Changes in heart rate are important for thermoregulation in the varanid lizard Varanus varius

Laboratory studies and a single field study have shown that heart rate in some reptiles is faster during heating than during cooling at any given body temperature. This phenomenon, which has been shown to reflect changes in peripheral blood flow, is shown here to occur in the lizard Varanus varius (lace monitor) in the wild. On a typical clear day, lizards emerged from their shelters in the morning to warm in the sun. Following this, animals were active, moving until they again entered a shelter in the evening. During their period of activity, body temperature was 34-36 degrees C in all six study animals (4.0-5.6 kg), but the animals rarely shuttled between sun and shade exposure. Heart rate during the morning heating period was significantly faster than during the evening cooling period. However, the ratio of heating to cooling heart rate decreased with increasing body temperature, being close to 2 at body temperatures of 22-24 degrees C and decreasing to 1.2-1.3 at body temperatures of 34-36 degrees C. There was a significant decrease in thermal time constants with increasing heart rate during heating and cooling confirming that changes in heart rate are linked to rates of heat exchange.     

Prostaglandins are important in thermoregulation of a reptile (Pogona vitticeps)

The effectiveness of behavioural thermoregulation in reptiles is amplified by cardiovascular responses, particularly by differential rates of heart beat in response to heating and cooling (heart-rate hysteresis). Heart-rate hysteresis is ecologically important in most lineages of ectothermic reptile, and we demonstrate that heart-rate hysteresis in the lizard Pogona vitticeps is mediated by prostaglandins. In a control treatment (administration of saline), heart rates during heating were significantly faster than during cooling at any given body temperature. When cyclooxygenase 1 and 2 enzymes were inhibited, heart rates during heating were not significantly different from those during cooling. Administration of agonists showed that thromboxane B(2) did not have a significant effect on heart rate, but prostacyclin and prostaglandin F(2alpha) caused a significant increase (3.5 and 13.6 beats min(-1), respectively) in heart rate compared with control treatments. We speculate that heart-rate hysteresis evolved as a thermoregulatory mechanism that may ultimately be controlled by neurally induced stimulation of nitric oxide production, or maybe via photolytically induced production of vitamin D.     

The influence of thermoregulation on behavioural recovery from exercise in a lizard

1. Past work on the thermal preferences of Dipsosaurus dorsalis (Biard & Giard) has indicated that intense, exhaustive exercise causes these lizards to select a body temperature (33·5 °C) which is cooler than their preferred activity temperature of 40°C during the first 1–2 h of exercise recovery.
2. In order to test the hypothesis that the thermal regime selected by exhausted D. dorsalis is beneficial to the process of exercise recovery, lizards were forced to undergo both exhaustive and sprinting exercise at their preferred body temperature of 40°C. The peak speeds attained and the total distances travelled by these animals during these two different exercise protocols were measured and the animals were then forced to undergo a second bout of either sprinting or exhaustive exercise, following a 30–330 min recovery at either 20°C, 40°C or under a variable thermal regime which duplicated that selected by animals following exercise.
3. Animals recovering at a constant 40°C regained their ability to repeat exhaustive activity in less than 85 min, while animals recovering under the other two thermal regimes required between 85 and 100 min of recovery to be able to repeat this activity. Animals recovering at both 40°C and under the variable thermal regime regained their ability to repeat sprint behaviour within 60 min of recovery, while animals recovering at 20°C required more than 100 min of recovery to be able to repeat sprint behaviour.
4. These results formed the basis of the conclusion that the post-exercise behaviour selected by D. dorsalis retards the rate at which the animals recover their ability to repeat exhaustive exercise when compared with recovery at a constant 40°C but does not retard their ability to repeat sprint exercise.     

Thyroxine affects behavioral thermoregulation but not growth rate among populations of the western fence lizard (Sceloporus occidentalis)

