The Metabolic Effort and Duration of Ecdysis in Timber Rattlesnakes: Implications for Time-Energy Budgets of Reptiles.

AbstractTemperate reptiles are often considered to be low-energy systems, with their discrete use of time and energy making them model systems for the study of time-energy budgets. However, the semifrequent replacement and sloughing of the epidermis is a ubiquitous feature of squamate reptiles that is often overlooked when accounting for time and energy budgets in these animals. We used open-flow respirometry to measure both the energetic effort of ecdysis and the duration of the associated metabolic upregulation (likely related to behavioral changes often reported for animals in shed) in wild-caught timber rattlesnakes (Crotalus horridus). We hypothesized that total effort of skin biosynthesis and physical removal would be related to body mass and expected the duration of the process to remain static across individuals at a fixed temperature (25°C). We provide both the first measurements of the cost of skin biosynthesis and physical removal in a reptile and the highest-resolution estimate of process duration recorded to date. We found that skin biosynthesis, but not the cost of physical removal of the epidermis, was related to body mass. Shed cycle duration was consistent across individuals, taking nearly 4 wk from process initiation to physical removal of the outermost epidermal layer. Total energetic effort of ecdysis was of sizeable magnitude, requiring ∼3% of the total annual energy budget of a timber rattlesnake. Energetic effort for a 500-g snake was equivalent to the amount of metabolizable energy acquired from the consumption of approximately two adult mice. Ecdysis is a significant part of the time-energy budgets of snakes, necessitating further attention in studies of reptilian energetics.

Gradual and Discrete Ontogenetic Shifts in Rattlesnake Venom Composition and Assessment of Hormonal and Ecological Correlates.

Ontogenetic shifts in venom occur in many snakes but establishing their nature as gradual or discrete processes required additional study. We profiled shifts in venom expression from the neonate to adult sizes of two rattlesnake species, the eastern diamondback and the timber rattlesnake. We used serial sampling and venom chromatographic profiling to test if ontogenetic change occurs gradually or discretely. We found evidence for gradual shifts in overall venom composition in six of eight snakes, which sometimes spanned more than two years. Most chromatographic peaks shift gradually, but one quarter shift in a discrete fashion. Analysis of published diet data showed gradual shifts in overall diet composition across the range of body sizes attained by our eight study animals, while the shifts in abundance of different prey classes varied in form from gradual to discrete. Testosterone concentrations were correlated with the change in venom protein composition, but the relationship is not strong enough to suggest causation. Venom research employing simple juvenile versus adult size thresholds may be failing to account for continuous variation in venom composition lifespan. Our results imply that venom shifts represent adaptive matches to dietary shifts and highlight venom for studies of alternative gene regulatory mechanisms.

Metabolic cost of venom replenishment by Prairie Rattlesnakes (Crotalus viridis viridis).

Snakes demonstrate a great deal of variation in the amount of venom injected in both predatory and defensive strikes. There is some evidence that snakes can adaptively meter venom dosage. An underlying assumption in the evolution of venom metering is that the production of venom is energetically costly. To date there has been very little research that has quantified the metabolic costs associated with venom production. We used open-flow respirometry to test for significant differences between Prairie Rattlesnakes (Crotalus v. viridis) that had venom extracted and control snakes that did not. Although previous studies demonstrated high metabolic costs for venom production, we found that snakes that had venom extracted did not have significantly higher metabolic rates than control snakes. The pattern of metabolic deviation from baseline measurements was similar for both treatments, and on average snakes that had venom extracted only exhibited a 1.1% increase over baseline compared to a 2.5% increase in control snakes. Our data suggest that venom is not energetically costly to produce and that perhaps other costs associated with venom can better explain the variability in venom expenditure.