RESEARCH PAPER Ant–Acacia interaction: Chemical or physical defense? Shereen M. ELBANNA Zoology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt Correspondence Shereen M. Elbanna, Zoology Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt. Email: shelbana@gmail.com Received 24 December 2010; accepted 12 April 2011. doi: 10.1111/j.1748-5967.2011.00330.x Abstract Two different ant–Acacia ecosystems at two different sites were investigated for comparing their lifestyles. One ecosystem is at St. Katherine’s Protectorate and the other at Ismailia Province, Egypt. The defense mechanisms that each Acacia tree use against browsers were investigated. Seasonal and daily abundances of ants and other herbivores on two Acacia trees were studied. The study indicated different defense mechanisms used by these two Acacia species: Acacia use both physical and chemical defense mechanisms. The efficiency of both mechanisms in defend- ing Acacia against herbivores and why Acacia trees preferred one mechanism to the other are discussed. Key words: Acacia–ant interaction, alkaloids, chemical defense, insect–plant interaction. Introduction Herbivores can dramatically impact the growth and repro- duction of plants. In response, plants demonstrate an array of direct and indirect defense strategies that help protect them from herbivores (Berenbaum & Zangerl 1992). Direct defense strategies include physical structures such as thorns and barbs as well as the production of chemicals that can make foliage poisonous or otherwise unpleasant for herbivorous predators (Denno et al. 2000). Indirect defense strategies can involve a symbiotic relationship with a species that can protect against potential predators or can involve the release of odors that attract species that are natural enemies of the plant’s herbivorous attacker (Abrams et al. 1996). The latter defense strategy is used by some plants such as Gomphocarpus sinaicus, which attract ladybirds, a type of beetle, that defend the plant against aphids (Elbanna 2007). Acacia trees are an excellent model to study plant defense strategies as they use a variety of such strategies. For example, most Acacia species have long thorns along their branches, protecting against browsing animals (Cossalter 1986). An extreme example of this defense strategy is found on the Bull’s Horn acacia. The Bull’s Horn acacia gets its name from its enlarged stipules that form large, sharp, two- pronged barbs that resemble bull’s horns. Thorns are an effective deterrent against large grazing animals but do little to protect the tree against herbivorous insects. As further protection against herbivores, some Acacia have developed a symbiotic relationship with specific ant species. The obligate mutualism between Acacia and the ant species Pseudomyrmex spinicola in Central America is one of the best-documented cases of symbiosis (Janzen 1983). This group of symbiotic Acacia, known collectively as “Swollen Thorn” Acacia, provides a nesting site and food source for the ants. In return, the ants patrol the tree and act as guards against herbivorous mammals and insects. Ants carve out nests within the swollen thorns along the branches (Young 1990). Nectar is secreted from the base of the leaves year-round, providing a steady food source for the ants. Additionally, small orange beads that are rich in fat are found at the top of leaflets. These beads are collected by the ants, cut up and fed to their larvae. Thus, ants are provided with a rich food source without destroying the leaves or branches of the tree. In return, these ant inhabitants provide a protective army against herbivorous attack. Ants will “gang-up” on any her- bivore that threatens the plant and bite them. Insects quickly retreat, and even giraffe and rhino have been observed to become irritated and move away from these aggressive ants (Janzen 1967). Additionally, ant species release an alarm pheromone when they are disturbed, causing other ants to Entomological Research 41 (2011) 135–141 © 2011 The Author Entomological Research © 2011 The Entomological Society of Korea and Blackwell Publishing Asia Pty Ltd