An orb‐weaver spider eludes plant‐defending acacia ants by hiding in plain sight

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Guards and thieves: antagonistic interactions between two ant species coexisting on the same ant-plant

Abstract.  1. The simultaneous occupation of a rare understorey ant-acacia Acacia mayana by its guarding ant Pseudomyrmex ferrugineus , and an apparent opportunist parasite of the mutualism, the generalist ant Camponotus planatus is described. The two ant species occur together in 30.7% of the 26 mature A. mayana plants [23.5% of all trees ( n  = 34)] surveyed, but C. planatus is absent from saplings below 1 m in height ( n  = 8).
2. While P. ferrugineus shows behaviour compatible with effective host-tree defence, C. planatus does not attack phytophagous insects and appears ineffective as an ant-guard. Camponotus planatus does, however, occupy swollen thorns (pseudogalls) on the host tree, and harvests nectar from extrafloral leaf nectaries. It is proposed that C. planatus is a parasite of the Acacia–Pseudomyrmex mutualism.
3. Camponotus planatus does not harvest the second trophic reward produced by the tree for its Pseudomyrmex ant-guards, protein-rich food (Beltian) bodies. Camponotus planatus lack the specialised larval adaptations needed to use Beltian bodies as brood food, suggesting that this resource is potentially more resistant to exploitation by generalists than extrafloral nectar.
4. In competition for access to nectaries, C. planatus effectively displaced P. ferrugineus in 99.8% of encounters. These results suggest not only that C. planatus is a parasite of this mutualism, but also that it is able to effectively counteract the aggression shown to other insects by the resident ant-guards.     

Polymerase chain reaction primers for polymorphic microsatellite loci in the ant Pseudomyrmex gracilis (Formicidae: Pseudomyrmecinae)

The elongate twig ant Pseudomyrmex gracilis is widely distributed in the New World ranging from southern USA to Brazil and northern Argentina. To investigate its within-colony relatedness, we developed primers for nine polymorphic microsatellite loci. High levels of within-population variation were observed, with number of alleles ranging from two to 13, and heterozygosity from 0.28 to 0.90. The loci showed no deviations from Hardy-Weinberg equilibrium and no linkage disequilibrium.     

Respiration,worker body size,tempo and activity in whole colonies of ants

Ants are social, and their metabolism should be measured on at least two levels: (i) the individual workers and brood of which the colony is composed and (ii) the colony in its entirety. Whole colony respiration, tempo (size‐free running speed in body lengths per second) and whole colony activity were simultaneously measured for 15 species of ants in four subfamilies, and these data are related to average worker and whole‐colony weight, activity, percentage brood and percentage fat. Across all 15 species, whole colony respiration rate (μL CO₂ h^?1) is linearly related to whole colony live weight (log–log slope = 1.0). Colonies composed of large workers respire less than colonies composed of an equal live weight of small workers, and colonies with high tempos respire more than lower tempo colonies of equal weight. The tempos and respiration rates of smaller ants tend to be higher, and a higher tempo exacts a cost in higher respiration independent of the effect of small body size. Individual worker respiration (μL CO₂ h^?1) scales to worker live weight with an exponent of 0.76. Whole colony specific respiration rate (μL CO₂ g^?1 h^?1) is unrelated to colony live weight. The regressions of respiration rates against colony and worker dry weight, lean weight and metabolic weight have similar slopes to those of live weight but different intercepts. Respiration is not related to worker percentage fat, percentage brood or activity. Ant ecology, tempo, body size, polymorphism and colony size are discussed in relation to respiration.     

Isolation and characterization of microsatellite loci in the plant-ant Pseudomyrmex ferrugineus (Formicidae: Pseudomyrmecinae) and cross-testing for two congeneric species

To investigate the population structure of the obligate plant-ant Pseudomyrmex ferrugineus, we developed primers for 12 microsatellite loci. We tested the variability of the markers on 11 individuals from each of two populations (totalling 22 individuals) and found two to 12 alleles per locus and population. No deviations from Hardy-Weinberg equilibrium were detected. Observed and expected heterozygosities at each locus ranged from 0.00 to 0.50 and from 0.08 to 0.46, respectively. We also investigated suitability of these primers in two congeneric species.     

Plant‐Inhabiting Ant Utilizes Chemical Cues for Host Discrimination

The Neotropical ant Pseudomyrmex triplarinus is involved in an obligate and complex symbiotic association with Triplaris americana trees. The ants inhabit trunk and branch domatia and respond aggressively to foreign invaders. Their degree of host specificity and basis for recognition of host trees has not been studied. We determined that, in contrast to T. americana seedlings, heterospecific seedlings set around the host trees suffered continuous pruning. Ants also removed 80–100 percent of heterospecific leaves attached to the trunk in contrast to only 10–30 percent of conspecific leaves. True species specificity was demonstrated by the selective removal of leaves from Triplaris poeppigiana pinned to host trees. This selectivity was also observed in a matrix‐independent bioassay using leaf cuticular extracts on glass microfiber strips. Strips treated with leaf wax extracts from host trees and pinned to the trunk of host trees received only 42 percent of the number of ant visits recorded on solvent‐treated controls by the end of the experiment. Strips treated with extracts of a related species, T. poeppigiana, received 64 percent of the number of ant visits compared with solvent‐treated controls. These experiments also suggest that P. triplarinus recognizes surface chemicals of their host tree, independent of the texture or architecture of the carrier material; although these factors may still play some role in recognition. This is the first study that we are aware of to investigate the mechanism of host discrimination related to pruning behavior. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Look before leaping: foraging selectivity of capuchin monkeys on acacia trees in Costa Rica

