Feeding response of the Aldabra giant tortoise (Geochelone gigantea) to island plants showing heterophylly


*Jens M. Olesen, Department of Ecology & Genetics, Aarhus University, Ny Munkegade Block 540, DK-8000 Aarhus C, Denmark. E-mail: jens.olesen@biology.au.dk


Aim  Heterophylly is present in many plant species on oceanic islands. Almost all of these plants are island endemics, and heterophylly may have evolved as a response to feeding from large insular browsers such as giant tortoises and flightless birds. We tested this anti-browser hypothesis by feeding Aldabra giant tortoises (Geochelone gigantea) with leaves of native Mauritian plants to see if they distinguished between juvenile and adult leaves and between heteophyllous and homophyllous species.

Location  Mauritius.

Methods  In a choice experiment we recorded feeding response of four captive Aldabra giant tortoises to 10 species of Mauritian plants, of which seven were heterophyllous and three homophyllous.

Results  In general, juvenile leaves of heterophyllous species showed convergence in shape and midrib coloration. Homophyllous foliage was preferred to heterophyllous, and among heterophyllous species adult foliage was preferred to juvenile.

Main conclusions  Several Mascarene heterophyllous plants show convergence in morphology of juvenile leaves and these are avoided by giant tortoises. This indicates a strong selection history from large browsers such as the giant tortoises. The Mascarene example is in accordance with several other comparable cases of plant-large browser interactions from other archipelagos.