Variation in crocodilian dorsal scute organization and geometry with a discussion of possible functional implications

Dermal ossifications, including osteoderms, are present in many vertebrates and are frequently interpreted as a defense against predators. Nevertheless, osteoderms remain ubiquitous in adult crocodilians while being absent in hatchlings, even though adults rarely experience predation. In other biological systems, increased variation, particularly fluctuating asymmetry, have proven useful for identifying biological structures likely to have evolved under relaxed selection, which in turn may inform their function. Therefore, using the keratinous scutes as proxies for the underlying osteoderm morphology, I investigated the average intraspecific variability of geometry and fluctuating asymmetry in dorsal scutes in five species of crocodilians. I first tested for differences in variability of scute length and width, then for differences in bilateral fluctuating asymmetry of scute number, before finally investigating scute distribution patterns for each species compared to hypothetical rectangular and hexagonal scute arrangements. The American crocodile, Crocodylus acutus, shows significantly more asymmetry than other species, which is consistent with relaxed selection on osteoderms in this species. A suspected decrease in intraspecific aggression within Crocodylus acutus, in conjunction with the inferred relaxed selection, suggests that, in general, crocodilian osteoderms function primarily as defensive armor in aggressive encounters with conspecifics. The smooth-fronted caiman, Paleosuchus trigonatus, exhibits increased variation in scute dimensions linked to the mediolateral offset of osteoderms in adjacent rows, possibly resulting in a more rigid carapace. Unfortunately, comparative data on crocodilian behavior, physiology, and development is extremely limited and restricts the ability to explore other potential explanations for the patterns observed, highlighting the need for more research on rare and cryptic crocodylians.