Acacia trees in Costa Rica have an obligate mutualism with three species of Pseudomyrmex ants, which vigorously defend their host tree from insect and mammalian herbivores. Depending on the size and species of ant colony, individual acacia trees may be differentially protected. For animals able to discern between weakly and highly aggressive ant colonies, costs of ant stings from less active colonies might be offset by nutritional value acquired from feeding on acacia fruit or ant larvae in swollen thorns. We examined foraging selectivity of capuchin monkeys on acacia trees in Santa Rosa National Park, Costa Rica. We measured four characteristics of the acacia trees from which capuchins fed and of acacias immediately adjacent to those in which the monkeys fed: diameter at breast height (DBH), accessibility, species of closest tree and ant species present. We found that capuchins prefer to forage in acacias that are large and accessible. We also made two measurements of ant colony activity on each tree, one before and one after disturbing the ant colony. We found that the three species of mutualistic ants differ in baseline activity levels and that mutualistic ants are more active than non-mutualistic ant species found in acacia trees. We also found that capuchins foraged more frequently in trees colonized by non-mutualistic ants, but the explanatory value (r ²) of this model was low. Furthermore, monkeys did not discriminate between acacias on the basis of baseline ant activity or the ant colony’s response to disturbance. We conclude that these monkeys select acacia trees in which to forage based on characteristics of the trees rather than the ants. In addition, our study suggests that white-faced capuchins act as predators on the acacia ants but they probably benefit the dispersal and reproductive success of acacia trees. Capuchins may in fact function as an additional mutualistic partner for acacia trees via seed dispersal, but they must overcome the ants’ defense of the trees to do so.     

HOW TO PREVENT CHEATING: A DIGESTIVE SPECIALIZATION TIES MUTUALISTIC PLANT-ANTS TO THEIR ANT-PLANT PARTNERS

Mutualisms often involve reciprocal adaptations of both partners. Acacia ant-plants defended by symbiotic Pseudomyrmex ant mutualists secrete sucrose-free extrafloral nectar, which is unattractive to generalists. We aimed to investigate whether this extrafloral nectar can also exclude exploiters, that is nondefending ant species. Mutualist workers discriminated against sucrose whereas exploiters and generalists with no affinity toward Acacia myrmecophytes preferred sucrose, because mutualist workers lacked the sucrose-cleaving enzyme invertase, which is present in workers of the other two groups. Sucrose uptake induced invertase activity in workers of parasites and generalists, but not mutualists, and in larvae of all species: the mutualists loose invertase during their ontogeny. This reduced metabolic capacity ties the mutualists to their plant hosts, but it does not completely prevent the mutualism from exploitation. We therefore investigated whether the exploiters studied here are cheaters (i.e., have evolved from former mutualists) or parasites (exploiters with no mutualistic ancestor). A molecular phylogeny demonstrates that the exploiter species did not evolve from former mutualists, and no evidence for cheaters was found. We conclude that being specialized to their partner can prevent mutualists from becoming cheaters, whereas other mechanisms are required to stabilize a mutualism against the exploitation by parasites.     

Ants Induce Domatia in a Rain Forest Tree (Vochysia vismiaefolia)1

In Amazonian rain forest trees of Vochysia vismiaefolia (Vochysiaceae), ants were found to induce twig structures that resembled classical ant domatia. This phenomenon is novel for ant‐plants, which commonly develop domatia without the activity of ants. Eight species of ants were recorded inside the domatia of six individual trees, but domatia were most numerous and characteristic when induced and inhabited by an undescribed species of Pseudomyrmex on two trees. To investigate the mechanism of domatium growth, we drilled holes into young twigs: the expansion of the twig diameter surrounding the holes was significantly accelerated, comparable to domatia formation. Domatia induction is discussed as a putative step in the evolution of ant‐plants.     

Can floral repellents pre‐empt potential ant–plant conflicts?

Ants, by consuming floral nectar, are potential parasites of plant–pollinator mutualisms, the persistence of which depends on mechanisms preventing ants from visiting flowers. Here I report the existence of such a mechanism which, uniquely, appears general in its effects. I show that two acacia–ant mutualists are repelled by floral tissue chemicals from their own host-plants as well as those from 13 other plant genera, only one of which associates symbiotically with ants. Furthermore, 18 of 25 ant species, from several subfamilies representing degrees of ant–plant interaction, are repelled by acacia floral chemicals. Thus floral ant repellents are widespread among plants, repel most ant species, and can prevent ants from parasitizing plant–pollinator mutualisms.