We investigated whether thyroxine influences hatchling growth rate of the western fence lizard (Sceloporus occidentalis) throught its effects on thermoregulatory behaviors. We reared control and thyroxine-injected hatchlings from three populations of S. occidentalis that differ in growth rate in a thermal gradient. We also measured the daily changes in body temperature and activity level (proportion of time spent out of retreat sites) of control and thyroxine-injected lizards. Previous studies have shown that within and among population differences in growth rate of the western fence lizard are correlated with the maintenance of high activity levels (proportion of time spent outside of retreat sites) and high body temperatures throughout the day. Growth rate was not influenced by injections of thyroxine. However, injections of thyroxine did elevate average daily body temperature and daily activity. Although administration of thyroxine uniformly increased the probability of activity throughout the day, it did not appear to alter the daily changes in activity. Previous studies have shown that the slower-growing hatchlings from northern populations exhibit a decline in activity during the later part of the thermal cycle, whereas the faster growing southern hatchlings maintain the same level of high activity throughout the thermal cycle. The decline in activity of northern populations was not prevented by thyroxine injection used in our current study. Northern lizards receiving exogenous thyroxine were still less active later in the day compared to early in the day, even though activity level throughout the day was elevated. Thus, the effects of thyroxine on temperature regulation observed in our study (general increase in activity level) appear to be unrelated to those aspects of temperature regulation (e.g., daily changes in behavioral thermoregulation) that are correlated with among population differences in growth rate. We also raised hatchlings in a cycling thermal regime (forced thermal cycle of 34°C:15°C, 12L:12D) where behavioral thermoregulation is not possible. The growth rate of lizards forced to cycle between 34°C:15°C on a daily basis was significantly lower than those lizards allowed to behaviorally thermoregulate, further underscoring the importance of the circadian pattern of thermoregulation for growth.Abbreviations GR growth rate - MR metabolic rate - SMR standard metabolic rate - SVL snout-vent length - T₄ thyroxine - T _b body temperature - T _e environmental temperature     

Temperature-dependent rate of clutch production in a tropical lizard (Paroedura picta: Gekkonidae): Intraspecific test of the metabolic theory of ecology

Recently, a general model based on scaling of metabolic rate and reaction kinetics that predicts dependence of various biological rates on temperature and body size has been proposed as a core of the “Metabolic Theory of Ecology” (MTE). However, its thermal component has been rarely explicitly tested and its usefulness for prediction of thermal effect on key life-history traits such as reproductive rate and hence fitness is still questionable. Here, we tested its applicability to temperature-dependent rate of clutch production in a tropical gecko. The thermal effects on reproductive rates in reptiles are only poorly known and difficult to estimate, because most species lay clutches largely infrequently. Females of the Madagascar ground gecko (Paroedura picta) lay clutches in unusually short intervals, which allowed us to use this species as a model for estimation of dependence of rate of clutch production on temperature. We kept adult females at three constant temperatures (24, 27, 30 °C) and recorded their reproductive characteristics. Increasing temperature positively influences rate of clutch production, but in a manner not predicted by a simple model of reaction kinetics. The results in P. picta suggest that predictions of fitness consequences of shifts in thermal environment can be more complicated than expected under the general relationship of the MTE.     

Observations on thermoregulation in the western slender glass lizard, Ophisaurus attenuatus attenuatus (Sauria: Anguidae)

Based upon thermocouple implantation, thermal preferenda and voluntary maxima for the head and body were studied in three individuals of Ophisaurus attenuatus attenuatus collected in Texas. Calculated grouped data for the head and body mean preferred temperature were 31.2°C (range 29.O° -34.7°C) and 31.8°C (range 30.3° -34.5°C) respectively. Gaping or panting were found to be absent in the O. a. attenuatus examined in this study.     

Wavelet analysis of nonstationary heart rate variability in a head-up tilt-table test

Heart rate variability during the head-up tilt test (HUT) was investigated using an automated setup of dynamic positioning in the gravity field. The wavelet theory was applied to find the dynamics of changes in the spectral properties of rhythmograms during the transitional steps of HUT. Quantitative parameters (spectral integrals, instantaneous maximal frequencies, nonstationarity factors) that describe the transients of such nonstationary HUT were calculated.     

Experimental test for premunition in a lizard malaria parasite (Plasmodium mexicanum)

Premunition in Plasmodium spp. is the prevention of superinfection by novel genotypes entering an already established infection in a vertebrate host. Evidence for premunition was sought for the lizard malaria parasite, P. mexicanum, in its natural host, the fence lizard, Sceloporus occidentalis. Clonal diversity (= alleles for the haploid parasite) was determined with the use of 3 microsatellite markers. Both naturally infected lizards (N = 25) and previously noninfected lizards (N = 78) were inoculated intraperitoneally (IP) with blood from donor infections and followed over a 3-mo period. Compared to the success of clonal establishment in all the naive lizards (78/78 successful), clones entering preexisting infections had a significant disadvantage (9/25 successful). The number of preexisting clones (1-2 vs. 3-4) within recipient infections had no effect on the success of superinfection. Infections that excluded entering novel clones did not have higher initial asexual parasitemia, but had a higher initial density of gametocytes, suggesting they were older. Infections allowing superinfection experienced a higher final parasitemia.     

