415 A New Case of Ants Nesting within Branches of a Fig Tree: the Case of Ficus subpisocarpa in Taiwan by A. Bain1, 2, B. Chantarasuwan3, L.S. Chou1, M. Hossaert McKey2, B. Schatz2 & F. Kjellberg2* ABStrACt Ficus is one of many plant genera involved in interactions with ants. The interaction is however little documented. We show here that ants, belong- ing mainly to the genus Crematogaster, nest in hollow internodes of young branches of Ficus subpisocarpa, a monoecious fig species studied in taiwan. The ants feed on the mutualistic fig-pollinating wasps as well as on parasitic non-pollinating fig wasps. Nevertheless fig-wasps may not constitute a sufficient food source to ensure permanent presence of ants on the tree as the ants were observed to be frequently associated with hemipterans such as coccids and aphids. Fig wasps seem to constitute a reliable and sufficient food source on some dioecious Ficus species. On the contrary, in monoecious Ficus species, resident ants have always been observed to tend homopteran in addition to feeding on fig wasps. Frequent fruiting, prolonged fruit ripening period, ramiflory and rapid growth could constitute traits facilitating strong associa- tion based on fig-wasps' consumption of the monoecious F. subpisocarpa with ants. Despite these traits, ants were observed to tend hemipterans, and F. subpisocarpa does not seem to have evolved specialized morphological traits to facilitate the association. Key words: ant-plant interaction, community ecolog y, Asian biodiversity, myrmecophytism, ant foraging. 1 Institute of Ecolog y and Evolutionary Biolog y, College of Life Sciences, National taiwan University, 1, Sec. 4, roosevelt rd., taipei, 10617 taiwan. 2Centre d’Ecologie Fonctionnelle et Evolutive CEFE, UMr 5175 CNrS, 1919 route de Mende, 34293 Montpellier France 3Netherlands Centre for Biodiversity Naturalis (section NHN), Leiden University, P.O. Box 9514, 2300 rA Leiden, The Netherlands and Thailand Natural History Museum, National Science Museum, Pathum Thani, 12120, Thailand. * Correspondence: finn.kjellberg@cefe.cnrs.fr 416 Sociobiolog y Vol. 59, No. 2, 2012 INtrODUCtION A major challenge of ecolog y is the assessment of species' communities and the understanding of the determinants of their structure and organization (Fortuna & Bascompte 2006; Bascompte & Jordano 2007; Proffit et al. 2007; Bascompte 2009; Cavender-Bares et al. 2009; Ings et al. 2009; Vázquez et al. 2009). Various groups of insects have inspired the development of theories in community ecolog y. They have long been recognized as convenient models for testing hypotheses in this domain (Heil & McKey 2003). Among them, ants (Hymenoptera: Formicidae) can be central for such perspectives, as they often play a major role in interaction networks as predators (Lach et al. 2009), seed dispersers (Giladi 2006), pollinators (rico-Gray & Oliveira 2007), mutualistic defenders of their host-plants (Heil & McKey 2003; Heil 2007) and exploiters of various mutualisms (Schatz et al. 2006, 2008). More specifically, ants have been widely studied for their interactions with plants. Ant-plant protection mutualisms have been reported in 100 plant genera and 40 ant genera, mainly in tropical regions (Davidson & McKey 1993; Speight et al. 2008). The high diversity of this type of interaction even within genus (Webber et al. 2007) is explained by easy development of mutualistic interac- tions. Indeed, the strong potential protection against herbivores provided by ant colonies may substantially increase plant fitness (rosumek et al. 2009). Specialized ant-plant interactions have been used as ecological and evolutionary models to understand selective factors affecting plants (Heil & McKey 2003). Some of these specialized interactions have been shown to be structured by chemical mediation (ranganathan & Borges 2009; Schatz et al. 2010) and support complex networks of plurispecific interactions involving herbivores, parasites, predators and secondary mutualists (Palmer et al. 2008; Schatz et al., 2008, 2010; Blatrix et al. 2009; Goheen & Palmer 2010). Among the numerous genera of plants associated with ants, the interaction between Ficus and ants has been poorly studied. Ficus is one of the 100 plant genera presenting what has been interpreted as structures specifically evolved to host ant nests (Speight et al. 2008), with only a single case hitherto reported for this genus (Maschwitz et al. 1996). Fig species are known for their obligate species-specific nursery