Glenn J. Tattersall

The thermogenesis of digestion in rattlesnakes

SUMMARY Some snakes have a feeding regime characterized by the infrequent ingestion of relatively large meals, causing impressive increments in post-prandial metabolism. Metabolism remains elevated for many days, while digestion proceeds, resulting in considerable investment of time and energy. Snakes actively adjust thermoregulatory behavior to raise their body temperature during digestion, exhibiting a post-prandial thermophilic response that accelerates digestion at the expense of higher metabolic rates. In the present study, we investigated the possibility that endogenously derived heat, originating as a byproduct of the post-prandial increase in metabolism, could itself contribute to the elevated body temperature during digestion in the South American rattlesnake Crotalus durissus. We assessed heat production, at a constant environmental temperature, by taking infrared (IR) images of snakes during fasting and after being fed meals varying from 10% to 50% of their own body masses. Our results show clearly that digesting rattlesnakes have significantly increased body temperatures, even when precluded from adjusting their thermoregulatory behavior. The feeding-derived thermogenesis caused the surface body temperature of rattlesnakes to increase by 0.9–1.2°C, a temperature change that will significantly affect digestive performance. The alterations in body temperature following feeding correlated closely with the temporal profile of changes in post-prandial metabolism. Moreover, the magnitude of the thermogenesis was greater for snakes fed large meals, as was the corresponding metabolic response. Since IR imaging only assesses surface temperatures, the magnitude of the thermogenesis and the changes in deep core temperature could be even more pronounced than is reported here.

Hypoxia progressively lowers thermal gaping thresholds in bearded dragons, Pogona vitticeps.

SUMMARY Most animals, including reptiles, lower body temperature (Tb) under hypoxic conditions. Numerous physiological and behavioural traits significant to the regulation of Tb are altered by hypoxia in ways that suggest an orchestrated adjustment of Tb at a new and lower regulated level. We examined this matter in bearded dragons, Pogona vitticeps, a species of reptile that naturally exhibits open mouth gaping at high temperatures, presumably in order to promote evaporation and thus prevent or avoid further increases in Tb. The threshold for the onset of gaping (assessed as the temperature at which lizards spent 50% of their time gaping) was reduced from 36.9°C in normoxia to 35.5°C at 10% and 34.3°C at 6% O2. The overall magnitude or degree of gaping, measured qualitatively, was more pronounced at lower temperatures in hypoxia. Females consistently had lower gaping threshold temperatures than did males, and this difference was retained throughout exposure to hypoxia. In addition to gaping, evaporative water loss from the cloaca may also play a significant role in temperature regulation, since the ambient temperature at which cloacal discharge occurred was also reduced significantly in hypoxia. The results reported herein strongly support the view that hypoxia reduces temperature set-point in lizards and that such changes are coordinated by specific behavioural thermoeffectors that modulate evaporative water loss and thus facilitate a high potential for controlling or modifying Tb.

Decreased precision contributes to the hypoxic thermoregulatory response in lizards.

SUMMARY The decrease in body temperature (Tb) observed in most vertebrate classes in response to hypoxia has been attributed to a regulated decrease in set-point, protecting organs against tissue death due to oxygen depletion. Hypoxia, however, imparts particular challenges to metabolic function which may, in turn, affect thermoregulation. In ectotherms, where thermoregulation is mainly behavioural, stressors that influence the propensity to move and respond to temperature gradients are expected to have an impact on thermoregulatory control. Using low oxygen as a potent stressor, we evaluated the variability and level of thermoregulation of inland bearded dragons. To examine the source of thermoregulatory variability, we studied their behaviour in an electronically controlled temperature-choice shuttle box, a constant temperature dual-choice shuttle box, and a linear thermal gradient. A significant increase in the size of the Tb range was observed at the lowest oxygen concentration (4% O2), reflecting a decrease in thermoregulatory precision in the temperature-choice shuttle box. This was also accompanied by a drop of ∼2–4°C in Tb, the drop being greatest in situations where Tb must be actively defended. Situations that force the lizards to continually choose temperatures, rather than passively remain at a given temperature, lead to an increase in the variability in the manifested Tb, which is further exaggerated in hypoxia. This study reveals that a decrease in thermoregulatory precision caused by a diminished propensity to move or effect appropriate thermoregulatory responses may be a contributing component in the lowering of selected body temperatures observed in many hypoxic ectotherms.

