” Inborn template Final song Aims of the study Rhythm is common in courtship displays. Insects, birds and humans use singing, dancing or drumming to attract the other sex. Our model explores the possibility that using rhythm as an indicator of mate quality was a driving force for the evolution of rhythm in sexual signals. We focused on bird song as an example of rhythmic behavior because female birds can assess a male's mate quality through his singing. We propose, by way of a model, that it is easier to judge a partner’s quality on a rhythmic song than on a non-rhythmic song. This might be the reason that so many signals take a rhythmic form: regular, rhythmic displays heighten the salience of errors. Mistakes due to neural disfunctioning or developmental instability can be easily perceived by the female. The title means "Peep peep peep, I love you" and is derived from the song "Guildo hat euch lieb", by Guildo Horn And The Orthopaedic Stockings, Eurovision Song Contest 1998 III. Judging 0 1 0 1 0.50 01 expected heard Female template Male song, heard by female Score x x x x x x x x x population M 0101010111 M 0010100010 M 1010001110 F 0100100101 egg F 0110101100 M 0101001011 IV. Recombination The female hears subset of males and chooses the one with the highest score. The number of offspring depends on male mate quality. The offspring get a mutated template from a parent. Expected reward Unexpected no reward 0.5+0+1+0.5+1+0+1+0.5+0+0 = 4.5 + 0.7 = 5.2 Results and conclusion We computed across 20 preference tables which song templates were best discriminating with regard to males of high or low quality. The most discriminating song template was rhythmically alternating (1010101010 or 0101010101) in all but four cases. These results indicate that a rhythm- preferring female better can better tell high quality males from low quality males, and thereby have more offspring, implying that rhythmic signals do seem to be useful quality indicators. Will the song templates eventually evolve towards this optimum? The model shows that rhythmic songs are more discriminating with regard to the type of mistakes that male birds make in comparison to non-rhythmic songs. In our framework, this effect is not strong enough to drive evolution to perfectly rhythmically alternating templates. However, once predominating, the rhythmic templates were more stable against evolutionary change than monolithic templates. Rhythmic displays may have evolved as indicators of mate quality by means of their conspicuous properties. Other selective pressures or historical contingencies favoring rhythm remain to be investigated. Number of alterations (y-axis) plotted against generations (x-axis), each line averaged over 5 generations. After initializing the whole population with one of the two extremes, 1010101010 (9 alternations) and 1111111111 (0 alternations), we found that the discriminability effect was strong enough to lead the templates away from the monolithic template, but also they evolved slightly away from pure alternation. deletioninsertion Perceptual noise Two types of mistakes: Preference table We constructed both coevolutionary and optimization models of populations of artificial birds that produce and evaluate songs for mate choice. The expression of the song both has a genetically transmitted and a condition-dependent component. We defined rhythm as regularly repeated notes. The line of research we advocate fits with the research devoted to finding the correlation between irregularity and developmental stress or genetic imperfection. Is a rhythmic birdsong the sonic equivalent of a peacock’s tail, through which a male can reveal his quality?