Herbivore-deterring secondary compounds in heterophyllous woody species of the Mascarene Islands

Heterophylly (juvenile-adult leaf dimorphism) is widespread among native woody species of the Mascarene Islands, but the causes to this phenomenon have so far not been fully explained. The absence of mammals and dominance of now extinct giant tortoises and flightless birds are characteristics of the original Mascarene fauna. The present study investigates the hypothesis that the distinct morphology and colouration of juvenile leaves signalled unpalatability to browsing giant tortoises or birds. Juvenile and adult leaves of 28 heterophyllous woody species endemic to the islands of Mauritius and Rodrigues were screened for possible chemical defences against herbivory. The screening comprised the following classes of secondary compounds: alkaloids, cyanogenic constituents, saponins, simple phenolics, tannins and anthocyanins. The screening showed that there are no consistent significant differences between juvenile and adult leaves regarding any of the studied secondary compound classes, with the exception of the level of anthocyanins, which was significantly higher in juvenile leaves. This difference was also clearly reflected in a generally high degree of red patterning of juvenile leaves. Based on comparisons with floras of other oceanic islands once inhabited by tortoises and birds, and an evaluation of feeding habits of these herbivores, we suggest that the reduced lamina and partly red colouration of juvenile leaves may be an evolutionary response to selective herbivory by birds.