Embryonic motility and hatching success of Ambystoma maculatum are influenced by a symbiotic alga

To augment O2 supply through the jelly mass and egg capsule, embryonic yellow-spotted salamanders (Ambys- toma maculatum (Shaw, 1802)) take advantage of a unicellular alga, Oophila ambystomatis. Convective currents from sur- face cilia, however, may also enhance O2 transport, whereas muscular contractions could either enhance delivery or contribute to O2 consumption. Embryonic motion is, therefore, potentially vital to salamander development. We examined embryonic motility across multiple developmental stages, survivorship, and hatching timing in response to different algal levels by rearing salamander egg masses under three different diel light cycles: 24 h dark, 12 h light, and 24 h light per day. Embryos raised in continuous light hatched synchronously and at slightly earlier developmental stages than embryos raised in the dark or in 12 h light per day. We removed eggs at multiple stages to examine embryonic rotation and mus- cular contraction rates under 180 min periods of both light and dark. Rotational movements occurred more frequently in alga-free than in algae-inhabited eggs, and more frequently in algae-inhabited eggs in the dark than in light. At later de- velopmental stages, muscular contractions were more frequent in embryos from algae-inhabited egg masses in light than those in the dark; thus embryos with less O2 reduced muscular activity, thereby reducing energy consumption when O2 availability was compromised.

Thermal games in crayfish depend on establishment of social hierarchies

SUMMARY An unequal resource distribution is commonly seen in dominance hierarchies, in which the individual with the higher status is more successful in obtaining the resource. One possible resource is preferred temperature. When situations allow, ectotherms regulate their body temperature by behaviourally selecting different environmental conditions, achieving, when possible, a preferred temperature. Using a shuttlebox, the preferred temperature for Procambarus clarkii was determined to be 23.9°C with upper and lower voluntary escape temperatures of 25.9 and 21.8°C, respectively. If this preferred temperature zone (21.8–25.9°C) was valued as a resource, given the choice between a preferred temperature and a non-preferred temperature, crayfish should compete over the preferred temperature, with the dominant individual of dyadic pairs achieving the preferred temperature more often than the subordinate. Using a dual-choice experimental tank, competition over a binary temperature choice between rank-established paired crayfish was determined under both warm and cold challenge conditions (warm vs preferred temperature and cold vs preferred temperature, respectively). In naive pairings, similar levels of competition over the preferred temperature occurred in both warm and cold challenge trials, as predicted by game theory. In established pairings, however, dominant crayfish gained significantly greater access to preferred temperature in both warm and cold challenge conditions. These results demonstrate that crayfish engage in a cost–benefit assessment during their initial agonistic contests over temperature, but as hierarchies mature, these thermal games are decided by the dominant animal gaining primary access to the temperature resource.

Constant set points for pH and PCO2 in cold-submerged skin-breathing frogs

The low temperatures encountered by overwintering frogs result in a large downregulation of metabolism and behaviour. However, little is known about acid-base regulation in the extreme cold, especially when frogs become exclusive skin-breathers during their winter submergence. Blood and muscle tissue acid-base parameters (pH, P(CO2), bicarbonate and lactic acid concentrations) were determined in submerged frogs exposed to a range of low temperatures (0.2-7 degrees C). At overwintering temperatures between T = 0.2 and 4 degrees C plasma pH and P(CO2) were maintained constant, whereas intracellular pH regulation resulted in larger pH-temperature slopes occurring in the presumably more active heart muscle (deltapH/deltaT = -0.0313) than in the gastrocnemius muscle (deltapH/deltaT = -0.00799). Although blood pH was not significantly affected by submergence between 0.2 and 4 degrees C (pH = 8.220-8.253), it declined in the 7 degrees C frogs (pH = 8.086), a decrease not linked to the recruitment of anaerobiosis. Plasma P(CO2) and pH in the cold appear to be regulated at constant levels, implying that cutaneous CO2 conductance in submerged frogs is adjusted within the range of overwintering temperatures. This is likely geared toward facilitating the uptake of oxygen under conditions of greater metabolic demand, however there remains the possibility that acid-base balance itself is maintained at a constant set point at the frogs natural overwintering temperatures.