What's that smell?

Investigation into carrion and faecal mimicry is not the most popular research topic in evolutionary biology. As mammals, it is ingrained in us to avoid such things because of the potential diseases and pathogenic microbes that may be harbored within them.

It is proposed that carrion and faecal mimicry in flowers has evolved to not only attract pollinators, but also to deter herbivorous mammals (Lev-Yadun and Gutman, 2013). Carrion and faecal mimicry is also hypothesised to give a false warning to herbivores of nearby predators. Flowers mimic the scent of carrion which would likely be surrounded by carnivores, or of predator’s urine or faecal matter to fake the recent passing of a carnivore. This fear of predation has been known to influence the herbivores behavior in such a strong way that vegetation structure can be completely changed (Lev-Yadun and Gutman, 2013). Some hypotheses even go as far as to say that carrion odor may assist plants by attracting large carnivores, which may then defend them from herbivores (Lev-Yadun et al., 2009). Research has shown that the mimicking flowers rely heavily on the volatiles used by flies as cues to locate carrion and faeces (Johnson and Jürgens, 2010). It has been discovered that the oligosulphides emitted by the Mediterranean Arum flower are identical to those emitted by animal carrion and trigger the same electrophysiological response in fly antennae (Johnson and Jürgens, 2010). This shows amazing mimicry of the chemical cues given by carrion to successfully exploit carrion flies.

The primary purpose of the odorous emissions is a hotly debated topic, for example the Rafflesia genus only emits odours during its reproductive season, leading to the conclusion that the attraction of pollinators is the primary function of the chemical emissions (Lev-Yadun et al., 2009). Despite this conclusion scientists argue as to whether the evolution of odours was primarily to repel herbivores, or for the attraction of pollinators. 

Rafflesia spp. adapted from http://liseblog.dk/thailand-2012/

Australia - Deception 'hotspot'

The Australian continent is known for its profound concentration of deceptive organisms. Orchids are the favored deception example in the plant kingdom, with sexual deception resulting in over 11 genera and hundreds of species; complementing Australia’s reputation as the deception ‘hotspot’ of the world.  An interesting paper released late last year by Herberstein et al. (2013), suggests and discusses a few distinct reasons why this might be so.

“Is the prevalence of some deceptive systems a reflection of species diversity?”

This hypothesis questions whether the diversification of deceptive species could be a function of overall species diversity. If this were the case then the overall species diversity of plants would be largely proportional to the diversity of the corresponding deceptive species. Using orchids again as an example, only ~5% of worldwide species occur in Australia, which is a very low species diversity and does not explain the high number of deceptive species. Using orchids and other animal and plant examples, Herberstein et al. (2013) concluded that deceptive systems are not a reflection of species diversity.

“Does deception evolve readily in Australia?”

This hypothesis investigates Australia’s environmental conditions, and its isolation and potential for invasion. Australia is mostly dry, with poor soils and frequent fires, phenomena which have been used to explain the production of nutrient poor biomass (can result in low rates of herbivory) along with ample amounts of sap and nectar (which can lead to pollination by larger animals). These environmental restrictions may have increased the selection for behavioral strategies that minimise the costs to survive, e.g. floral rewardlessness in orchids, as well as fragmentation of species due to fire regimes promoting selection for gene flow across larger distances. If harsh environmental conditions are a driver for the evolution of deception, that is where a high number of deceptive systems are likely to occur. Due to limited information in the literature, Herberstein et al. (2013) urge further research into the frequency of deception in harsh environments to help support or disprove this hypothesis.

Australia has been subject to a long history of evolutionary isolation and recent invasions, which may explain a radiation of deception. If this were the case, then it can be predicted that most deceptive species have only recently arrived in Australia, and seek to exploit naïve and endemic species. Australia’s Mediterranean and tropical climates have given rise to high levels of endemism and biodiversity; which provides a broad variety of potential deception targets. Isolated populations are typically vulnerable to exploitation by invasive species, the mixing of invasive species with existing species can lead to new symbiotic relationships; and perhaps even deception relationships.

“Does Australia’s intellectual and research culture encourage discovery of deception?”

Herberstein et al. (2013) discuss their final hypothesis of high levels of Australian deceptive species as being the result of recent research popularity. They predict that the popularity of certain research areas leads to a higher than expected reporting of similar phenomena; including the discovery and investigation of new deceptive species. For example, behavioral ecology in Australia is thriving, especially when the number of research institutions are compared with other nations. Over the years 2010 and 2011 8% of papers published in Behavioral Ecology were authored by Australian researchers and institutions, compared with 16.5% of papers being from the United Kingdom, which has 3 times the number of institutions as Australia. However, Herberstein et al, (2013) issue caution when presenting this hypothesis. This is because a more thorough investigation and analysis on the number of publications on deceptive and non-deceptive systems is needed for a comprehensive evaluation of the high number of Australian deceptive species due to potential publication bias.

Descriptives of the higher education environments in Australia, Europe, United Kingdom, United States of America, Canada, and Japan, indicating Australia’s surprisingly large proportion of behavioral ecologists and consequent contribution to the journal Behavioral Ecology given the relatively few research institutes. Adapted from Herberstein et al. (2013).