Essay, Research Paper
Janzen, F.J. 1993. An Experimental Analysis of Natural Selection on Body Size of Hatchling Turtles. Ecology. 74: 332-341
The purpose of the experiment was to evaluate the effects of natural selection on hatchling turtle size. The author tested whether turtle hatchlings with bigger mass, body size, and plastron length would exhibit greater survivorship during movement from the nest to the water habitat.
In conducting this experiment eggs from 17 different Chelydra serpentina clutches were collected from a National Wildlife Refuge area in Whiteside County, Illinois in the month of June 1990. The collected eggs were then incubated in two different environments. Either a dry -150kPa or a wet -950kPa environment, both environments maintained a temperature of 27.5.C. The containers were rehydrated twice weekly and the eggs rotated daily. Once the turtles hatched the date and body size of each individual was recorded. Approximately three weeks after hatching, the turtles were given a number and prepared for release and recapture.
The recapture process used a drift fence constructed of 50m of 15cm high aluminum flashing support, eighteen lidless, 25cm high plastic containers that were implanted into the sand at 3m intervals. The turtles were then arranged randomly in a line facing the nest and released at sunset, all were released together, since C. serpentina hatch synchronously. The collection pits were checked at sunrise, midday, and sunset. The experiment was terminated when two successive monitoring periods failed to produce a hatchling.
The findings showed that the incubating conditions modified the phenotypes of hatchling snapping turtles. Those incubated in the wet environment were larger and took longer to hatch than those incubated in the dry environment. Nearly 60% of the (66 of 112) hatchlings were recaptured. The use of normal distribution to calculate bypass rate indicated that approximately two hatchlings probably missed the fence. There were no signs of predators around the collection pits, so the turtles were not attacked after reaching the drift fence. Survivorship was related to three different measures of body size and shape. Mean hatchling mass: wet environment was 8.63g with a standard deviation (SD)of 1.38 and the dry environment was 7.60g with a SD of 1.24. Mean plastron length: the wet environment was 20.60mm with a SD of 1.32 and in the dry environment 20.06mm with a SD of 1.35. The carapace length: wet environment 29.39mm a SD of 1.88 and dry environment 27.73mm with a SD of 1.78. The larger hatchlings had increased survivorship over the smaller hatchlings in all three of the variables. One calculated variable, PC1, the overall size and mass, was measured at 0.327. This accounts for almost 85% of variation in body size, which produces a change in body size of approximately one-third of a standard deviation. PC3, plastron length was measured at 0.282 and PC4, trade-off between hatchling mass and width was 0.162. PC3 and PC4 account for less than 5% of variation in body size. Other variables, I, opportunity for selection was 0.703 and h2, the heritability of body mass, was 0.72 + 0.34 for the wet environment and 0.17 + 0.28 for the dry environment.
The results of the experiment indicate that larger hatchlings may have a survival advantage over smaller hatchlings during migration from the nest to the aquatic habitat. The use of different water potentials permitted the analysis of survivorship among larger and smaller hatchling turtles. The moderate strength of natural selection and low heritability of hatchling size implies that evolution would not occur rapidly among C. serpentina.
In reviewing the presentation, I felt that the results section was very confusing I could not understand how the author analyzed each of the variables accounted for. Nor did the author ever describe what each principle component (PC) was, how it was measured, and what significance it played in determining the effects of natural selection on the hatchlings. The author did however address the issue of predation upon the hatchlings. The author explained that the use of clay or model hatchlings might give us a better understanding of how/if hatchling size has an effect on predation. Finally, the methods section was very clear in exactly what took place, how long, and why it was being done. With the exception of the results section the presentation was easy to follow and right to the point.