Allometric analysis of brain cell number in Hymenoptera suggests ant brains diverge from general trends

Abstract

Many comparative neurobiological studies seek to connect sensory or behavioural attributes across taxa with differences in their brain composition. Recent studies in vertebrates suggest cell number and density may be better correlated with behavioural ability than brain mass or volume, but few estimates of such figures exist for insects. Here, we use the isotropic fractionator (IF) method to estimate total brain cell numbers for 32 species of Hymenoptera spanning seven subfamilies. We find estimates from using this method are comparable to traditional, whole-brain cell counts of two species and to published estimates from established stereological methods. We present allometric scaling relationships between body and brain mass, brain mass and nuclei number, and body mass and cell density and find that ants stand out from bees and wasps as having particularly small brains by measures of mass and cell number. We find that Hymenoptera follow the general trend of smaller animals having proportionally larger brains. Smaller Hymenoptera also feature higher brain cell densities than the larger ones, as is the case in most vertebrates, but in contrast with primates, in which neuron density remains rather constant across changes in brain mass. Overall, our findings establish the IF as a useful method for comparative studies of brain size evolution in insects.

Keywords: allometry; brain evolution; isotropic fractionator.

Figure 1.

Phylogeny (adapted from [35]) of hymenopteran genera included in phylogenetically controlled brain-to-body mass regression analysis (* =not included). Symbols (diamond, circle, star, etc.) associated with family names are used in subsequent figures. The size of grey dots logarithmically represents the brain and body mass of the respective genera's specimen. Arrows point at images of some of the examined species. Note that the species vary considerably in body size as indicated by the length of the respective scale bars, each representing 2 mm. The inset demonstrates the size range of the sampled species, representing the largest (Pepsis thisbe) and the smallest (Leptopilina hetero) species and, for comparison, a honeybee (Apis mellifera), all reproduced at the same scale (scale bar, 10 mm). The images of Neodiprion and Calliopsis have been modified with permission from originals provided by Andrey Ponomarev (©Andrey Ponomarev, http://insecta.pro) and, respectively, Heather Horn (©Heather Horn; www.pollinatorsnativeplants.com). (Online version in colour.)

Figure 2.

Brain size scaling across Hymenoptera. (a) Brain-body mass regression analysis for Hymenoptera from PGLS. (b) Sample-level values used to calculate mean values for (c). (c) Brain mass and nuclei number regression from PGLS. Family indicated by shape, superfamily by colour (light blue, Tenthredinoidea, dark blue, Formicoidea; green, Apoidea; red, Pompiloidea; pink, Vespoidea; orange, Ichneumonoidea). Separate analyses of Apoidea (green, subscript A) and Formicidae (blue, subscript F) indicated by dotted lines. Saturated symbols represent species included in PGLS. For specific species used in each analysis see the electronic supplementary material, table S1; for samples sizes used in each analysis, see the electronic supplementary material, table S4. (Online version in colour.)

Figure 3.

Brain cell density scaling with body size in Apoidea (a) and Formicoidea (c) and brain cell density differences among Apoidea (b) and Formicoidea (d) species. Apoidea family symbols in (a) and (b) from figure 1. Grey symbols in (a) and (c) represent individual data points. Pairwise comparisons from Tukey's HSD following least-squares regression for brain mass (b,d, ranked by body mass). For full species names, see the electronic supplementary material, table S1. For model statistics, see the electronic supplementary material, table S5. Species listed in grey in (a) not included in PGLS. Grey dots in (c) and (d) not included in statistical analyses because either estimate is from one specimen (c) or because BM was not measured (d). Letters denote statistically significant differences among groups (p < 0.05). (Online version in colour.)

Figure 4.

Comparisons of insect brain allometries with estimates from vertebrate studies. Brain-body mass (a,b) and brain mass–neuron number comparison (c) of Hymenoptera and vertebrates; logarithmical plots. Vertebrate data in (a) are from a compilation by [45], in (b) compiled from ([,,–51]; electronic supplementary material, table S7) and neuron data in (c) are from ([52]; mammals) and from ([53]; birds). In (a,b), we included a large sample of ant data (green crosses; 62 species from [19] and two species from [54]) to supplement our own data (magenta squares in a–c). Note that vertebrate data in (c) represent neurons only, whereas our hymenopteran data include glia (which are much less numerous in insects than in birds and mammals). Slope lines in (a,c) are linear regressions whereas slopes in (b) include more complex regression models (for details, see the electronic supplementary material, table S7). (Online version in colour.)