Résumé
Climate oscillations produce dramatic changes in species distribution, even in the tropics. The ant–plant
Leonardoxa africana africana hosts and feeds the ant
Petalomyrmex phylax, which provides protection against herbivores in return. Both partners of this symbiosis present a recent southward range expansion. To test whether the higher investment in sexuals (and thus lower investment in protective workers) previously documented on the colonization front is compensated by a more effective protective behaviour, we compared ant behavioural investment in plant defence between two populations, one in the core of the range and one on the colonization front. We induced ant patrolling activity by artificially damaging leaflets and measured this activity by counting patrolling ants and calculating the increase relative to constitutive patrolling activity measured on control (undamaged) leaflets. Contrary to our expectation, ant behavioural investment in plant defence was lower on the colonization front. Thus, production of fewer workers is not compensated by more protective behaviour of each. Instead, both traits contribute to a phenotype that is less mutualistic as a whole. By favouring increased allocation to dispersal, range expansion can shape ant behavioural traits and potentially the outcome of mutualism.
► A plant–ant species showed a more dispersive strategy at colonization front, including a lower relative investment in workers. ► We showed that ants invested less in plant defence behaviour at the colonization front. ► The global ant strategy is thus less mutualistic at colonization front. ► Range expansion can shape ant behavioural traits and potentially the outcome of mutualism.