Thinking small: miniaturization might have facilitated adaptation to new niches in tarantulas

Abstract

The interplay between ecology and morphology holds significant importance in the field of evolutionary ecology. Miniaturization, a prevalent trend across diverse branches of the tree of life, often emerges as a consequence of adaptation to specific ecological niches. However, the causes and outcomes of miniaturization can vary significantly among different taxa. In our study, we utilized tarantulas as a model system to explore whether microhabitat choices are linked to morphological characteristics. Specifically, we examined whether the transition to a non-burrowing, ground-dwelling lifestyle is associated with miniaturization. Our findings indicate that the evolution of morphological traits in tarantulas has primarily been influenced by stabilizing selection, with no discernible difference in morphotypes associated with fossorial or arboreal habits. Nevertheless, we observed that whenever tarantulas opportunistically transitioned to a non-burrowing, ground-dwelling lifestyle—such as living under rocks, logs, or within crevices—they seem to have undergone body-form miniaturization and a reduction in the ratio of forelimb to hindlimb length. This study underscores the potential for repeated shifts to unique ecological niches to disrupt the conventional trajectory of morphological evolution, resulting in the emergence of novel phenotypes.