Scaling of morphological characters across trait type, sex and environment: a meta-analysis of static allometries

Biological diversity is to a large extent a matter of variation in size. Proportional (isometric) scaling where large and small individuals are magnified versions of each other is often assumed the most common way morphological traits scale relative to overall size within species. However, the many traits showing non-proportional (allometric) scaling have motivated some of the most discussed hypotheses on scaling relationships in biology, like the positive allometry hypothesis for secondary sexual traits and the negative allometry hypothesis for genitals. I evaluate more than 3200 allometric parameters from the literature and find that negative allometry, not isometry, is the expected scaling relationship of morphological traits within species. Slopes of secondary sexual traits are more often steeper compared to other traits, but slopes larger than unity is also common for traits not under sexual selection. The steepness of the allometric slope is accordingly a weak predictor of past an...