Activity 4
Frog model: In this activity we will be using an energy balance model that takes into account the characteristics of a frog and the microclimate surrounding it. For a given set of conditions, the model calculates the animal’s body temperature and its water loss rate.
Unlike reptiles, frogs have wet skin, and this complicates the energy balance. If a lizard walks from a cool, shady position to a sunny spot with a higher air temperature, the lizard will receive more energy and its body temperature will rise. If a frog hops from a cool, shady position to a sunny spot with a higher air temperature, it will receive more energy, but it will also evaporate at a higher rate, resulting in a loss of energy. The net effect of these energy exchanges (that is, whether the body temperature will go up or down) is difficult to predict without using an energy balance model that incorporates all of the avenues of energy gain and energy loss. This link opens an Excel file that is an energy balance mode for a wet skinned animal, such as a frog.
This version of the model is set up for a 50g Litoria caerulea (green tree frog) with climatic conditions measured at a site near Darwin. Note that the 8 columns (simulations) correspond to different hours of the day (as shown in the green row near the middle of the spread sheet). Eight different hours are shown (0 = midnight, 3am, 6am, 9am, 12 noon, 15 (= 3pm), 18 (= 6pm), and 21 (9pm).
The output from the model is shown in the light blue region at the bottom of the page. The first row in this section is the calculated body temperature of the frog, and the other 4 rows are different ways to express water loss. The time to 70% hydration is the time (in hours) that it would take under these conditions for the frog to lose 30% of its body water, which is dangerously close to the limit for most frogs.
Litoria caerulea has a moderately high cutaneous resistance of 14 s/cm, and this is shown in the second row. You can change any of the values of the variables in this model, but the ones we are most interested in are: body mass, cutaneous resistance, and air temperature. The first pink row determines the way radiation is used in the model. If this is set as Yes (Y), then the model calculates both solar radiation and infrared (or ‘thermal’) radiation. This setting can be used for both day and night, but at night the calculated solar radiation would be zero. If No (N) is used for a given time, it assumes that the frog is in a tree hollow or in a burrow, so the model only incorporates thermal radiation based on the temperature of the surrounding surface (so, no solar radiation and no radiant exchange with the sky). Note that this model for Litoria caerulea is set up assuming the frog is outside at night, but in a tree hollow during the day. Note also, that at 6am, when the frog moves inside the tree hollow, the warm, humid conditions inside the tree result in condensation (dew) forming on the frog, so it gains water from the air (represented by a negative water loss). This unusual avenue of water gain has been demonstrated for this species at a site near Darwin. It is particularly important during the dry season when there are few sources of water for frogs.
Activity: Record the air temperatures, the body temperatures, and the water loss rates shown for the given set of environmental conditions.
Then explore the results of a climate change scenario by resetting the air temperature at 3°C higher and record the resulting body temperatures and water loss rates. Does this temperature increase result in lethal conditions with respect to water loss (assuming water loss > 30% is lethal)? The Critical Thermal Maximum Temperature (just below the lethal body temperature) for this species is 37.5°, so does this climate change scenario result in dangerously high body temperatures? Think about the effects of changing environmental conditions across a “normal” day (the original conditions in the model) as compared to the effects due to the climate change scenario.
You will be asked to report on your results in Assignment 1.
You can explore other scenarios with this model by changing any of the variables. In Assignment 1, we will explore energy balance models for other frogs and explore the effects of body size and cutaneous resistance.