pollination mutualisms with fig wasps ( Janzen 1979; Kjellberg et al. 2005). Specificity is reinforced by a physical filter (matching 417 Bain, A. et al. — A New Case of Ants Nesting in Fig trees between pollinator head shape and ostiole structure; van Noort & Compton 1996) and chemical mediation (emission of volatile compounds responsible for pollinating wasp attraction by receptive figs; Grison et al. 1999; Proffit et al. 2007, 2008, Hossaert-McKey et al. 2010). Ficus are increasingly investi- gated for their role in supporting a complex network of interactions involving a community of species-specific parasitic chalcid wasps (regionally up to 25 parasitic species for a single Ficus species, Bouček et al. 1981), but also several species of ants, all of them capable of detecting and using chemical signals emitted by figs (Chen et al. 1999; Kjellberg et al. 2005; Proffit et al. 2007; ranganathan et al. 2010; Schatz et al. 2008, 2010). Within that network, ants have been shown to severely reduce the prevalence of parasites and to be a structuring factor for various other insect species (other predators, herbivores, visitors) (Schatz & Hossaert-McKey 2003; Schatz et al. 2006, 2008, Wei et al. 2005). Ants have often been reported to be predators of fig wasps. They have also been observed to participate in fig-seed dispersion (Kaufmann et al. 1991; roberts & Heithaus 1986; Laman 1995, 1996) and they are involved in ant-homopteran-fig tree tritrophic interactions (Compton & robertson 1988, 1991; Dejean et al. 1997, ranganathan et al. 2010). Fig wasps may constitute a major source of food for ants. Indeed, Schatz et al. (2008) demonstrated on dioecious Ficus species that fig wasps on male trees provided a sufficiently abundant and reliable resource to allow continuous presence of dominant ants on the trees. However, whether fig wasps may constitute a sufficient resource in monoecious Ficus species is still an open question. Indeed, these species produce figs less frequently than dioecious ones and often present more synchronized crops (e.g. Shanahan et al. 2001). Hemipterans could constitute an alternative resource that could allow the continuous presence of tending ants on monoecious fig trees (Schatz et al. 2008). However, the presence of hemipterans and tending ants on monoe- cious fig species has been mainly reported from Africa and Madagascar (F. sur in South Africa [Compton & robertson 1988, 1991; Zachariades 1994]; F. vallis-choudae in Cameroon [Dejean et al. 1997], and several fig species in Madagascar, Malawi, South Africa, Zambia, and Zimbabwe [Cushman et al. 1998]) while the most convincing data on fig wasps providing sufficient food for ants in dioecious figs stems from Borneo (Schatz et al. 2008). Hence, to confirm that continuous ant presence on monoecious fig trees is systematically 418 Sociobiolog y Vol. 59, No. 2, 2012 associated with the presence of hemipterans, we need to record whether in other parts of the world, ants on monoecious Ficus are systematically asso- ciated with hemipterans. For ants, a fig tree may constitute an appropriate support presenting disper- sed feeding sites, located exclusively in the arboreal stratum. Optimization of fig wasp capture efficiency could lead ants to inhabit this stratum in order to reduce distance between their nest and this food source in accordance with central place foraging theory (Stephens & Krebs 1986; Bell 1991; Schatz et al. 2008). Diverse reports of arboreal ant nests within fig trees support this hypothesis, and suggest that ants effectively find sufficient food sources wi- thin fig trees. Examples include nest location among the leaves of F. fistulosa for Oecophylla smaragdina and of F. sur for O. longinoda (Thomas 1988; Schatz et al. 2008), between grouped figs of the cauliflorous F. botryoides for unidentified Formicidae (Dalecky et al. 2003) and cauliflorous figs of F. fistulosa for Crematogaster sp. (Schatz et al. 2008), within figs of F. sycomo- rus by Cardiocondyla wroughtoni (Lupo & Galil 1985), in large persistent stipules (up to 80 x 20 mm) of F. paracamptophylla often tenanted by ants (Corner 1976), in ant gardens on F. paraensis and F. trigona for Camponotus femoratus and Azteca cf. traili (Davidson 1988; Benzing 1991) and within the dead parts of branches on F. carica by Crematogaster scutellaris (Schatz & Hossaert-McKey 2003). Moreover, one species of Ficus presents what appear to be structures specifically evolved to host ant nests (Maschwitz et al. 