Assessing stress physiology of cane toads in Florida: tradeoffs with dispersal

Abstract

Physiological tradeoffs occur in organisms coping with their environments, which are likely to increase as populations reach peripheries of established ranges, potentially due to stress. Invasive species offer opportunities to study tradeoffs that occur, such as immune responses and defensive behaviors. The cane toad (Rhinella marina) is a well-known invasive species. Populations near the expanding edge of the Australian invasion have altered immune responses, increased leg and poison (parotoid) gland size, and decreased likelihoods of fleeing when disturbed compared to toads from longer established core populations, although this has not been well-documented for Florida populations. The research of this dissertation focuses on evaluating how immune, defensive, and endurance levels of cane toads from a northern edge (New Port Richey (NPR)) and southern core (Miami) population in Florida differ due to location in Florida. Core population individuals injected with LPS showed greater metabolic increases compared to their baseline rates that were higher compared to those from the edge population, and core individuals had different circulating leukocyte profiles compared to saline-injected cane toads while edge individuals did not. There was also a significant interaction between location and time on circulating corticosterone (CORT) levels following injections with saline or LPS, with CORT decreasing more with time in core population toads. We found that residual body indices increased in individuals from higher latitude populations, and relative parotoid gland size increased with increasing toad size. There was no effect of latitude on the allometric relationship between gland size and toad size. We observed an increase in likelihood of secretion by cane toads in the field with increasing latitude. Edge population individuals were less willing to move and did not travel as far in the track as those from the core population; however, there was no effect of locality on the distances traveled by toads in a treadmill. Although lactate levels decreased with time, there was also no effect of locality on lactate metabolism. The CORT responses of these toads observed in the field and in lab indicates that differential stress responses contribute to the tradeoffs observed with immunity, behavior, and dispersal.