The energetics of sexual selection in the grey treefrog, Hyla versicolor

Advertisement call characteristics and calling effort were quantified for male gray treefrogs, Hyla versicolor, to determine (1) if individuals differ in calling behavior, (2) which aspects of calling behavior were linked to mating success, and (3) if calling behavior is constrained by energy availability. Individual male frogs, marked uniquely with toe-clips, were recorded and scored for mating success (amplexus) nightly during the 1994 and 1995 breeding seasons at sites within the Cuyahoga Valley National Park. Year-to year variation was observed in the number of calling males and the number of nightly choruses in the calling season, apparently due to air temperature and rainfall variations between the two seasons. Call duration and call intensity were inversely correlated with air temperature, and call intensity increased with body mass of frogs in 1994. These variables were not correlated in 1995, possibly because of lower temperature variation in that year. Although substantial inter-individual variability was found in calling behavior (call rate, call duration, call intensity) and chorus tenure (hours of calling per night, number of nights calling), only the number of nights calling (chorus tenure) demonstrated a significant relationship with mating success, i.e., the most persistent males mated most often.;During the 1996 and 1997 seasons, I assessed the effects of foraging success on calling effort through a diet-enhancement experiment. Ten to twelve individual frogs were periodically given a bucco-pharyngeal injection of a cricket slurry of standardized caloric content; ten to twelve control frogs were given water of the same volume. At two different locations, "diet-enhanced" frogs called on approximately 20% more nights and spent on average 6 additional minutes calling nightly in comparison controls. These results suggest that prey availability, foraging ability and/or rate or efficiency of food processing are linked to mating success in these frogs. However, even "diet-enhanced" frogs did not return on 35-40% of nights of chorusing, indicating that other constraints affect chorus tenure including foraging time and duration of elevated metabolic rates during digestion and absorption.;Previous workers have hypothesized that chorus attendance schedules are determined by corticosteroid cycling, e.g. the Energetics-Hormone Vocalization Model (Wingfield, 1994; Emerson, 2001). Here, I propose the Hormone-SDA-Vocalization Model for regulation of chorus tenure in prolonged breeding frogs with high calling activity. In this model, high corticosteroid levels induced by calling cause a short term stress reaction that leads to cessation of calling and commencement of feeding. Subsequently, however, elevated post-prandial metabolic rate (Specific Dynamic Action or SDA) limits energy expenditures for other activities and prevents the return of frogs to the chorus until metabolism approaches routine, pre-feeding levels. Hence, the time spent away from the breeding pond represents responses to hormone levels in combination with foraging time and duration of SDA. This model implies that foraging abilities and the speed and efficiency of energy processing in male anurans with high call rates may be shaped by sexual selection to reduce time away from the chorus.