![]() An additional possibility is to rely on some form of coercion/manipulation of the partner without disrupting the mutually beneficial outcomes of the relationship ( Grasso et al., 2015 Heil, 2015a). However, possible conflicts can be managed and mutualism stabilized in different ways, from special rewards for cooperatives and sanctions for cheaters to strict specificity in partner choice ( Douglas, 2008, 2010). These conflicts challenge the maintenance of mutualisms and selection may favor exploitation or the abandonment of such relationships. The balance of nature was challenged by the evolutionary theory based on natural selection elaborated by Darwin (1859), according to which “natural selection cannot possibly produce any modification in a species exclusively for the good of another species.” From a more recent evolutionary point of view, mutualistic relationships hide an apparent paradox since each species tends to maximize its own fitness when interacting with another and unrelated partners may have conflicts of interests ( Sachs, 2015). ![]() Mutualism, regarded as reciprocal cooperation between species, was therefore perfectly framed in this theory. According to this theory natural systems tend to remain in a stable equilibrium where natural forces prevent species from becoming too abundant or becoming extinct ( Egerton, 1973). ![]() Mutualisms have often been reported as evidence of the ancient classical theory of “balance of nature” that is rooted in Greek philosophy and mythology. Mutualism has a pivotal role in the functioning of all current ecosystems and in key events of the evolutionary history of life on our planet, such as the evolution of eukaryotic cells, colonization of land by plants and the radiation of angiosperms ( Bronstein et al., 2006 Douglas, 2010 Bronstein, 2015). Mutualistic inter-species relationships, i.e., relationships in which interacting species reciprocate benefits received, are very common in all kingdoms of living organisms since virtually every species is involved in one or more such relationships. This review highlights the recent background of the “manipulation” hypothesis, discussing it in the framework of new ecological and evolutionary scenarios in plant–animal interactions, as a stimulus for future research. Recent evidence motivated some authors to use the term “manipulation” of animals by plants in nectar-mediated mutualistic relationships. Such substances can be considered plant cues to exploit mutualism. Several substances other than sugars and amino acids have been found in nectar and some affect the foraging behavior of insects and potentially increase the benefits to the plant. It implies the evolution of strategies to maximize the benefits obtained by one partner while still providing the reward/service to the other partner. A more subtle way of exploiting mutualism was recently highlighted. Cheating (or deception or exploitation), namely obtaining the reward/service without returning any counterpart, is however, well-known in mutualistic relationships, since the interacting partners have conflicting interests and selection may favor cheating strategies. Plant–animal relationships involving the two types of nectar have therefore been used for a long time as text-book examples of symmetric mutualism: services provided by animals to plants in exchange for food provided by plants to animals. In both cases nectar is a source of carbon and nitrogen compounds that feed animals, the most abundant solutes being sugars and amino acids. Floral nectar is presented inside the flower close to the reproductive organs and rewards animals that perform pollination while visiting the flower. Extra-floral nectar is involved in so-called indirect defense by attracting animals (generally ants) that prey on herbivores, or by discouraging herbivores from feeding on the plant. It has been known for centuries that floral and extra-floral nectar secreted by plants attracts and rewards animals. ![]() 3Department of Agrifood Production and Environmental Sciences, University of Florence, Florence, Italy. ![]()
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