Claw morphology and its ecological relevance in arthropods

Claws are mostly pointed and curved structures at the tip of the limbs and they occur in a wide variety of animals, like insects, spiders, lizards, cats, birds etc. They can fulfil various functions, as for example climbing, digging, grasping prey or fighting. Despite their presence in many animals, little is known about the factors influencing claw shape and size. Studies of claw characteristics in birds, lizards and some dinosaurs demonstrated that the angle of a claw may be related to the lifestyle of an animals, for example tree-living animals possess stronger curved claws than ground-dwelling animals. A recent study investigating coastal mites even showed that shape and size of their claws are strongly related to the substrate the animals walk on. Mites are very small arachnids which can be found in every habitat and thus they hold on to a wide range of surface structures. Therefore, they are ideal candidates to investigate the relationship between claw shape and lifestyle of the animals. In this project we will use different methods to find out if claws are strongly shaped by the ecology of the animal and if yes, what is the nature of this correlation. For this purpose, we will first study the claws of a variety of mites from different habitats to assess if shape is strongly influenced by the lifestyle. In a second step, we will perform tests with living mites to see if performance and claw features are related, for example if animals with stronger curved claws are able to get a better grip or move faster. In a further step, we will photograph the claws of mites from various habitats and create digital 3D-models based on these photographs. By using a specific computer program, these models will then be subject to a stress test, i.e. differen t virtual forces will be applied to the claw to see when and how they would break. The outcomes from this project will have implications for many other animal groups as well, because many of them possess claws and live in the same environments. Additionally, the results could provide important clues for bionic research which uses biological structures to solve technical problems. For example, several research companies are presently developing robots that should be able to climb walls and to advance into cavities of collapsed buildings in disaster areas. The present study could provide nice models for claw-like structures improving the locomotion of these robots.

Arthropods, such as insects or spiders, possess claws at the ends of their legs to hold on to the ground during locomotion. In this project, the characteristics of claws of moss mites were investigated in relation to their habitats. Moss mites are tiny arachnids that live mainly in the soil, where they play an important role in decomposition and nutrient cycling. On the coasts, they can be found in tidal areas where they are exposed to daily flooding by the tides and strong wave action. It could be shown that animals that are strongly exposed and live on rocky substrates have stronger curved and robust claws than animals that live on softer ground, such as mangrove roots. In comparison with mites from typical terrestrial habitats, such as forests or meadows, the mites from the coast have relatively larger claws. On the coast, it is vital for the mites to hold on to the ground, because if they are washed away with the waves, this usually means the end of their lives. This so-called 'evolutionary pressure' is so strong that even the young animals already show optimal claw shapes, depending on the ground on which they live. Related species generally look very similar, but this does not apply to the claws if closely related species live in different habitats, such as rocky cliffs and mangrove forests. In terrestrial habitats, the shape of the claws does apparently not so strongly dependent on the substrate, but over forty studied species still have very differently shaped claws. Attachment to the ground is not essential for survival in terrestrial environments and the claws are also used for other activities such as fast running, digging or climbing. Species that live in trees and spend most of their time climbing vertically have strongly curved but also blunter claws, while species that run quickly across the forest floor have slender and very pointed claws. It has also been shown that the shape of the claws can change during the development of an animal. If the animal changes its life strategy during the transition from the juvenile stage to the adult animal, i.e. if the juvenile stages show different habitats or habits than the adults, then the claw shape also changes. The results of this project could be of significance for bionics, as nature has adapted claw shapes ideally to their environment and their respective activities. Machines or robots that require claw-like structures could therefore be equipped with the perfect claws from the outset without having to go through lengthy test phases.