PLANT ANIMAL INTERACTIONS
Thiago J. Izzo · Heraldo L. Vasconcelos
Cheating the cheater: domatia loss minimizes the effects of ant castration in an Amazonian ant-plant
Received: 7 January 2002 / Accepted: 11 July 2002 / Published online: 20 August 2002 © Springer-Verlag 2002
Abstract We studied the relationship between Hirtellamyrmecophila (Chrysobalanaceae), a common but littlestudied Amazonian ant-plant that produces leaf-pouches as domatia, and its obligate ant partner, Allomerus octoarticulatus. Field observations revealed that H. myrmecophila drops domatia from older leaves, a characteristic that is unique among myrmecophytes. The physiological mechanism for abortion of domatia is currently unknown, but thischaracteristic allows for the existence, within the same plant, of branches with and without ants. Older branches generally bear only old leaves with no domatia and therefore have no ants, whereas younger branches have leaves of various ages. Ants forage mainly on new leaves, and experimental removal of ants showed that A. octoarticulatus is crucial for defense of these leaves against insect herbivores.However, A. octoarticulatus also acts as a castration parasite, severing the plant’s inflorescences. Mature flowers and fruits were only found on older branches with no ants, and flower production was 8 times greater on plants whose ants were experimentally removed than on control plants. Given the reproductive costs inflicted by its mutualistic partner, we suggest that abortion of domatia is astrategy developed by H. myrmecophila to minimize the effects of cheating by A. octoarticulatus. These results support the view that evolutionary conflicts of interest between mutualistic species often impose selection for cheating on the partner, as well as for mechanisms to retaliate or to prevent super-exploitation. Opposing selection pressures, operating independently on the two partners, probablyhelp to maintain the evolutionary stability of this mutualistic relationship.
T.J. Izzo · H.L. Vasconcelos (✉) Coordenação de Pesquisas em Ecologia, Instituto Nacional de Pesquisas da Amazônia (INPA), C.P. 478, 69011-970 Manaus, AM, Brazil H.L. Vasconcelos Present address: Instituto de Biologia, Universidade Federal de Uberlândia, C.P. 593, 38400-902 Uberlândia, MG, Brazil, e-mail:email@example.com, Tel.: +55-34-32182243, Fax: +55-34-32182243
Keywords Allomerus · Myrmecophytes · Mutualism · Hirtella · Herbivory
Myrmecophytes, also known as ant-plants, are plants that have evolved obligate, mutualistic relationships with ants (Janzen 1966; Beattie 1985; Benson 1985; Davidson and McKey 1993). To house ants, these plants have evolved special hollow structures, know asdomatia, in which ants nest (Janzen 1966; Beattie 1985; Benson 1985). Many ant-plants also provide food to their associated ants, in the form of nectar or food bodies (Janzen 1966; Baudoin 1975; Janzen 1975; O’Dowd 1980; Beattie 1985; Vasconcelos 1991). In exchange, ants often protect plants against herbivores (Janzen 1966; McKey 1984; Benson 1985; Vasconcelos 1991; Davidson and McKey 1993; Fonseca1994; Federle et al. 1998), against encroaching vines and competing plants (Janzen 1966; Benson 1985; Davidson and McKey 1993; Federle et al. 1998), or provide nutrients essential for plant growth (Janzen 1966; Treseder et al. 1995). However, not all associated ants are mutualistic. Some ant species act like parasites by utilizing domatia and food rewards without providing benefits (Janzen 1975;McKey 1984; Gaume and McKey 1999), while others prune the reproductive or vegetative structures of their host-plants, thus negatively affecting plant growth and reproduction (Yu and Pierce 1998; Stanton et al. 1999; Yu 2001). Among ant species protecting myrmecophytes from herbivores (Vasconcelos 1991; Davidson and McKey 1993; Fonseca 1994), many do not forage or nest off their hosts, and for these...