Rates of attraction of cane toad tadpoles to chemicals

Chemical cues produced by late-stage embryos of the cane toad (Rhinella
marina) attract older conspecific larvae, which are highly cannibalistic
and can consume an entire clutch. To clarify the molecular basis of this
attraction response, we presented captive tadpoles with components present
in toad eggs. As previously reported, attractivity arises from the
distinctive toxins (bufadienolides) produced by cane toads, with some
toxins (e.g., bufagenins) much stronger attractants than others (e.g.,
bufotoxins). Extracts of frozen toad parotoid glands (rich in bufagenins)
were more attractive than were fresh MeOH extracts of the parotoid
secretion (rich in bufotoxins), and purified marinobufagin was more
effective than marinobufotoxin. Cardenolide aglycones (e.g.,
digitoxigenin) were active attractors, whereas C-3 glycosides (e.g.,
digoxin, oubain) were far less effective. A structure-activity
relationship study revealed that tadpole attractant potency strongly
correlated with Na+/K+ ATPase inhibitory activity, suggesting that
tadpoles monitor and rapidly react to perturbations to Na+/K+ ATPase
activity.