Water loss under controlled humidity and temperature regimes in relation to bird eggshell thickness
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The African Penguin (Spheniscus demersus) is an Endangered species occurring along the Indian and South Atlantic coasts of South Africa. There are many suspected causes for the precipitous decline in their numbers. Previously, eggshell thinning associated with increased concentrations of persistent organic pollutants has been found for the African Penguin. The combination of thinner shells and higher temperatures due to global climate change may cause accelerated water loss through the eggshells, thereby possibly harming the embryo. A method had to be developed to measure water loss across eggshells under controlled temperature and humidity treatments. This I did in a previous study where I converted a chicken egg incubator, and successfully tested the effectiveness using chicken eggshells. Thinner eggs lost water faster than thicker shells at higher temperatures, moderated by humidity. I used the African Penguin eggshells from a previous POPs study to measure the effects of temperature and humidity on water loss. I chose temperature and humidity treatment based on current climate data, and selected two higher temperatures (32°C and 37°C) and commensurate humidity settings to simulate future climate change scenarios. The eggshells lost water faster at higher temperatures, and less water was lost at higher relative humidities irrespective of temperature, as was the case for chicken eggshells. However, eggshell thickness did not play a role. Thinner and thicker eggs lost water at the same rate at all temperature and humidity treatments. Water loss across the eggshells was therefore regulated by the shells, irrespective of their thickness. I noticed from the data that the standard deviations increased at higher temperatures. Plotting the percentage coefficient of variation (%CVs) of the water loss at each relative humidity against their respective temperatures showed that the water loss regulation (low %CVs) was lost at around 32°C, and increased towards 37°C. An optimal 15°C temperature window for effective water loss regulation was indicated by the model. Since previous studies have not investigated the combination of temperature, humidity, and eggshell thickness in relation to water loss, this study suggests an optimal temperature/humidity window related to water loss. Not all shells responded in this way, suggesting a variability upon which natural selection can act. One additional factor was tested and that was the effect of the oily eggshell extraneous materials (EEM). EEM-covered eggshells unexpectedly showed a significant faster water loss than the clean shells. I cannot adequately explain this, as ‘wicking’ of water through an oily layer is counter intuitive. This issue requires further testing. An aspect to keep in mind was that the tests were carried out at an air pressure of 80.7 kPa at an altitude of 1300 m above sea level. The normal air pressure at sea level where the penguins nest is 101 kPa. Differences in pressure may affect the amounts of water loss, but cannot explain lack of effect of eggshell thickness on water loss. Although this study started off with a concern raised by thinner eggshells associated with increase POPs concentrations in the eggs, I could find no significant water loss effect related to differences in eggshell thickness. This finding does not negate that thinner eggs are not a problem since the eggs still need to withstand other forces such as body pressure exerted by the incubating parents. Thinner eggs are also more susceptible to infection. The effect of water loss affected by temperature and humidity through African Penguin eggshells on their population size remains to be determined. However, I showed that increased temperatures are likely to increase water loss rates, thereby placing metabolic stress on the developing embryo. The compensatory abilities of the embryo might be exceeded at such higher temperatures. Climate change through increasing summer temperatures therefore, will add pressure on African Penguins though accelerated water loss.