Sex and weaponry: The distribution of toxin‐storage glands on the bodies of male and female cane toads (Rhinella marina)

Abstract

The parotoid macroglands of bufonid anurans store (and can expel) large volumes of toxic secretions and have attracted detailed research. However, toxins also are stored in smaller glands that are distributed on the limbs and dorsal surface of the body. Female and male cane toads (Rhinella marina) differ in the location of toxin‐storage glands and the extent of glandular structures. Female toads store a larger proportion of their toxins in the parotoids than males as well as (to a lesser extent) in smaller glands on the forelimbs. Males have smaller and more elongate parotoids than females, but glands cover more of the skin surface on their limbs (especially hindlimbs) and dorsal surface. The delay to toxin exudation in response to electrostimulation varied among glands in various parts of the body, and did so differently in males than in females. The spatial distribution of toxin glands differs between the sexes even in toads that have been raised under standardized conditions in captivity; hence, the sexual dimorphism is due to heritable factors rather than developmentally plastic responses to ecological (e.g., habitat, predation risk) differences between the sexes. The selective advantages of this sexual dimorphism remain unclear. A priori, we might expect to see toxin widely dispersed across any part of the body likely to be contacted by a predator; and a wide distribution also would be expected if the gland secretions have other (e.g., male–male rivalry) functions. Why, then, is toxin concentrated in the parotoids, especially in female toads? That concentration may enhance the effectiveness of frontal displays to deter predation and also may facilitate the transfer of stored toxins to eggs.

Skin coloration as a possible non-invasive marker for skewed sex ratios and gonadal abnormalities in immature common toads (Bufo bufo)

“…It is possible that the density of pigment cells may differ between male and female skin and might be measurable by skin biopsy (although this method may be more expensive or complicated, and more invasive than photographing, it might perhaps provide more accurate information on sex). Additionally, other aspects of sexual dimorphism may also become detectable before sexual maturation, as has been found for toxin gland size in juvenile toads (Chen et al, 2017).…”

Variation in size and shape of toxin glands among cane toads from native-range and invasive populations

“…Cane toads provide an excellent system in which to study the factors influencing investment into anti-predator defences: the toad’s toxins are expensive to produce 16 , the magnitude of toxin stores is easily quantified 17 , and recent range expansions have modified the toad’s rate of dispersal as well as its exposure to predation in both aquatic and terrestrial phases of the life-history 19 . Variation in dimensions of the parotoid macroglands among individuals is not attributable to recent expulsion of toxins (i.e., as measured over 5 days) and exhibits significant heritability (present study), consistent with the hypothesis that geographic variation in gland morphology reflects adaptive responses to novel challenges.…”

“…These toxins have cardio-acceleratory and vasopressor effects on vertebrates 52 , 56 , such that ingestion can be lethal for predators 57 – 59 . In cane toads ( Rhinella marina ), this toxin is contained both within the parotoids and a series of smaller glands that are distributed on the dorsal surface and limbs 17 . The cane toad uses the toxin as a passive defence, exuding it when stressed (Fig.…”

“…Like many amphibians, toads synthesise potent chemical defences in the form of steroidal bufotoxins 12 – 14 that are energetically expensive to produce 15 , 16 . Quantifying investment into those chemicals is facilitated by the fact that most toad toxins are produced and stored in parotoid (shoulder) macroglands, whose dimensions can be easily and accurately measured on live animals 17 , 18 (Fig. 1 ).…”

Shifts in the foraging tactics of crocodiles following invasion by toxic prey

“…Treatment (toad) baits were non-lethal as internal organs and parotid glands had been removed. Toad baits hence had very little toxin and were unlikely to induce taste aversion 21 . We randomly assigned baits to either side of the station so that visiting animals could choose between both bait types at each location.…”