The influence of habitat rugosity on limb length: the size-grain hypothesis applied across taxa

The size-grain hypothesis (SGH) maintains that as terrestrial walking organisms decrease in size, their realised environmental rugosity increases and becomes less planar. Therefore, it is hypothesised that the advantage of long limbs (i.e. increased efficiency of movement) may decrease in smaller organisms, where it is important that interstitial habitats can be effectively exploited using relatively shorter limbs. Positive allometric relationships between limb length and body size have been previously found in several ant taxa. We tested a corollary of the SGH by examining limbs in three taxa which were all capable of flight, hypothesizing that organisms in planar environments would have relatively longer limbs (either wings or legs) than organisms that utilise rugose environments. The lengths of bird and bat wings from differing habitats were obtained from the literature, and measurements of limb length, leg length, and pronotum width were taken from dung beetles caught in two different habitat types, one rugose, the other planar, in Skukuza, Kruger National Park. The strongest support for the SGH came from the bird taxa, where birds in a rugose environment (i.e. forest) had relatively shorter wings than birds from three planar environments (grassland, open coast and marine). Although there were signals of emerging support for SGH in bats, there were no significant trends present. It was proposed that this arose due to the use of forearm length as a proxy for wing size, and failed to capture total bat wing length and variability. Dung beetles also showed relatively little support for the SGH. Since dung beetles utilise two forms of locomotion, the trade-offs constraining their morphology are more complex than those that can be explained by the SGH.