Embryos in the fast lane: high-temperature heart rates of turtles decline after hatching

Abstract

In ectotherms such as turtles, the relationship between cardiovascular function and temperature may be subject to different selective pressures in different life-history stages. Because embryos benefit by developing as rapidly as possible, and can "afford" to expend energy to do so (because they have access to the yolk for nutrition), they benefit from rapid heart (and thus, developmental) rates. In contrast, hatchlings do not have a guaranteed food supply, and maximal growth rates may not enhance fitness--and so, we might expect a lower heart rate, especially at high temperatures where metabolic costs are greatest. Our data on two species of emydid turtles, Chrysemys picta, and Graptemys pseudogeographica kohnii, support these predictions. Heart rates of embryos and hatchlings were similar at low temperatures, but heart rates at higher temperatures were much greater before than after hatching.

Figure 1. Thermal dependence of heart rates of embryos and hatchlings in the turtles Chrysemys picta (a) and Graptemys pseudogeographica kohnii (b).

Heart rates were measured by non-invasive methods, and are expressed as mean ± standard error. Heart rates did not change with the developmental stage of embryos or with the age of hatchlings, but were greater for embryos than for hatchlings when the animals were tested at high temperatures. Specifically, Turkey's test indicated that significant differences occurred at 30 and 33°C in C. picta, and 25, 30 and 33.5°C in G. pseudogeographica kohnii.