1996). This study showed that 64% of the opened structures they called domatia of Ficus obscura var. borneensis (included in Ficus pisifera sensu Berg & Corner 2005) were inhabited by ants, belonging to eight species distributed into five genera (Camponotus, Cardiocondyla, Cataulacus, Crematogaster and Te- tramorium). Despite lack of description of ant behavior on this species, the presence of extrafloral nectaries and structures sheltering opportunistic ants on F. obscura var. borneensis suggests existence of a non-specific protection mutualism. Interestingly, in their field survey, Maschwitz et al. failed to detect similar structures in Ficus species belonging to over 20 other Malaysian fig species and they also failed to detect such structures in a survey of herbarium samples from 37 Australasian fig species. In this study conducted in the island of taiwan, we focused on the monoe- cious Ficus subpisocarpa. We describe here the presence of hollow structures 419 Bain, A. et al. — A New Case of Ants Nesting in Fig trees inhabited by ants in a fig tree associated with an plant-ant interaction. We determined the frequency of the hollow structures and the ant occupancy and we identified the observed ants to genus and describe traits of their presence. We also correlate ant presence and presence of hemipterans. We then discuss the insights into ant-fig interactions brought about by this discovery of a new case of ants inhabiting hollow structures in fig trees. MAtErIALS & MEtHODS Ficus subpisocarpa Gagnep. is a monoecious fig tree belonging to subsection Urostigma (sensu Berg & Corner 2005). It is a hemiepyphitic or terrestrial tree which grows up to 7m in height distributed in South Asia from the South of Japan to taiwan and to the Malay Peninsula and the Maluku Islands (Berg and Corner 2005). This species produces abundant crops, 2-4 times per year, of small cauliflorous figs, ripening over a 1-3 week period (Corlett 2006; Bain pers. obs. for taiwan). As most species of subsection Urostigma, F. subpisocarpa is a deciduous tree presenting rhythmic growth (Berg & Corner 2005). Hollowed stems hosting ants were incidentally noted on individual trees growing in taiwan when inspecting trees for ant presence. to better describe and quantify this trait we examined branches from a series of individual trees. Branches were collected on eight trees: six in taipei (National taiwan Uni- versity campus and Fuzhoushan Park), one 25 km north, at Bitou Cape on the North Coast of taiwan and one 350 km south, in Kenting in the extreme south of the island. Five to 16 apical branches were sampled for each tree at their junction with a main trunk. Indeed such branches usually grow directly from the trunk or from main branches and usually wither without reaching more 60 cm (Bain pers. obs.). For each branch we measured length, diameter of the branch at the cut- ting level, diameter at the middle of the apical internode and we counted the number of ramifications. Then the branches were split lengthwise, and the length of each hollow section (hereafter called cavity) was measured. Branch diameter was measured at the middle of the cavity as well as the distance from the branch apex. When ants were observed in the cavity, the inner diameter of the cavity was measured and the number of exit holes noted. The cavities were characterized as described in Fig. 1 into six categories: young cavities empty or attacked 420 Sociobiolog y Vol. 59, No. 2, 2012 by insect larvae, inhabited and previously inhabited cavities, mature cavities, and old cavities. The sequence of the cavities from the apex was noted and for trees 02, 03, 04 and 06, the precise internodes, numbered from the apex, where the structures were observed were also noted. The insect content of inhabited cavities was collected, identified and counted. Ants were identified to genus using Bolton (1994) and Lin & Wu (2003). Fig. 1. The different types of cavities. A) Young cavity: these cavities are characterized by their young tissues. Only the central part is hollowed, the inner pith has not dried out yet. B) Young attacked cavity: the structure of the cavity is the same as for young cavities but insect larvae are feeding inside. C) Inhabited cavity: ants are observed inside. All pith tissue has been removed. D) Entry holes of an inhabited cavity. E) Formerly inhabited cavity: all the pith has been removed but no ant live inside. Often fungi or mould have grown on the inner surface. F) Mature cavity: the inner pith