Can endocranial volume be estimated accurately from external skull measurements in great-tailed grackles (Quiscalus mexicanus)?

Abstract

There is an increasing need to validate and collect data approximating brain size on individuals in the field to understand what evolutionary factors drive brain size variation within and across species. We investigated whether we could accurately estimate endocranial volume (a proxy for brain size), as measured by computerized tomography (CT) scans, using external skull measurements and/or by filling skulls with beads and pouring them out into a graduated cylinder for male and female great-tailed grackles. We found that while females had higher correlations than males, estimations of endocranial volume from external skull measurements or beads did not tightly correlate with CT volumes. We found no accuracy in the ability of external skull measures to predict CT volumes because the prediction intervals for most data points overlapped extensively. We conclude that we are unable to detect individual differences in endocranial volume using external skull measurements. These results emphasize the importance of validating and explicitly quantifying the predictive accuracy of brain size proxies for each species and each sex.

Keywords: Bead method; Bird; Endocranial volume; Great-tailed grackle; Quiscalus mexicanus; Skull measurements.

Figure 1. Skull length, height, and width.

Measuring skull length (A), skull height (B), replicating the height that can be measured on live birds, and skull width (C) at the widest part of the braincase.

Figure 2. Bead method.

Skull holes are plugged with cotton and then the cranium is filled with glass beads.

Figure 3. Plots of the volumes of spheres and volumes calculated from CT scans for females and males.

Correlations between CT volumes and the volume of a sphere as calculated from linear measurements for female (A) and male (B) adults (small circles) and immatures (large circles), with the year the skull was collected represented by a red-blue spectrum (earlier years are redder and recent years are bluer). Note that regression lines only reflect the relationship between spherical volume and CT volume and do not correct for age or year (factors in the best-fitting model for both sexes) as in the GLMs. Skulls were aged as described in the methods.

Figure 4. Plots of the bead volumes and CT volumes for females and males.

Correlations between CT volumes and bead volumes for female (A) and male (B) adults (small circles) and immatures (large circles), with the year the skull was collected represented by a red-blue spectrum (earlier years are redder and recent years are bluer). Note that regression lines only reflect the relationship between bead volume and CT volume and do not correct for age (in the best-fitting male model) or year (in the best-fitting female model) as in the GLMs. Skulls were aged as described in the methods.