Lizard tricks: overcoming conflicting requirements of speed versus climbing ability by altering biomechanics of the lizard stride.

Adaptations promoting greater performance in one habitat are thought to reduce performance in others. However, there are many examples of animals in which, despite habitat differences, such predicted differences in performance do not occur. One such example is the relationship between locomotory performance to habitat for varanid lizards. To explain the lack of difference in locomotor performance we examined detailed observations of the kinematics of each lizard's stride. Differences in kinematics were greatest between climbing and non-climbing species. For terrestrial lizards, the kinematics indicated that increased femur adduction, femur rotation and ankle angle all contributed positively to changes in stride length, but they were constrained for climbing species, probably because of biomechanical restrictions on the centre of mass height (to increase stability on vertical surfaces). Despite climbing species having restricted stride length, no differences have been previously reported in sprint speed between climbing and non-climbing varanids. This is best explained by climbing varanids using an alternative speed modulation strategy of varying stride frequency to avoid the potential trade-off of speed versus stability on vertical surfaces. Thus, by measuring the relevant biomechanics for lizard strides, we have shown how kinematic differences among species can mask performance differences typically associated with habitat variation.

Water balance of field-excavated aestivating Australian desert frogs, the cocoon-forming Neobatrachus aquilonius and the non-cocooning Notaden nichollsi (Amphibia: Myobatrachidae).

Burrowed aestivating frogs of the cocoon-forming species Neobatrachus aquilonius and the non-cocooning species Notaden nichollsi were excavated in the Gibson Desert of central Australia. Their hydration state (osmotic pressure of the plasma and urine) was compared to the moisture content and water potential of the surrounding soil. The non-cocooning N. nichollsi was consistently found in sand dunes. While this sand had favourable water potential properties for buried frogs, the considerable spatial and temporal variation in sand moisture meant that frogs were not always in positive water balance with respect to the surrounding soil. The cocoon-forming N. aquilonius was excavated from two distinct habitat types, a claypan in which frogs had a well-formed cocoon and a dune swale where frogs did not have a cocoon. Cocoons of excavated frogs ranged in thickness from 19.4 microm to 55.61 microm and consisted of 81-229 layers. Cocooned claypan N. aquilonius were nearing exhaustion of their bladder water reserves and had a urine osmolality approaching that of the plasma. By contrast, non-cocooned N. aquilonius from the dune swale were fully hydrated, although soil moisture levels were not as high as calculated to be necessary to maintain water balance. Both species had similar plasma arginine vasotocin (AVT) concentrations ranging from 9.4 to 164 pg ml(-1), except for one cocooned N. aquilonius with a higher concentration of 394 pg ml(-1). For both species, AVT showed no relationship with plasma osmolality over the lower range of plasma osmolalities but was appreciably increased at the highest osmolality recorded. This study provides the first evidence that cocoon formation following burrowing is not obligatory in species that are capable of doing so, but that cocoon formation occurs when soil water conditions are more desiccating than for non-cocooned frogs.

Comparative morphology of Western Australian varanid lizards (Squamata: Varanidae).

Varanid lizards, which vary considerably in body mass both interspecifically and intraspecifically, are generally considered to be morphologically similar. However, significant and non-isometric variation in the relative appendage dimensions for 17 species of Western Australian goannas suggest that these lizards are not morphologically conservative. The first and second canonical variates clearly distinguish the two subgeneral Odatria and Varanus, and species are generally sexually dimorphic. The morphological variation observed among these 17 species of goanna is associated with foraging mode and ecology. However, no single or small group of morphological dimensions discriminates phylogenetic groups, sexes, or ecological groups, and body size is an important component in these analyses. J. Morphol. 233:127-152, 1997. © 1997 Wiley-Liss, Inc.

Aerial and aquatic respiration of the Australian desert goby, Chlamydogobius eremius.

Physiological, anatomical and behavioural adaptations enable the Australian desert goby, Chlamydogobius eremius, to live in mound springs and temporary aquatic habitats surrounding the south-eastern rim of the Lake Eyre drainage basin in the harsh inland of Australia. This study describes the desert goby's respiratory and metabolic responses to hypoxic conditions and its use of buccal air bubbles for gas exchange at the water surface. Oxygen consumption for C. eremius is significantly higher in water than in air under normoxic and hypoxic conditions. In water, total oxygen consumption (V(O(2))) increases from normoxic conditions (253 microl g(-1) h(-1)) to 8% ambient O(2) concentration (377 microl g(-1) h(-1)), then decreases with increasing hypoxia of 4% O(2) (226 microl g(-1) h(-1)) and at 2% O(2) (123 microl g(-1) h(-1)). In air (fish were moist but out of water), V(O(2)) progressively decreases from normoxic conditions to hypoxic conditions (21% O(2), V(O(2)) is 169 microl g(-1) h(-1) to 39 microl g(-1) h(-1) at 2% O(2)). These data indicate oxygen-conforming patterns with increasing hypoxia both in air and in water below 8% O(2). In water, opercular movement rates remain unchanged with increasing hypoxia (139 min(-1) at 21% O(2), 154 min(-1) at 8%, 156 min(-1) at 4% and 167 min(-1) at 2%) but in air, opercular movement rates are significantly lower than in water, corresponding with the lower metabolic rate (71 min(-1) at 21% O(2), 53 min(-1) at 8%, 96 min(-1) at 4% and 64 min(-1) at 2%). Chlamydogobius eremius can use a buccal air bubble for aerial O(2) uptake, most probably in response to increased aquatic hypoxia. In air, C. eremius relies more on the buccal bubble as an oxygen source with increasing hypoxia up to an ambient O(2) of 4% (7.1% of V(O(2)) at 21% O(2); 14.5% at 8% O(2); and 27.1% at 4% O(2)), then when the available supply of O(2) is further reduced, it decreases (15% of V(O(2)) at 2% O(2)) and respiration across the skin again makes a higher relative contribution. The Australian desert goby has a higher metabolic rate in higher salinities (336 microl g(-1) h(-1) in 35 ppt, 426 microl g(-1) h(-1) in 70 ppt) than in freshwater (235 microl O(2) g(-1) h(-1)), presumably because of the increased metabolic cost of osmoregulation. There was no significant difference in V(O(2)) for fish in air that had come from varying salinities.