Communication Through a Window of Error: Proportional Processing and Signal Categorization

Signal detection shapes ornament allometry in functionally convergent Caribbean Anolis and Southeast Asian Draco lizards

Visual ornaments have long been assumed to evolve hyper-allometry as an outcome of sexual selection. Yet growing evidence suggests many sexually selected morphologies can exhibit other scaling patterns with body size, including hypo-allometry. The large conspicuous throat fan, or dewlap, of arboreal Caribbean Anolis lizards was one ornament previously thought to conform to the classical expectation of hyper-allometry. We re-evaluated this classic example alongside a second arboreal group of lizards that has also independently evolved a functionally equivalent dewlap, the Southeast Asian Draco lizards. Across multiple closely related species in both genera, the Anolis and Draco dewlaps were either isometric or had hypo-allometric scaling patterns. In the case of the Anolis dewlap, variation in dewlap allometry was predicted by the distance of conspecifics and the light environment in which the dewlap was typically viewed. Signal efficacy, therefore, appears to have driven the evolution of hypo-allometry in what was originally thought to be a sexually selected ornament with hyper-allometry. Our findings suggest that other elaborate morphological structures used in social communication might similarly exhibit isometric or hypo-allometric scaling patterns because of environmental constraints on signal detection.

The ecological context and fitness impact of categorical perception: a comment on Green et al

Animals, including humans, must make sense of information, bombarding them from the enviroment, quickly and effectively. Categorical perception is one such process, and the evidence for it is increasing. We argue that the next step in the research of category-based decision making should be to study wild animals in the field and in behavioral contexts that have strong fitness-relevant impacts. We illustrate this point by the promise of categorical perception in the rejection of foreign egg colors by American robins (Turdus migratorius).

Why the Static Allometry of Sexually-Selected Traits Is So Variable: The Importance of Function

Sexually-selected traits often show positive static allometry, with large individuals bearing disproportionately large structures. But many other sexually-selected traits show isometry or even negative allometry, with trait size varying relatively little with body size. We recently proposed that the functions of these traits (as aggressive signals, weapons, courtship signals, and contact courtship devices) determine their allometries. Positive allometry is generally favored for aggressive signals because aggressive signals are selected to emphasize body size (and thus fighting prowess). In contrast, the biomechanics of force application in weapons only sometimes select for positive allometry; the content of courtship signals is even less often related to body size; and contact courtship devices are selected to be relatively invariant across body sizes. Here we summarize the arguments in favor of this “functional allometry” hypothesis and expand a comparative test of its predictions. Our results indicate that sexual traits have the allometric slopes predicted by our hypothesis, regardless of which body part bears the structure.

Is it one call or two? Perception of inter-pulse and inter-call intervals in the grey treefrog, Hyla versicolor

Female grey treefrogs, Hyla versicolor, treat a call with an anomalously large gap between call pulses as relatively unattractive. However, whether such a ‘call’ is treated (or perhaps perceived) as a single call or two separate calls, an issue relevant to the problem of perceptual segregation of calls of different males, was unclear. We tested whether there could be gaps shorter than a typical inter-call interval that would be sufficient to elicit the latter percept by offering females a choice between a 5-pulse call and 13-pulse call separated by a 1575-ms gap and these pulse strings separated by shorter gaps. Preliminary results suggested that perception of inter-call gaps was categorical, and the distributions of such gaps between males in choruses were compatible with this finding. However, when we also offered females call alternatives designed to specifically test the hypothesis of categorical perception, predictions were only sometimes met.