Innate immunity of Florida cane toads: how dispersal has affected physiological responses to LPS

Physiological tradeoffs occur in organisms coping with their environments, which are likely to increase as populations reach peripheries of established ranges. Invasive species offer opportunities to study tradeoffs that occur, with many hypotheses focusing on how immune responses vary during dispersal. The cane toad (Rhinella marina) is a well-known invasive species. Populations near the expanding edge of the Australian invasion have altered immune responses compared to toads from longer-established core populations, although this has not been well-documented for Florida populations. In this study, cane toads from a northern edge [New Port Richey (NPR)] and southern core (Miami) population in Florida were collected and injected with lipopolysaccharide (LPS) to compare immune responses. 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. In addition, LPS-injected core individuals had different circulating leukocyte profiles compared to saline-injected cane toads while edge individuals did not. There was a significant interaction between plasma bacteria-killing capability (BKA) and treatment, such that BKA decreased with time in saline compared to LPS-injected individuals, and saline-injected toads from the edge population had lower BKA compared to LPS-injected edge toads at 20 h post-injection. 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. The differential CORT response indicates that differential stress responses contribute to the tradeoffs observed with immunity and dispersal.