Open— and closed—system experimental designs were used to test comparative tolerance, survival, and potential natural selection in four species subjected to low dissolved oxygen concentration–the desert pupfish (Cyprinodon macularius), long—fin dace (Agosia chrysogaster), speckled dace (Rhinichthys osculus), and Gila sucker (Catostomus clarki). Different rankings of the species resulted for tolerance (closed—air system) and survival (open air system). Open—system experiments were designed that demonstrate mechanisms by which this environmental stressor produces catastrophic selection in nature. During interspecific competition, adaptive behavior and structure as well as physiological tolerance were decisive in survival.
In the marsupial, the potoroo, multiple regression analysis shows that ambient temperature makes a minor (2%) contribution towards variation in oxygen consumption with speed. This suggests that the heat generated during running is substituted for heat which would otherwise have to be generated for temperature regulation. Maximum levels of oxygen consumption are also temperature-independent over the range 5-25 degrees C, but plasma lactate concentrations at the conclusion of exercise significantly increase with ambient temperature. Adult potoroos show a linear increase in oxygen consumption with speed, and multiple regression indicates that the most significant factor affecting energy use during running is stride length. Juvenile potoroos have an incremental cost of locomotion about 40% lower than that predicted on the basis of body mass. The smaller animals meet the demands of increasing speed by increasing stride length rather than stride frequency, as would be expected in a smaller species. Our results show that juvenile potoroos diverge significantly from models based only on adult animals in incremental changes in stride frequency, length and the cost of transport, suggesting that they are not simply scaled-down adults.
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