has dried out all along the internode. G) Old cavity: secondary growth is beginning to compress the cavity 421 Bain, A. et al. — A New Case of Ants Nesting in Fig trees An additional set of trees were examined for the presence of ants and of hemipterans to determine whether there was a frequent association. One hun- dred ten branches distributed on 11 trees located in taipei were examined. Additionally herbarium samples originating from Japan, Vietnam and Thailand were examined for the presence of similar hollow structures. They included samples of Ficus subpisocarpa subpisocarpa, F. subpisocarpa pubi- poda C.C. Berg and the closely related F. geniculata. The herbarium samples examined were: Ficus subpisocarpa Gagnep., JAPAN: Kagoshima Pref., Yaku Is., 20 May 1984, t. Yahara, t. Nagamasu & t. Kawahara 10340 (L); ryukyu Islands, Misato-mura, Matsumo to-aza, 7 August 1951, E.H. Walker, S. tawada, t. Amano 6441 (L); VIEtNAM: Sai Wong Mo Shan (Sai Vong Mo Leng ), Lomg Ngong Village, Dam-ha, tonkin, July 18 – Sept 9, 1940, W.t. tsang 30348 (L). Ficus subpisocarpa Gagnep. subsp. pubipoda C.C. Berg. tHAILAND: ranong, Kaper, Laem Sohn National Park, sea level, 30 November 1996, J.F. Maxwell 96 – 1568 (L); Chiang Mai, Muang, Doi Sutep – Pui National Park, elevation 1350m, 26 March 2002, J.F. Maxwell 02 – 103 (L); Chaiyaphum, between Nam Phrom and Thungkamang, alt. 800m, 13 Dec 1971, C.F. van Beusekom, r. Geesink, C. Phengkhlai & B. Wongwan 4218 (L). Ficus geniculata Kurz. VIEtNAM: Cana province de Phanrang, 25 Oct 1925, M. Poilane 12491 (P); Ka rom pr. Phanrang, 7 Mar 1924, M. Poilane 9964 (P); tHAILAND: Lampang, Jae Home, Doi Jae Sawn National Park, alt. 425 m, 7 Jan 1992, J.F. Maxwell 92 – 16 (E); tak, Mueang, rd. No. 105, km 19 – 20, alt. 400 m, 21 Nov 2005, r. Pooma, C.C. Berg & M. Poopath 5734 (L). rESULtS Occurrence of cavities Sampled branch length ranged from 21 cm to 156 cm. ten percent of the studied branches presented no cavity, 36% presented a single cavity and 54% presented several cavities (table 1). The average length and diameter (at cutting level) of branches did not differ between branches with and without cavity (Mann-Whitney U test, NS). 422 Sociobiolog y Vol. 59, No. 2, 2012 On the 86 analyzed branches, a total of 204 cavities were observed (2.4 ± 2.0 cavities per branch or 4.2 ± 3.1 cavities per meter). The number of cavi- ties per branch varied significantly among trees from 0.90 to 5.0 (One-Way ANOVA F 6,79 = 11.85; p<0.001) (table 1). The different types of cavities We identified six kinds of cavities in F. subpisocarpa branches (Fig. 1): young cavities, intact or attacked by insect larvae, cavities inhabited by ants and cavities formerly inhabited by ants, mature cavities, old cavities. The most frequent hollow structures were mature cavities (45%), followed by young cavities (30%, 17% intact and 13% attacked by insect larvae) and inhabited cavities (15%). Young and mature cavities were observed on all trees. Cavities inhabited by ants were observed in six of the trees (75%), and a formerly in- habited cavity was observed on an additional tree. two-third of the inhabited cavities sheltered ant pupae or larvae. Out of the 32 inhabited cavities, 7 were located on branches that presented a young cavity attacked by insect larvae (22% of the inhabited cavities): protection, if any, is not perfect. Length of cavities The length of most cavities (75) ranged from 2 to 14cm (Fig. 2). Four cavi- ties measured less than 2cm and seven were longer than 22cm. Young cavities table 1. Cavity data per tree. Cavity types are defined according to Fig. 1. The two undetermined cavities correspond to cavities that were damaged during sampling. All trees located in taipei, except for tree 7 located at Bitou Cape and tree 8 located at Kenting. tree 1 2 3 4 5 6 7 8 Proportion Number of branches 13 10 10 16 12 10 10 5 Young healthy cavity 11 6 4 3 4 3 3 1 0.172 Young attacked cavity 3 6 5 0 6 4 1 1 0.127 Inhabited cavity 3 4 0 0 16 7 1 1 0.152 Formerly inhabited cavity 0 0 0 1 9 1 1 0 0.059 Mature unused cavity 7 17 3 22 24 15 2 2 0.451 Old cavity 0 3 0 0 3 0 0 0 0.029 Undetermined 0 0 0 0 0 0 1 1 0.010 total 24 36 12 26 61 30 9 6 423 Bain, A. et al. — A New Case of Ants Nesting in Fig trees were most frequent (46%) in the short length classes (L<4cm) and were overall significantly shorter than the other ones (One-Way ANOVA: F 1,233 = 12.72; p<0.001; average: 7.1±4.3cm). Mature cavities were most frequent in the seven following length classes (4cm