Ecological example of conditioned flavor aversion in plant-herbivore interactions: effect of terpenes of Eucalyptus leaves on feeding by common ringtail and brushtail possums
Lawler, IR, Stapley, J, Foley, WJ & Eschler, BM 1999, Journal of Chemical Ecology, vol. 25, no. 2, pp. 401-415.
The avoidance of Eucalyptus terpenes by common ringtail and brushtail possums is found here to be due to a conditioned flavour aversion. The possums avoid terpenes due to them acting as an indication of toxic diformylaphloroglucinol compounds (DFPCs), rather than due to being toxic themselves.
DFPCs are toxic compounds, but are non-volatile and difficult to smell, whereas terpenes give the strong, distinctive Eucalyptus smell. Terpene concentration has been correlated with DFPC concentration in Eucalyptus leaves, so animals are able to determine relative DFPC levels in foliage by consuming a small amount and using the terpene levels as an indicator. In this study, cineole was used as a representative terpene and added to a possum’s food, which was shown to reduce their food intake. This demonstrated that the possums were avoiding terpenes rather than DFPCs themselves. By increasing the levels of cineole without having DFPCs causing negative physiological effects, the possums became acclimatised to cineole and increased their food consumption. Then, DFPCs (specifically jensenone) were added as well as the cineole, which reduced the possums’ intake again. Finally, cineole was added to their diets by itself again and the possums demonstrated a reduction in food intake, demonstrating that the flavour aversion of cineole and terpenes is a conditioned effect due to their indication of the presence of DFPCs.
The study is first to identify both the taste and toxin in a flavour aversion interaction. Previous studies had found aversive compounds, and that the aversion was concentration-dependent relative to the compound with the strong taste/smell, but had been unable to characterise the flavoured compound. Authors had previously hypothesised that terpenes themselves were the primary cause of deterrence, but this study found that their smell is the important deterrent, rather than their toxicity. Other studies had also posited that plant species could use mimicry (high terpene levels but low DFPC levels) to reduce herbivory. The experiment demonstrated that conditioned flavour aversions such as these can be deconditioned relatively easily, suggesting that this would not be an effective method unless a large proportion of the trees were truly toxic to maintain the conditioning.
Terpenes causing conditioned flavour aversion to DFPCs means that terpene levels can be modified through varying Eucalyptus genotypes to achieve desired results. For example, commercial Eucalyptus plantations can plant high terpene, high DFPC genotyped trees to limit herbivory and therefore increase yield, whereas conservation areas can plant low terpene, low DFPCs strains to encourage marsupial predation. This study has developed our understanding of the feeding habits of Eucalyptus-eating marsupials, and may potentially contribute to our knowledge of the diets and food preferences of koalas.
Summarised by Laura Wait
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