Using whole-group metabolic rate and behaviour to assess the energetics of courtship in red-sided garter snakes

- Males modify their energetic expenditure relative to the likelihood of fitness payoffs.
- Courtship intensity is tailored in response to male competition and female behaviour.
- Presence of competitors and mates significantly increases metabolic rate.
- Aerobic factorial scope of male red-sided garter snakes is very high for an ectotherm.

Reproductive effort is an important aspect of life history as reproductive success is arguably the most important component of fitness. Males tend to compete for access to females and, in the process, expend their energetic capital on mate searching, male-male competition and courtship rather than directly on offspring. Red-sided garter snakes, Thamnophis sirtalis parietalis, are an exceptional model for studying energetic costs of courtship and mating as they fast during the spring mating season, which segregates the cost of energy acquisition from the cost of courtship and mating. However, measuring an individual male's metabolic rate during courtship is complicated by the fact that male courtship behaviour in red-sided garter snakes is dependent on both the detection of a female sexual attractiveness pheromone and on facilitated courtship (i.e. vigorous courtship is only exhibited in the presence of other males). Thus, traditional techniques of placing a mask over the head of individuals would prevent male courtship behaviour, and single animals placed in a flow-through chamber would not yield ecologically realistic levels of courtship, which are only seen in the context of a mating aggregation in this species. Because of these difficulties, we placed groups of males in a flow-through metabolic chamber together with a single female whose respiratory gases were vented outside the chamber to yield a whole-group metabolic rate during competitive courtship. We also measured the standard metabolic rates (SMR) of the males individually for comparison with active metabolic rates. Conservative estimates of peak group metabolic rates during courtship are 10-20 times higher than resting group metabolic rate, which was 1.88 times higher than SMR. These measurements, coupled with the fact that these males are aphagous during the breeding, indicates that costs of courtship may be high for males and has implications for the male mating tactics in this system.