Late spring and summer represent an important period for members of the Gray Treefrog and Cope’s Gray Treefrog species complex (Hyla versicolor, LeConte/chrysoscelis, Cope; hereafter treefrog). Most breeding takes place during late spring through mid-summer, and this energy-expensive period is followed by preparations for overwinter dormancy, another energetically expensive time (Smith et al., 2003; Briggler and Johnson, 2021). As such, the summer energy budget is an important component of the annual fitness of individuals in treefrog populations. This study explores the importance of this energy budget in terms of weight change in relation to both sexes over the breeding and winter preparatory time periods.

In northwest Missouri, treefrog breeding typically reaches its peak in early May and lasts through the end of June or early July, with small amounts of breeding extending even to August (Briggler and Johnson, 2021). Males will establish choruses at shallow ponds, or similar sites where fish are unlikely to prey on eggs, allowing females to approach and select mates based on site and call characteristics (Fellers, 1979; Schwartz et al., 2001). This is particularly energetically expensive for males, who may produce over 5000 calls a night (Wells and Taigen, 1986). However, females incur energy costs as well, traveling nightly to visit choruses and using fat supplies to produce eggs (Johnson et al., 2007; Rastogi et al., 2010). It should be no surprise then, that both males and females lose weight during this time (Rastogi et al., 2010).

During winter, dormancy typically occurs in leaf litter above the frost-line, where temperatures are moderated, but still often below freezing (Layne, 1999; Johnson et al., 2008). Cryoprotectants are secreted by the liver and take the form of glucose (Hyla chrysoscelis) broken down from glycogen (Hyla versicolor, Costanzo et al., 1992; Irwin and Lee, 2003). Regardless of the energy expenses incurred during breeding, treefrogs should rebuild nutrient supplies as soon as possible in preparation for winter dormancy. Indeed, Kovacs et al. (2007), in a study of the diet of congener Hyla arborea, reported higher feeding intensity in the period prior to dormancy (August-September).

The selective pressure to maintain nutrient levels during the breeding season, and to rebuild nutrient supplies for winter dormancy should be detectable in the form of weight shifts in individuals during the summer months. If true, this would suggest that current monitoring studies for treefrogs should include weight measurements during the summer. We hypothesized a detectable decline in weight during the main part of the breeding period (May – June) for a population of treefrogs in Nodaway County, Missouri. We further hypothesized that this would be followed by a detectable weight gain in the subsequent period (July – August).

Overall patterns of weight change matched our hypotheses for both sexes. We documented weight declines in May-June, followed by weight increases in July-August. Nevertheless, our results varied highly by year and sex. Significant weight loss during May-June appeared to be more commonly true for males than females. This difference between the two sexes likely reflects the heightened energy requirements males undergo by calling (Taigen and Wells, 1985). However, even for males, in some years significant weight loss was not detected. It is possible that male energy loss over this time is usually offset by foraging bouts during the day or during absences from chorusing (Murphy, 1994; Wells et al., 1995; Johnson et al., 2007). As such, measuring weights on a weekly basis would not be precise enough to detect energy losses in most years. Instead, we would be detecting only unusually poor periods where energy losses in the form of weight loss would last for a longer period. This would be true for females as well, but our results suggest that significant weight loss occurs in fewer years during this period than for males. Females during this period may travel to assess multiple breeding ponds and tend to locate at the periphery of a population’s distribution possibly for this purpose or to avoid foraging competition with males who are in higher densities around breeding ponds (Johnson et al. 2007). Despite the greater degree of movement and consequent energy cost this may require, it is likely only for a short period of time, perhaps as short as 48 hours in some populations (Johnson et al. 2007), and so was not detectable by this study (sampling two consecutive days per week).

Later in the summer (July-August), both sexes show significant weight gains in three of seven years, with overlap in years 2016 and 2020. During this period prior to cooler weather, one would expect increased foraging to offset any costs associated with breeding, but also to prepare for winter dormancy by increasing glycogen stores (Irwin and Lee, 2003; Costanzo et al., 1992). This would presumably result in weight gain as late summer progressed. Our inability to detect these weight gains in most years is again likely due to the coarseness of our sampling strategy, such that we were only able to detect major shifts in weight. Large scale changes in weight may only occur in some years, potentially high-quality years in terms of prey availability, offsetting competitive pressures.

One caveat to our analysis is that individual frogs were tracked at varying rates over the summer. Some were captured multiple times, some were captured just once. Those treefrogs that were reweighed more consistently over the course of the summer contributed more data to the analysis. If treefrogs that were likely to use PVC pipes for longer periods (allowing multiple measurements) differed from the larger population, it would bias our results. However, we were unable to find a significant difference between treefrogs captured only once and treefrogs captured multiple times, suggesting that this was not the case. A second caveat is that the conditions of our study meant that treefrogs could join or contribute a weight measurement at any time. This would be problematic if younger, and presumably lighter treefrogs joined later than older and heavier treefrogs because it would falsely imply a weight loss. We did on occasion see older females who had survived the winter start using pipes early, be measured a couple of times, then not be captured again, but this was rare and would be unlikely to bias results heavily. Other bias could include environmental factors such as general weather conditions and a short sampling period that excluded the months of March, April, September, and October.

Our results suggest that there are detectable weight shifts in some years during the summer, presumably reflecting a reproductive season (May – June) and winter preparatory season (July–August). This indicates that the summer period overall (May–August) in northwest Missouri is an ecologically important period for treefrogs, requiring the balancing of multiple goals, such as reproducing and preparing for winter. In addition, there is an inherent variability in annual conditions affecting treefrogs’ ability to meet these goals. We suggest monitoring protocols for populations should include weight measurements, especially during summer.