Thermal Advantages of Communal Egg Mass Deposition in Wood Frogs (Rana sylvatica)

Abstract

Embryos of most anurans develop within a special microhabitat consisting of the jelly secreted around the eggs by the female parent. The jelly envelope covering each embryo effectively provides a buffer between it and the surrounding water; moreover, the three-dimensional structure of the gelatinous egg mass is thought to represent an adaptation to thermal stresses and gas exchange requirements likely to be encountered by the developing embryos. Jelly masses of North American ranid frogs can be categorized into two species-specific types related to these environmental factors: globular-shaped and surface film forms (Moore, 1940). Frogs that breed early in the spring deposit their eggs in globular masses, which may facilitate the concentration and retention of heat (obtained from solar radiation) within the egg masses. Such traits may be particularly adaptive in the cold water characteristic of ponds in early spring. In contrast, frogs breeding in late spring and summer generally deposit their eggs in monolayer surface film masses. Because embryos of these species develop in much warmer conditions, their metabolic rates are higher than those of early-breeding species and the oxygen tension in the water is lower; thus this egg mass form may represent an adaptation to maximize surface area for gas exchange (reviewed in Salthe and Mecham, 1974). In some instances, this surface film structure may also functionally cool the eggs (Ryan, 1978). Wood frogs (Rana sylvatica) are the most northerly distributed ectothermic tetrapod in North America, with a range extending from north of the Arctic Circle to Georgia. In temperate regions, wood frogs breed in early spring, frequently before ice has completely melted from breeding ponds. Wood frogs deposit their egg masses in communal clumps within limited areas, thereby compounding insulation effects provided by their individual egg masses. Although several investigators have suggested that clumping of egg masses warms embryos (R. sylvatica: Herreid and Kinney, 1967; Hassinger, 1970; Howard, 1980; Seale, 1982; R. temporaria: Savage, 1950, 1961; Guyetant, 1966; Beattie, 1980; R. pipiens: Zweifel, 1968; Hassinger, 1970; Merrell, 1977; R. aurora, R. pretiosa: Licht, 1971), differences in temperatures of egg masses at various positions within clumps have not been investigated. We studied egg masses in two woodland ponds near Ithaca, New York during April 1978 and 1980. The ponds are shallow (less than 0.8 m depth) permanent pools, with little submerged or emergent vegetation. Most egg masses were deposited in clumps in the shallowest water (less than 0.25 m depth), attached to submerged vines, reeds, and twigs. Generally, all egg masses in a pond were deposited at one site, leading to the formation of one clump per pond. However, in 1978 at one pond, six clumps consisting of variable numbers of egg masses were formed, allowing us to compare characteristics of egg masses based upon clump size within the same pond. Clumps consisted of between 3 and 150 egg masses. To investigate the temperature of egg masses in relation to their positions within a clump, we measured temperatures of some egg masses (approx. 3 cm within each egg mass) both at the center and at the periphery of clumps, as well as those of single egg masses deposited outside clumps. We then compared these temperatures with that of the surrounding water, 10 cm outside the clump (5 cm below the water surface). All temperature data were obtained with a Schultheis quick-reading thermometer. To quantify the of each egg mass to the surrounding water, we visually estimated the percentage surface area of each egg mass not in direct contact with surrounding egg masses (on its lateral and ventral surfaces). Observations were made on clumps consisting of various numbers of egg masses, and in clumps on both sunny and overcast days. Although most egg masses within clumps were warmer than the surrounding water, egg masses in central locations within a clump were warmer than those at the edge of that clump. Temperature elevation of egg masses was inversely correlated with their exposed surface area. Moreover, egg mass temperatures increased (in all clump positions) as the size of the clump increased (Fig. 1). This finding suggests that the presence of additional egg masses in a clump provides added insulation, improving the heat retention capacity of the clumped egg masses. The correlation of temperature elevation of an egg mass with its surface area exposure was apparent both on sunny and on cloudy days. Figure 2 shows temperature elevation of egg masses as a function of their position within a clump of 36 egg masses on two successive afternoons. The first day was sunny and central egg masses were 3.0-3.2?C above ambient. Weather conditions the