Lizards exploit the changing optics of developing chromatophore cells to switch defensive colors during ontogeny.

Many animals undergo changes in functional colors during development, requiring the replacement of integument or pigment cells. A classic example of defensive color switching is found in hatchling lizards, which use conspicuous tail colors to deflect predator attacks away from vital organs. These tail colors usually fade to concealing colors during ontogeny. Here, we show that the ontogenetic blue-to-brown tail color change in Acanthodactylus beershebensis lizards results from the changing optical properties of single types of developing chromatophore cells. The blue tail colors of hatchlings are produced by incoherent scattering from premature guanine crystals in underdeveloped iridophore cells. Cryptic tail colors emerge during chromatophore maturation upon reorganization of the guanine crystals into a multilayer reflector concomitantly with pigment deposition in the xanthophores. Ontogenetic changes in adaptive colors can thus arise not via the exchange of different optical systems, but by harnessing the timing of natural chromatophore development. The incoherent scattering blue color here differs from the multilayer interference mechanism used in other blue-tailed lizards, indicating that a similar trait can be generated in at least two ways. This supports a phylogenetic analysis showing that conspicuous tail colors are prevalent in lizards and that they evolved convergently. Our results provide an explanation for why certain lizards lose their defensive colors during ontogeny and yield a hypothesis for the evolution of transiently functional adaptive colors.

How to use (and not to use) movement-based indices for quantifying foraging behaviour.

Movement-based indices such as moves per minute (MPM) and proportion time moving (PTM) are common methodologies to quantify foraging behaviour. We explore fundamental drawbacks of these indices that question the ways scientists have been using them and propose new solutions.To do so, we combined analytical and simulation models with lizards foraging data at the individual and species levels.We found that the maximal value of MPM is constrained by the minimal durations of moves and stops. As a result, foragers that rarely move and those that rarely stop are bounded to similar low MPM values. This implies that (1) MPM has very little meaning when used alone, (2) MPM and PTM are interdependent, and (3) certain areas in the MPM-PTM plane cannot be occupied. We also found that MPM suffers from inaccuracy and imprecision.We introduced a new bias correction formula for already published MPM data, and a novel index of changes per minute (CPM) that uses the frequency of changes between move and stop bouts. CPM is very similar to MPM, but does not suffer from bias. Finally, we suggested a new foraging plane of average move and average stop durations. We hope that our guidelines of how to use (and not to use) movement-based indices will add rigor to the study of animals' foraging behaviour.