Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

Abstract

A well-developed spatial memory is important for many animals, but appears especially important for scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent paradigm in which to study the integrative aspects of memory use within an ecological and evolutionary framework. One of the main tenets of this paradigm is that selection for enhanced spatial memory for cache locations should specialize the brain areas involved in memory. One such brain area is the hippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typically relating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is related to its function for spatial memory. Thus, the goal of this article is to evaluate volume as a main measurement of the degree of morphological and physiological adaptation of the Hp as it relates to memory. We will briefly review the evidence for the specialization of memory in food-hoarding animals and discuss the philosophy behind volume as the main currency. We will then examine the problems associated with this approach, attempting to understand the advantages and limitations of using volume and discuss alternatives that might yield more specific hypotheses. Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory in scatter-hoarding animals. However, volume may be only a coarse proxy for more relevant and subtle changes in the structure of the brain underlying changes in behaviour. To better understand the nature of this brain/memory relationship, we suggest focusing on more specific and relevant features of the Hp, such as the number or size of neurons, variation in connectivity depending on dendritic and axonal arborization and the number of synapses. These should generate more specific hypotheses derived from a solid theoretical background and should provide a better understanding of both neural mechanisms of memory and their evolution.

Figure 1.

Hippocampal and telencephalic volumes of black-capped chickadees and willow tits. A, Roth & Pravosudov 2009; B, Smulders et al. 1995; C, Hampton et al. 1995; D, Pravosudov & Clayton 2002; E, Healy & Krebs 1992; F, Brodin & Lundborg 2003; G, Cristol 1996. Filled circles, WITI; open circles, BCCH.

Figure 2.

Least-squared means of Hp volumes of European and North American corvids. European data from Lucas et al. (2004). North American data from Basil et al. (1996; filled circles) and Pravosudov & de Kort (2006; open circle). Error bars represent 1 s.d.