Among closely related species, larger mammals tend to have a longer face and proportionally smaller braincase. This putative ‘rule’ in mammalian macroevolution has been proposed for the first time in 2013 based on 3D geometric morphometrics of antelopes, fruit bats, tree squirrels and mongooses. To firmly demonstrate that this trend holds as a ‘rule’ requires expanding the analysis in more lineages and other mammalian orders: if supported in most groups, it may indeed become a new evolutionary ‘rule’ besides famous ones such as Bergmann’s and Allen’s. In this study, using statistical shape analysis and both standard and comparative methods on a sample of kangaroos, wallabies and other macropodine marsupials, we show that the ‘big size-long face’ pattern is indeed found also outside the placentals. This provides support to the hypothesis of an important role of size-related shape changes (i.e., allometry) in the origin of the exceptional disparity of mammals, that, only in terms of size, span more orders of magnitude than any other animal: from 3 to 4 g of a tiny bat to more than 100 tons in blue whales

Pentapedal locomotion is the use of the tail as a fifth leg during the slow gait of kangaroos. Although previous studies have informally noted that some smaller species of macropodines do not engage in pentapedal locomotion, a systematic comparative analysis of tail use during slow gait across a wide range of species in this group has not been done. Analysis of relative movement of the pelvis, tail, and joint angles of the lower limbs during slow gait, using 2D landmark techniques on video recordings, was carried out on 16 species of Macropodinae. We also compared the relative lengthening of the tibia using crural index (CI) to test whether hindlimb morphology was associated with pentapedal locomotion. Pentapedal locomotion was characterised by three features: the presence of the ‘tail repositioning phase’, the constant height of the pelvis and the stationary placement of the distal tail on the ground during the hindlimb swing phase. The mean CI of pentapedal species was significantly greater than that of non-pentapedal species (1.71 versus 1.36; P < 0.001). This lends support to the hypothesis that the use of pentapedal locomotion is associated with the relative lengthening of the hindlimb, which in turn is associated with body size and habitat preference within the Macropodinae.

Abstract. The western grey kangaroo, Macropus fuliginosus, is a large-bodied kangaroo that engages in pentapedal locomotion at low speeds and bipedal hopping at high speeds. The tail is thought to have functional roles in both of these modes of locomotion. In pentapedal locomotion the tail acts as a 'fifth limb' to support the body weight together with the forelimbs while the hind limbs are drawn forward. The tail has also been suggested to have a role as a counterbalance during bipedal hopping. On the basis of these functional roles for the tail in locomotion, the caudal musculature of the western grey kangaroo was dissected and described in this study. The arrangement of the caudal musculature showed particular adaptations for the role of the tail in both pentapedal locomotion and bipedal hopping.

The disjunct distribution of the quokka enabled this study to investigate cranial morphological variation in relation to insularity and latitude. Crania from mainland locations in south‐western Australia and from two islands were examined. Thirty‐eight three‐dimensional homologous landmarks were digitized on 110 quokka crania. The landmark data were first subjected to generalized Procrustes Analysis, followed by principal components analysis. General linear regression was used to test whether quokka cranial size obeys the Island rule and Bergmann's rule. Crania from the islands were found to be smaller than crania from the adjacent mainland. Crania of quokkas from cooler climates (higher latitudes) were larger than those from warmer climates (lower latitudes), and this trend was observed in both mainland and island samples. Multivariate regression of the combined principal components on the independent variable, latitude, showed shape of the snout of quokkas from cooler southern climates tended to be relatively longer and narrower than those from further north. General linear regressions was also used to ensure that latitude was significantly influencing shape (principal components) independent of age, sex and whether the crania were from the mainland or island populations. Results showed that 40.2% of cranial shape variation was significantly related to latitude, irrespective of age, sex and population type. The variation in size and shape of the quokka crania appears to reflect the eco‐geographic variation within the distribution of the species.

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