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We conducted a quantitative and qualitative chemical analysis of cane toad bufadienolides--the cardioactive steroids that are believed to be the principal cane toad toxins. We found complex shifts in toxin composition through toad ontogeny: (1) eggs contain at least 28 dominant bufadienolides, 17 of which are not detected in any other ontogenetic stage; (2) tadpoles present a simpler chemical profile with two to eight dominant bufadienolides; and (3) toxin diversity decreases during tadpole life but increases again after metamorphosis (larger metamorph/juvenile toads display five major bufadienolides). Total bufadienolide concentrations are highest in eggs (2.64 +/- 0.56 micromol/mg), decreasing during tadpole life stages (0.084 +/- 0.060 micromol/mg) before rising again after metamorphosis (2.35 +/- 0.45 micromol/mg). These variations in total bufadienolide levels correlate with toxicity to Australian frog species. For example, consumption of cane toad eggs killed tadpoles of two Australian frog species (Limnodynastes convexiusculus and Litoria rothii), whereas no tadpoles died after consuming late-stage cane toad tadpoles or small metamorphs. The high toxicity of toad eggs reflects components in the egg itself, not the surrounding jelly coat. Our results suggest a dramatic ontogenetic shift in the danger that toads pose to native predators, reflecting rapid changes in the types and amounts of toxins during toad development.

Summary1. In many anuran species, larvae modify their developmental trajectories and behaviour in response to chemical cues that predict predator risk. Recent reviews highlight a dearth of studies on delayed (post-metamorphic) consequences of larval experience. 2. We raised cane toad ( Bufo marinus ) tadpoles either under control conditions or in the presence of non-lethal predator cues (crushed conspecifics). 3. Exposure to these chemical cues massively reduced size at metamorphosis, as predicted by theory. Parotoid glands were larger relative to body size in post-metamorphic animals from the experimental treatment, suggesting higher investment in chemical defences. 4. Exposure to chemical cues from crushed conspecifics during larval life reduced total bufadienolide content of metamorphs, but increased amounts of one specific bufadienolide (bufalin). 5. Hence, cane toads respond to perceived predation risk in the aquatic environment by metamorphosing at a smaller size and modifying their investment in defensive toxins during post-metamorphic life. 6. Phenotypically flexible responses to larval conditions vary among amphibian taxa, and can involve significant carry-over effects into post-metamorphic life.

Photochemical extrusion of dinitrogen from diazo ketones 1-4 matrix isolated in argon at 10-15 K produces a-ketocarbenes 5, 6, and 8. UV-vis, infrared, and electron spin resonance spectroscopy identify the -ketocarbenes, which are further characterized by trapping with carbon monoxide and dioxygen. Excitation (T0-Tt) of the -ketocarhenes leads to rapid ring contraction of 5-*9 and slow ring contraction of 6-*10 and 8-*12. This trend parallels the increasing degree of strain in the product ketenes. Shorter wavelength irradiation causes rapid ring contraction of 6-*10 and 8-1-12. The stepwise

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