Morphology of parotoid glands in cane toads

If optimal investment in anti-predator defences depends on predation risk,
invading new regions (and thus, encountering different predators) may
favour shifts in that investment. Cane toads offer an ideal system to test
this prediction: expensive anti-predator toxins are stored mainly in
parotoid glands whose dimensions are easy to measure, and toad invasions
have changed the suites of predators they encounter. Although plasticity
may influence parotoid morphology, comparisons between parents and progeny
revealed that gland dimensions were highly heritable. That heritability
supports the plausibility of an evolved basis to variation in gland
dimensions. Measurements of 3,779 adult toads show that females have
larger glands than males, invasive populations have larger glands than in
the native-range, and that parotoid sexual size dimorphism varies strongly
among invaded areas. Geographic variation in parotoid morphology may be
driven by predation risk to both adult toads and offspring (provisioned
with toxins by their mother), with toxins allocated to eggs exacerbating
the risk of cannibalism but reducing the risk of interspecific predation.
Investment into chemical defences has evolved rapidly during the cane
toad’s international diaspora, consistent with the hypothesis that
organisms flexibly adjust resource allocation to anti-predator tactics in
response to novel challenges.