Differences in morphology and in composition and release of parotoid gland secretion in introduced cane toads (Rhinella marina) from established populations in Florida, USA

Abstract

Cane toads are highly toxic bufonids invasive in several locations throughout the world. Although physiological changes and effects on native predators for Australian populations have been well documented, Florida populations have received little attention. Cane toads were collected from populations spanning the invaded range in Florida to assess relative toxicity, through measuring morphological changes to parotoid glands, likelihood of secretion, and the marinobufagenin (MBG) content of secretion. 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. Individuals from southern and northern populations did not vary significantly in the quantity of MBG contained in their secretion. Laboratory-acclimated cane toads receiving injections of epinephrine were more likely to secrete poison with increasing dose, although there was no difference in likelihood of secretion between southern and northern populations. This suggests that differences between populations in the quantities of epinephrine released in the field, due to altered hypothalamic sensitivity upon disturbance, may be responsible for the latitudinal effects on poison secretion. Our results suggest that altered pressures from northward establishment in Florida have affected sympathetic sensitivity and defensive mechanisms of cane toads, potentially affecting risk to native predators.

Keywords: Rhinella marina; bufadienolide; epinephrine; invasion; phenotypic plasticity; sympathetic sensitivity.

Figure 1

Cane toad populations sampled in 2018 for morphological measures and likelihood of secretion. Locations are listed above images of collected cane toads, with images depicting representative toads from each sampled location being recorded for gland sizes following mass, sex, and SVL being recorded. Toads were collected along a south to north gradient, from Homestead to DeLand, FL. Sites indicated by a star indicate populations sampled in 2019 for MBG concentration in poison secretions

Figure 2

Poison secretion data collected from 9 cane toad populations spanning a south to north gradient in FL (n = 21, 25, and 13 for Homestead, Ft. Myers, and DeLand, and n = 20 for all other populations). Panel (a) represents percentages of toads secreting poison following 1 hr of capture and handling. The likelihood of a toad secreting poison following collection in the field increased significantly (z173 = 3.24, p < .01) with increasing latitude, from the most southern population (Homestead) to the most northern (DeLand, FL). Panel (b) depicts a toad secreting poison immediately following capture from a northern FL population (DeLand)

Figure 3

Percentages of laboratory‐acclimated cane toads collected from Miami (south) (n = 28) and New Port Richey (north) (n = 30) during 2019 secreting poison following injections with increasing doses of epinephrine (µmol ((g body mass‐1)). The likelihood of a toad secreting poison increased significantly with increasing dose (z55 = 2.71, p = .01), although there was no effect of location on this likelihood (p > .05)