Effect of thyroxine and noradrenaline on thermoregulation, cardiac rate and oxygen consumption in the monitor lizard Varanus albigularis albigularis

1. 1. At the preferred body temperature (35°C) resting metabolic rate was 0.155 ± 0.015 ml O₂/g·h and heart rate was 54 ± 11 beats/min. Spontaneous activity at this body temperature caused a two-fold increase in heart rate and a six-fold increase in O₂ consumption. Maximum values being 0.86 ml/g·h with an O₂ pulse of 13.6 × 10⁻⁵ ml/g·beat.

2. 2. Pre-treatment for seven days with thyroxine caused a 27% increase in resting metabolic rate and a 63% increase in the thermal gradient between core and ambient temperature at the preferred body temperature.

3. 3. Noradrenaline reduced heart rate but had no effect at the dosage recommended on metabolic rate at body temperatures of either 35 or 15°C, suggesting that non-shivering thermogenesis is absent in lizards. The evolutionary implications of these results have been briefly discussed.

The role of prostaglandins and the hypothalamus in thermoregulation in the lizard, Phrynocephalus przewalskii (Agamidae)

Typically, small lizards rely heavily on behavioral thermoregulation rather than physiological mechanisms to control their rates of warming and cooling. We tested the hypothesis that prostaglandins participate in mediating the cardiovascular response to heating and cooling and temperature regulating neurons in the hypothalamus of the small lizard Phrynocephalus przewalskii. In vivo and in vitro treatments, heart rates (HRs) were all found to be higher during heating than during cooling, hysteresis was distinct below 30 and 26°C, respectively. In vivo, as administration of COX inhibitor, there were no differences in HR between heating and cooling at any body temperature and administration of agonist prostaglandins only produced a significant effect on HR below 25°C. Single-unit activity was recorded extracellularly in vitro with microelectrodes, found the firing rate of the continuous unit increased 23% when the temperature of the artificial cerebrospinal fluid dropped from 30–20°C. We conclude that prostaglandins appear to play only a limited role in modulating heart activity in Phrynocephalus przewalskii and suggest that cold-sensitive neurons in the preoptic and anterior hypothalamus (PO/AH) are involved in thermoregulatory control during heating or cooling.     

Intrinsic rate of spermatogenesis in free-ranging feral pigs (Sus scrofa sp)

The aim of this research was to evaluate the intrinsic rate of spermatogenesis in adult free-ranging feral pigs. Twelve adult male free-ranging feral pigs were captured, sedated, and orchidectomized, and then were released and observed to complete recovery and return to their natural environment. Fragments of the testes were embedded in plastic resin and used to prepare slides for histometric analysis. Characteristics investigated included cell populations in the seminiferous epithelium in stage 1 of the cycle of the seminiferous epithelium, intrinsic rate of spermatogenesis and Sertoli cell index. The efficiency coefficient of spermatogonial mitosis was 7.59, the meiotic index was 3.03, the overall yield of spermatogenesis was 23.97 and the cell loss ratio during the meiotic prophase was 1.04. Each Sertoli cell supported an average of 0.92 type A spermatogonia, 7.01 primary spermatocytes in preleptotene/leptotene, 7.30 primary spermatocytes in pachytene and 22.16 round spermatids. In conclusion, the results of the present study indicate that the supporting capacity of Sertoli cells in free-ranging feral pigs is among the greatest values reported for most domestic animals, and the overall yield of spermatogenesis is comparable to that reported in wild boars.     

HOBOs, Tidbits and lizard models: the utility of electronic devices in field studies of ectotherm thermoregulation

1. A replicated experiment tested the null hypothesis that stand-alone electronic temperature recording devices produce sets of operative temperatures similar to those produced using lizard models.
2. Commercially available electronic temperature recording devices (HOBO XT™ with external probe and Tidbit™) produced sets of operative temperatures nearly identical to models designed to mimic the size, shape, scale architecture and colour of two species of common North American lizards.
3. Tidbits™ performed better than external probes.
4. These results suggest that electronic devices (especially Tidbits™) can be substituted for models in many applications and that size, morphology, scale architecture and colour contribute very little to temperature change in small-sized life-like models widely used in field-based studies on the thermal ecology of vertebrates.
5. Small differences between temperatures recorded by electronic devices and detailed lizard-shaped models fitted with thermal probes suggest that these models may nevertheless be necessary for certain kinds of studies.     

