Rates of oxygen consumption and blood lactate levels were measured in tammar wallabies (Macropus eugenii) trained to hop on a treadmill. In addition, the work required to overcome wind resistance during forward locomotion was measured in a wind tunnel. Up to approximately 2.0 m/s, rates of oxygen consumption increased linearly with speed and were not significantly different from rates of oxygen consumption for a quadruped of similar body mass. Between 2.0 and 9.4 m/s, rates of oxygen consumption were independent of hopping speed, and between 3.9 and 7.9 m/s, the range over which samples were obtained, blood lactate levels were low (0.83 +/- 0.13 mmol.min-1.kg-1) and did not increase with hopping speed. The work necessary to overcome drag increased exponentially with speed but increased the energy cost of locomotion by only 10% at the average speed attained by our fast hoppers. Thus, during hopping, the energy cost of locomotion is effectively independent of speed. At rates of travel observed in the field, the estimated energy cost of transport in large macropods is less than one-third the cost for a quadruped of equivalent body mass. The energetic savings associated with this unique form of locomotion may have been an important physiological adaptation, enabling large macropods to efficiently cover the distances necessary to forage in the semiarid landscapes of Australia.

Owing to the inherent difficulties of studying bluefin tuna, nothing is known of the cardiovascular function of free-swimming fish. Here, we surgically implanted newly designed data loggers into the visceral cavity of juvenile southern bluefin tuna (Thunnus maccoyii ) to measure changes in the heart rate ( f H ) and visceral temperature (T V ) during a two-week feeding regime in sea pens at Port Lincoln, Australia. Fish ranged in body mass from 10 to 21 kg, and water temperature remained at 18-198C. Pre-feeding f H typically ranged from 20 to 50 beats min K1. Each feeding bout (meal sizes 2-7% of tuna body mass) was characterized by increased levels of activity and f H (up to 130 beats min K1 ), and a decrease in T V from approximately 20 to 188C as cold sardines were consumed. The feeding bout was promptly followed by a rapid increase in T V , which signified the beginning of the heat increment of feeding (HIF). The time interval between meal consumption and the completion of HIF ranged from 10 to 24 hours and was strongly correlated with ration size. Although f H generally decreased after its peak during the feeding bout, it remained elevated during the digestive period and returned to routine levels on a similar, but slightly earlier, temporal scale to T V . These data imply a large contribution of f H to the increase in circulatory oxygen transport that is required for digestion. Furthermore, these data oppose the contention that maximum f H is exceptional in bluefin tuna compared with other fishes, and so it is likely that enhanced cardiac stroke volume and blood oxygen carrying capacity are the principal factors allowing superior rates of circulatory oxygen transport in tuna.

No abstract

SUMMARY To maximize the period where body temperature (Tb)exceeds ambient temperature (Ta), many reptiles have been reported to regulate heart rate (fh) and peripheral blood flow so that the rate of heat gain in a warming environment occurs more rapidly than the rate of heat loss in a cooling environment. It may be hypothesized that the rate of cooling, particularly at relatively cool Tbs, would be further reduced during postprandial periods when specific dynamic action (SDA) increases endogenous heat production (i.e. the heat increment of feeding). Furthermore, it may also be hypothesized that the increased perfusion of the gastrointestinal organs that occurs during digestion may limit peripheral blood flow and thus compromise the rate of heating. Finally, if the changes in fh are solely for the purpose of thermoregulation, there should be no associated changes in energy demand and, consequently, no hysteresis in the rate of oxygen consumption(V̇O2). To test these hypotheses, seven individual Varanus rosenbergi were heated and cooled between 19°C and 35°C following at least 8 days fasting and then approximately 25 h after consumption of a meal (mean 10% of fasted body mass). For a given Tb between the range of 19-35°C, fh of fasting lizards was higher during heating than during cooling. Postprandial lizards also displayed a hysteresis in fh, although the magnitude was reduced in comparison with that of fasting lizards as a result of a higher fh during cooling in postprandial animals. Both for fasting and postprandial lizards,there was no hysteresis in V̇O2 at any Tb throughout the range although, as a result of SDA,postprandial animals displayed a significantly higher V̇O2 than fasting animals both during heating and during cooling at Tbs above 24°C. The values of fh during heating at a given Tb were the same for fasting and postprandial animals,which, in combination with a slower rate of heating in postprandial animals,suggests that a prioritization of blood flow to the gastrointestinal organs during digestion is occurring at the expense of higher rates of heating. Additionally, postprandial lizards took longer to cool at Tbs below 23°C, suggesting that the endogenous heat produced during digestion temporarily enhances thermoregulatory ability at lower temperatures, which would presumably assist V. rosenbergiduring cooler periods in the natural environment by augmenting temperature-dependent physiological processes.

Oxygen consumption (VO2), CO2 production (VCO2), and minute ventilation (VE) together with breathing pattern were measured in echidnas during normoxia and hypoxia. In normoxia, VO2, VCO2, and VE were all found to be approximately 30% of the allometric prediction for a eutherian. As a consequence VE/VO2 and VE/VCO2 are as predicted for a mammal. This is in contrast to previous reports on the echidna in which the VE was shown to be low and the echidna, subsequently, to be in a state of hypoventilation. It is possible that the difference between this and previous studies is related to the resting state of the echidna; echidnas in this study adopted a curled-up "sleeping" posture, and measurements were made without tactile disturbance. Breathing pattern was typical of a semifossorial species in that inspiration time to total breath time was short when compared with the normal eutherian value. In graded hypoxia VE increased [threshold fractional concentration of inspired O2 (FIO2) = 0.125], predominantly the result of changes in frequency achieved through a shortening in expiration time. In acute hypoxia (FIO2 = 0.10) VE/metabolic rate showed a tendency to increase, mainly because of the increase in VE. Approximately 50% of the increase in VE could be attributed to the 25% increase in VO2 and VCO2 that occurred in acute hypoxia. Given that the general mammalian response to hypoxia is a drop in metabolic rate, possible reasons as to why the echidna does not decrease metabolic rate in hypoxia are discussed.