Saturday, January 21, 2012

Dazzle vs. neatness of finish

Fascinating result in this week's Science (poss. gated) suggesting that bowerbirds might be using optical illusions (viz. forced perspective) to attract mates.

The males of avenue-species bowerbirds—including great bowerbirds—construct elaborate bowers composed of two stick walls. The bowers are aligned to run from north to south, and the ends are filled with “gesso” (a collection of gray to white shells, stones, and bones), upon which colored objects are placed and thrown. [...]. Females enter bowers from their south end and watch the male at the north end carry on a display that includes vocalizations, movements, and the tossing of colored objects in front of the gesso. Many males never succeed in attracting females to their bowers, and only a select few do most of the mating after females visit and inspect their bower.[...]

In a previous report, Endler et al. (3) discovered that the arrangement of the gesso is not random: Smaller objects are arranged near the opening of the display court, and larger ones more distant from the opening. When the researchers reversed the gradient, the bowerbirds rapidly restored it to its initial distribution (but not the individual objects, just the gradient). This study showed that male bowerbirds, for whatever reason, care about the gradient. The interesting observation was that the bower creates a condition conducive to creating experiences of “forced perspective,”[...]. 

Kelley and Endler provide insight into why the males care so much about the gradient. They report that the males most adept at crafting forced-perspective illusions are most likely to achieve mating success. [...] The projected gradients, as seen from within the bower, are more predictive of mating success than the actual physical gradients. [...] Have male bowerbirds mastered the laws of perspective and learned to manipulate them to achieve lascivious ends? 

Although this possibility is intriguing, the current data are not yet sufficiently rich to sustain this remarkable hypothesis. The data provide compelling evidence that the quality of the gradient, from the vantage point of the female, predicts mating success, but the visual fact of the more uniform texture, not an illusion, may be the only factor determining her preference. Indeed, it has been previously reported that males with more symmetrical bower avenues have a higher mating success rate.
The -- immediately obvious -- argument for this view and against forced perspective qua depth-illusion is that the stones are arranged the wrong way round; they make the bower look smaller rather than bigger, and there cannot be any advantage to that as one could simply construct a smaller bower instead. (Also I don't know if bowerbirds have much depth-perception, though I'm sure this is something the authors have thought about.) But, upon reading the actual article [Science 335, 335 (2012)] it becomes clear that what the authors really want to claim is that it is the confusingness of the illusions that matters:
Any of these seven effects might hold the female’s attention longer than if absent, and still longer if the illusions interact. For example, females will not mate unless they have spent more than about 55% of their total time in the avenue watching the male display.
I'm curious to see how they test these hypotheses against each other. ("Common sense" is against the dazzle hypothesis but then it is also against the existence of bowerbirds.)

(I should remark in passing that the bowerbird has always struck me as a good symbol for most tumblrs and a certain kind of blog.)

(See also: Electric Brae.)

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