Asymmetric metabolism of hypothalamic catecholamines alternates with side of ovulation in a lizard (Anolis carolinensis).

We determined levels of monoamines and their metabolites in 2 hypothalami dissected from the right and left hemibrains of 15 females during the right-left alternating ovulatory cycle of Anolis carolinensis. Tissue contents of the following were measured using HPLC and electrochemical (coulometric) detection: dopamine (DA) and its metabolite 2,4-dihydroxyphenylacetic acid (DOPAC), norepinephrine (NE) and its metabolites 3-methoxy-4-hydroxyphenylglycol (MHPG) and 3,4-dihydroxyphenylglycol (DHPG), and serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). An asymmetry ratio (AR) was determined by subtracting hypothalamic content (pM/mg) on the larger ovary (LO) side from that on the smaller ovary (SO) side, divided by the sum of the 2 sides (AR = SO - LO/SO+LO). The Ar of MHPG and DHPG both decreased as the largest follicle in the LO grew during the cycle, from greater than 0 (content higher on the SO side) at the beginning of the cycle to less than 0 (content higher on the LO side). The average content of MHPG in the 2 sides significantly increased during the cycle. There were no significant asymmetric changes in hypothalamic DA or DOPAC. The average content of DA increased during the cycle, whereas the content of DOPAC, as well as DOPAC/DA, did not change. The average content of 5-HT increased, and the average metabolite ratio of 5-HIAA/5-HT decreased during the cycle without significant asymmetries. The metabolite ratios of NE and DA, but not 5-HT, were asymmetric on the same side in a given female.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diencephalic neuronal populations projecting axons into the basal plate in a lizard (Gallotia galloti).

Lizard diencephalic populations sending axons into the basal plate were studied by the in vitro HRP technique in the lizard Gallotia. Retrograde labeled cells were concentrated in distinct neuronal groups within alar plates of prosomeres p1 and p3, whereas the alar plate of p2 was poorly labeled. Efferent fibers from alar p1 and p3 populations entered the basal plate of the diencephalon along topologically dorsoventral courses, bifurcating thereafter into longitudinal ascending (rostral) and descending (caudal) trajectories. Thus, diencephalic segments p1 and p3 have alar cell populations contributing to the longitudinal premotor connectivity of the neural axis , whereas the alar p2 segment projects via the fasciculus retroflexus, the efferent tract of the epithalamus. However, the axons from the habenular complex bifurcate within or adjacent to the floor plate and not within the basal plate.     

Life in the fast lane: The free-ranging activity, heart rate and metabolism of an Antarctic fish tracked in temperate waters

The fish species Notothenia angustata inhabits temperate waters (10 °C), yet retains physiological traits that show it once existed at sub-zero temperatures. We determined the free-ranging activity, heart rate and metabolism of N. angustata and compared it with Notothenia coriiceps, an ecologically and morphologically congeneric cousin that still inhabits sub-zero waters. Firstly, the association between heart rate (fH) and oxygen consumption (MO₂) was first determined in the laboratory. The fish were then released into their respective environments and fH recorded by a miniature archival electronic tag, from which the metabolic rate of the free-ranging fish was extrapolated. Free-ranging activity of wild fish was monitored throughout the study using implanted acoustic tags and a static hydrophone array. Results showed that the difference in standard metabolic rate (SMR) between N. angustata and N. coriiceps fitted the Arrhenius model for inter-species thermal sensitivity in fish (Q₁₀ = 1.76). However, the inter-species disparity in total metabolic rate (TMR) was far greater. This was attributable to N. angustata swimming at higher speeds and covering a 5-fold greater area over 24 h compared with N. coriiceps. As a result, activity (and associated feeding) comprised a far larger portion of TMR in N. angustata (27.9%) than for N. coriiceps (5.7%). We conclude that the increased time spent swimming by N. angustata was presumably to forage for food to acquire sufficient energy to fuel its elevated SMR. This resulted in a much greater inter-species difference in TMR than may be predicted by the disparity in environmental temperature.