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

Logan CJ, Palmstrom CR - PeerJ (2015)

Bottom Line: 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 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: SAGE Center for the Study of the Mind, University of California Santa Barbara , Santa Barbara, CA , USA ; Department of Zoology, University of Cambridge , Cambridge , UK.

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.

No MeSH data available.


Bead method.Skull holes are plugged with cotton and then the cranium is filled with glass beads.
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fig-2: Bead method.Skull holes are plugged with cotton and then the cranium is filled with glass beads.

Mentions: Endocranial volume was measured by pouring 1 mm diameter glass beads (BioSpec Products, catalog number 11079110) into the cranium through the foramen magnum until full. The skull was repeatedly shaken to settle the beads and then filled again until the beads reached the posterior foramen magnum without falling out (Fig. 2). The volume was calculated by pouring the beads out of the skull and into a graduated cylinder (5 ml in 0.1 ml graduations, World Precision Instruments, Inc., Sarasota, Florida, USA, catalog number CG-0160; note that 1 ml = 1 cm3). In cross-species comparisons, there is mixed evidence about whether pouring the beads into a graduated cylinder introduces error when compared with pouring the beads onto a scale and converting their mass into volume (4% difference: Miller, 1997, 0% difference: Isler et al., 2008). The skulls in this study were measured with an average error of 2% (i.e., there was a 2% difference in volume between two sets of volume measurements carried out on a subset of 12 skulls), which is small in comparison to the variance between skulls (intra-class correlation coefficient = 0.94; Hutcheon, Chiolero & Hanley, 2010). Therefore, the error should not affect the power to detect a correlation with the more precise CT method (intra-class correlation coefficient = 0.98) of measuring skulls.


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

Logan CJ, Palmstrom CR - PeerJ (2015)

Bead method.Skull holes are plugged with cotton and then the cranium is filled with glass beads.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4465945&req=5

fig-2: Bead method.Skull holes are plugged with cotton and then the cranium is filled with glass beads.
Mentions: Endocranial volume was measured by pouring 1 mm diameter glass beads (BioSpec Products, catalog number 11079110) into the cranium through the foramen magnum until full. The skull was repeatedly shaken to settle the beads and then filled again until the beads reached the posterior foramen magnum without falling out (Fig. 2). The volume was calculated by pouring the beads out of the skull and into a graduated cylinder (5 ml in 0.1 ml graduations, World Precision Instruments, Inc., Sarasota, Florida, USA, catalog number CG-0160; note that 1 ml = 1 cm3). In cross-species comparisons, there is mixed evidence about whether pouring the beads into a graduated cylinder introduces error when compared with pouring the beads onto a scale and converting their mass into volume (4% difference: Miller, 1997, 0% difference: Isler et al., 2008). The skulls in this study were measured with an average error of 2% (i.e., there was a 2% difference in volume between two sets of volume measurements carried out on a subset of 12 skulls), which is small in comparison to the variance between skulls (intra-class correlation coefficient = 0.94; Hutcheon, Chiolero & Hanley, 2010). Therefore, the error should not affect the power to detect a correlation with the more precise CT method (intra-class correlation coefficient = 0.98) of measuring skulls.

Bottom Line: 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 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: SAGE Center for the Study of the Mind, University of California Santa Barbara , Santa Barbara, CA , USA ; Department of Zoology, University of Cambridge , Cambridge , UK.

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.

No MeSH data available.