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.
Highlights
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 evaluated the bead method of generating endocranial volume, where glass beads are poured into the skull and out into a graduated cylinder, to increase the value of our research by determining whether this widely used method accurately predicts actual endocranial volume as estimated by computerized tomography (CT) scans in this species
We can rule out that males and females were measured with different levels of accuracy, which might have caused the poor correlations between bead volumes/linear measures and CT volumes for males in the analyses below
Summary
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. Intraspecies brain size comparisons are rare perhaps due to the difficulty of obtaining data on a number of factors (e.g., biometric measurements, reproductive success, dominance rank, position in the social network, cognitive abilities, etc.) for the same individuals Acquiring such data is key for understanding what contributes to the evolution of brain size among individuals, as well as across species (Gonda et al 2013, Logan & Clutton-Brock 2013, Thornton & Lukas 2012). We investigated whether endocranial volume, a proxy for brain size (Iwaniuk & Nelson 2002), can be approximated using measurements of the external skull in great-tailed grackles (Quiscalus mexicanus, JF Gmelin, 1788) Finding such a proxy would greatly ease the collection of data on brain sizes since head measurements can be taken on live birds, allowing for correlations with any number of other factors on which data are gathered on this species in the field and the lab. We evaluated the bead method of generating endocranial volume, where glass beads are poured into the skull and out into a graduated cylinder, to increase the value of our research by determining whether this widely used method accurately predicts actual endocranial volume as estimated by CT scans in this species
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