The Role of Extrafloral Nectar Amino Acids for the Preferences of Facultative and Obligate Ant Mutualists
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Plants in some 300 genera produce extrafloral nectar (EFN) to attract ants as a means of indirect defence. Among Mesoamerican Acacia species, obligate myrmecophytes produce EFN constitutively to nourish symbiotic ant mutualists, while non-myrmecophytes induce EFN secretion in response to herbivore damage to attract non-symbiotic ants. Since symbiotic Acacia ants entirely depend on the host-derived food rewards while non-symbiotic ants need to be attracted to EFN, this system allows comparative analyses of the function of EFN components in ant nutrition and attraction. We investigated sugar and amino acid (AA) composition in EFN of two myrmecophytes (Acacia cornigera and Acacia hindsii) and two related non-myrmecophyte species (Acacia farnesiana and Prosopis juliflora). AA composition allowed a grouping of myrmecophytes vs. non-myrmecophytes. Behavioural assays with obligate Acacia inhabitants (Pseudomyrmex ferrugineus) and non-symbiotic ants showed that AA composition affected ant preferences at high but not at low AA/sugar ratios. Most interestingly, behavioural responses differed between the two types of ants tested: Symbiotic ants showed a clear preference for higher AA concentrations and preferred nectar mimics with those four AAs that most significantly characterised the specific nectar of their Acacia host plant. In contrast, non-symbiotic ants distinguished among nectars containing different sugars and between solutions with and without AAs but neither among nectars with different AA/sugar ratios nor among mimics containing different numbers of AAs. Our results confirm that both AAs and sugars contribute to the taste and attractiveness of nectars and demonstrate that the responses of ants to specific nectar components depend on their life style. AAs are a chemical EFN component that likely can shape the structure of ant–plant mutualisms.
KeywordsAcacia Ant–plant interaction Pseudomyrmex Mutualism Nectar
We thank Juan Carlos Silva Bueno and Ralf Krüger for their help in the field. We are grateful to Elfriede Reisberg, Werner Kaiser and Eva Wirth at University of Würzburg for kindly helping with the amino acid analyses. Contributions by Constanza Quiroz helped to improve the manuscript. Financial support by the DFG (grant He 3169/4-2) and the CONACyT (Consejo Nacional de Ciencia y Tecnología) is gratefully acknowledged. MGT is supported by a PhD fellowship from the DAAD (German Academic Exchange Service).
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