Brain composition and olfactory learning in honey bees

Abstract

Correlations between brain or brain component size and behavioral measures are frequently studied by comparing different animal species, which sometimes introduces variables that complicate interpretation in terms of brain function. Here, we have analyzed the brain composition of honey bees (Apis mellifera) that have been individually tested in an olfactory learning paradigm. We found that the total brain size correlated with the bees' learning performance. Among different brain components, only the mushroom body, a structure known to be involved in learning and memory, showed a positive correlation with learning performance. In contrast, visual neuropils were relatively smaller in bees that performed better in the olfactory learning task, suggesting modality-specific behavioral specialization of individual bees. This idea is also supported by inter-individual differences in brain composition. Some slight yet statistically significant differences in the brain composition of European and Africanized honey bees are reported. Larger bees had larger brains, and by comparing brains of different sizes, we report isometric correlations for all brain components except for a small structure, the central body.

Fig. 1

Photomicrographs of a bee brain. Three sections taken at different depth from the frontal brain surface: 100 μm (A), 300 μm (B) and 480 μm (C) showing the different brain components examined. The area boxed in (B) is enlarged in (D) outlining the different brain structures and highlighting the regions measured in the study. al, antennal lobe; ca, mushroom body calyx; cb, central body; lo, lobula; me, medulla; mbl, mushroom body lobe (includes vertical lobe, medial lobe and peduncle); otr, other brain neuropils (including the subesophageal ganglion (seg)); so, somata regions; the compound eye (ey), the ocelli and ocellar neuropil (oc) and the lamina (la) were not included in the analysis; scale bar applies to (A–C).

Fig. 2

Correlations between the bees’ head widths and their body weights (head and thorax weight) (A) and their total brain volumes, respectively (B); N = 121.

Fig. 3

Correlations between brain components (absolute volumes). (A) shows the slopes of the linear correlations for the different brain components (y-axis not given; it differs for each component). Note almost isometrical increase of most brain components with total brain size, except for the central body. Detailed graphs including data points are given for the medulla (B) and central body (C) (note almost 50-fold difference in y-axis). Data describing these correlations are given in Table 1. Broken lines indicate true isometric correlation (slope = 1). mb, mushroom body; somata, the volume occupied by cell bodies; other, neuropil other than optic and antennal lobes, mushroom and central body. N = 101.

Fig. 4

Correlations between pairs of brain components. The two mushroom body calyces (A), the two optic lobes (B) and the combination of mushroom body lobes and antennal lobes (D) show significant correlations when comparing their relative size. The combined volume of medulla and lobula (‘optic lobes’) correlates negatively with the relative volume of the antennal lobes (C).

Fig. 5

Comparison of head plus thorax weight (A), head width (B), learning performance (C) and total brain volume (D) of European bees (EHB; N = 64) and Africanized bees (AHB; N = 57). Diamonds indicate means and 95% confidence intervals; statistically significant differences (t-test) are indicated.

Fig. 6

Correlation between brain (A) or brain component size (B–D) and the learning performance of all bees excluding outliers (N = 101). Volumes are absolute (A and C) or relative to total brain volume (B and D). Linear fits (lines), r², p and α values (corrected for multiple comparisons) indicated for each brain component. Abscissa represents the percentage of correct (learned) proboscis extensions in response to odor presentations (0%, no learning; 100%, response to all odor presentations).