How invasive species recognize novel competitors during range expansion remains poorly understood. We investigated this in the red imported fire ant (Solenopsis invicta, RIFA) during its northward spread in China. We assessed interspecific aggression toward three native ants using two geographic contexts: sampling native ants from different latitudes (testing Tetramorium bicarinatum and Polyrhachis dives) and comparing populations between species that occurred both within and outside their invaded range (testing Pheidole nodus and Polyrhachis dives). Results suggest aggression can be modulated by competitor identity, origin, and coexistence history. Aggression toward ground-dwelling T. bicarinatum decreased with increasing latitude, suggesting a "recognition lag" at the invasion front. Conversely, aggression toward arboreal Polyrhachis dives was higher in the non-invaded frontier than in the invaded core, indicating that the RIFA actively identifies novel threats but develops "neighborly tolerance" following coexistence. Furthermore, higher aggression toward ground-dwelling species (e.g., Pheidole nodus) confirmed that niche overlap may dictate competitive recognition. These findings highlight the RIFA's behavioral plasticity; the transition from proactive threat assessment to tolerance likely facilitates successful colonizing and gaining ecological dominance in newly invaded areas.
Strumigenys SMITH, 1860 is a diverse and cosmopolitan ant genus divided into several species groups. Among these, the Strumigenys gundlachiand Strumigenys louisianae-groups stand out for including some of the most widespread and commonly found Neotropical ants, many of which exhibit morphological variation suggestive of cryptic-species complexes. This study investigated the cytogenetics of six species from these groups. Chromosome numbers and karyotype formulae for the S. gundlachi-group were: 2n = 26 (20 m + 6sm) for Strumigenys crassicornis MAYR, 1887, 2n = 16 (8 m + 8sm) for Strumigenys aff. stenotes (BOLTON, 2000), 2n = 18 (16 m + 2sm) for Strumigenys denticulata MAYR, 1887, and 2n = 18 (18 m) for Strumigenys subedentata MAYR, 1887. For the S. louisianae-group, two populations of Strumigenys louisianae ROGER, 1863 exhibit cytogenetic variation (2n = 4, 4 m; 2n = 20, 18 m + 2sm) and distinct external morphologies from the type-locality populations. While all studied species revealed a single 18S rDNA-bearing chromosome pair, the localization of these clusters varied interspecifically. Repetitive sequences, including (TTAGG)n and (GA)n, are aligned with typical ant patterns. We combined a morphological study to our cytogenetic data to investigate the taxonomy of those populations currently within S. louisianae. Our results support the reinstatement of Strumigenys clasmospongia BROWN, 1953 stat.rev. (2n = 20) as a valid species and the description of a new species for the S. louisianae-group, Strumigenys mineira sp.n. (2n = 4). We also provide a taxonomic key for species of the S. louisianae-group. Our results underscore the effectiveness of integrative approaches in uncovering cryptic diversity within the genus.
Genetic and environmental cues have been suggested to affect cuticular hydrocarbon (CHC) profiles, in which the profile signature mediates aggressive behaviors in ants. Paradoxically, conflicting evidence exists, and none of these hypotheses has received decisive support. These cues have been studied independently without much attention to their potential mutual relationship. This study investigated the mutual relationships of cuticular hydrocarbons, genetics, and bacterial endosymbionts in mediating aggressive behavior in 14 colonies of the invasive African big-headed ant, Pheidole megacephala, in urban and forest areas of Taichung, Taiwan. Behavioral assays revealed that workers displayed aggression toward those from other colonies, indicating the absence of a supercolony in Taichung. However, urban ants did not necessarily exhibit aggression toward forest ants, indicating that environmental cues between these habitats play a minor role in mediating agonistic interactions. The aggression level corresponded to differences in dimethyl alkane. The dimethyl-alkane distances were significantly associated with genetic distances. Co-inertia analysis demonstrated a strong relationship between dimethyl alkanes and the bacterial endosymbionts of ants. In particular, colonies infected with Wolbachia exhibited a higher abundance of peaks corresponding to long-chain dimethyl alkanes ranging from carbon numbers C33 to C39. Based on these findings, we suggest that ant CHCs, shaped by heritable bacterial endosymbionts such as Wolbachia, lead to colony-level differences in dimethyl-alkane profiles and, thus, the observed agonistic interactions in P. megacephala regardless of habitat type.
Arboreal ants are ecologically dominant in tropical forests due to their primarily herbivorous diet and aggressive behavior. This aggression is known to be influenced by the availability of carbohydrate and protein resources. However, how nutrient availability interacts with habitat variation to shape ant behavior remains poorly understood. Here, we investigated the trophic positions of the Asian weaver ant, Oecophylla smaragdina (FABRICIUS, 1775), and tested how carbohydrate supplementation affects their behavior across different habitat types in tropical Xishuangbanna, China. We hypothesized that weaver ants exhibit different trophic positions across habitats, influencing their activity and aggression, and that carbohydrate supplementation enhances these behavioral traits. To test these hypotheses, we collected weaver ants from a natural habitat (rainforest) and two types of plantations (fruit garden and agroforestry with rubber trees) for nitrogen and carbon stable isotope analyses. Behavioral responses were assessed by offering sucrose solution and distilled water (control) in each habitat. This study found that: 1) Ants in the three habitats did not differ in trophic position, but their resource utilization varied; 2) ants in the fruit garden, where carbohydrates are likely less constrained, displayed lower foraging recruitments and reduced persistence at baits over time; and 3) short-term carbohydrate supplementation did not significantly alter ants` overall aggression across habitats but reduced activity levels, including foraging recruitments and persistence at baits. These findings challenge previous assumptions that sugar feeding consistently enhances the aggression and activity of ants. Our results offer new insights into how carbohydrate availability modulates the behavior of ecologically dominant arboreal ants across different habitats, with implications for their ecological roles and potential applications in biological control.
Ants are among the most abundant terrestrial animals and play key roles in ecosystems across the globe. Their taxonomic and ecological diversity makes them a prime taxon for contributing to our understanding of the patterns and processes of speciation. However, studies on ants, such as those on ant diversification or taxonomy, often do not explicitly consider how their results inform or update the broader framework of speciation and reproductive isolation. Yet, the wealth of taxonomic, biogeographical, behavioral, and genomic studies on ants could significantly contribute to our understanding of both micro- and macroevolutionary patterns of speciation. Here, we combine microevolutionary studies on reproductive isolation and hybridization with macroevolutionary work on ant diversification to review what is known about ant speciation. We also discuss how two key features, sociality and haplodiploidy, could impact speciation and hybridization in ants. We find that, although key innovations as drivers of ant diversification have been studied, gaps exist in our understanding of reproductive isolating mechanisms in ants. However, general population features or traits, such as co-evolution within mutualistic or parasitic relationships, suggest that reproductive isolating mechanisms studied in ants may be generalizable across the tree of life. We suggest that ants could serve as valuable systems to study open questions in speciation, especially hybridization and divergence with gene flow, the genomic basis of intrinsic postzygotic isolation, and species interactions as drivers of reproductive isolation between species.
