Although BMR appears to be highly, positively correlated with encephalization across placental mammals (Martin 1981
; Isler & Van Schaik 2006
), here BMR is not correlated with encephalization within Carnivora. Isler & Van Schaik (2006)
also failed to find a significant correlation within Carnivora. This is not surprising, as Carnivora is exceptionally varied ecologically, and BMR has been shown to scale differently across many ecological variables (e.g. diet, habitat, substrate) (McNab 2008
). The lack of consistent correlations within constituent mammalian subclades, e.g. Carnivora, argues against a direct link between relative BMR and encephalization. Rather, the apparent relationship appears to connect disparate clades, within which no consistent relationship exists.
That encephalization does not correlate with BMR or reproductive life history would appear to argue against the MEH. However, Martin (1996)
noted that increasing total energy investment via prolonging the gestational period may represent a different means to increased encephalization. Yet, no systematic patterns relating encephalization to life history across Carnivora exist. Increased encephalization above the basal allometry (herpestoids and viverrids among living taxa: Finarelli & Flynn 2009
) are not accompanied by predictable shifts in life history. For example, both canids and musteloids have significantly higher encephalization residuals than herpestoids/viverrids (Mann–Whitney tests: p
< 0.001 and p
= 0.002, respectively), yet significantly shorter gestation times than expected for a herpestoid/viverrid of similar mass (p
= 0.005 and p
= 0.003, respectively). It is therefore unlikely that the MEH sufficiently explains observed changes in carnivoran encephalization allometries, and the causal mechanism behind these changes in allometry remains elusive.
In general, there is little structure between encephalization and the timing of maternal offspring investment across Carnivora. Three significantly different correlations were reconstructed for gestation time. Two of these are significant, but in opposite senses: positive for Musteloidea, negative for Nandinia–Canidae–Felidae–Ursidae. Similarly, with the two correlations for litters per year, the correlation for Canidae + Feliformia was significantly negative, but the arctoid correlation was not significant. Weaning time was not significantly correlated with encephalization in any manner. Thus, altering the tempo of life history does not appear to be reflected in changes in adult encephalization.
However, these results are not necessarily fatal to a concept of maternal investment informing adult encephalization. Several significant correlations exist in the mean-centred residuals (), demonstrating a relationship between reproductive strategy and encephalization, despite the fact that these have subsequently been obscured by among-clade adaptations. Mean-centred residuals for neonatal mass, weaning mass and litter size are significantly correlated with encephalization. The weaning mass correlation appears to reflect the fact that relatively large neonates also tend to be relatively large at weaning: the partial correlation of encephalization with neonatal mass, holding weaning mass constant, is significant (partial r = 0.349, p = 0.011), whereas the partial correlation for weaning mass, controlling for neonatal mass, is not (partial r = −0.014, p = 0.923).
From this, two important correlations emerge: encephalization is correlated positively with neonatal mass and negatively with litter size ( and ). These correlations point to a conserved carnivoran reproductive strategy, taxa with higher encephalization tend to have fewer, larger offspring, a strategy which remains intact even after multiple independent changes in encephalization allometries (Finarelli & Flynn 2009
). A trade-off between litter size and neonatal mass after correcting for body mass has been noted in analyses of mammalian life histories, and is sometimes referred to as ‘offspring quality’ (Smith & Fretwell 1974
; Bielby et al. 2007
). That encephalization correlates significantly with offspring quality demonstrates the importance of maternal energetic investment in the offspring in shaping adult encephalization, even if it is not directly expressed in correlations observed across Carnivora as a whole.
Figure 1. Bi-plots of mean-centred variable residuals: encephalization (x-axis) is plotted versus (a) litter size and (b) neonate mass. The negative correlation with litter size and positive correlation with neonatal mass point to a conserved reproductive strategy (more ...)
Hierarchical (taxon-level) effects are evident in the relationship between encephalization and both neonatal mass and litter size (Pagel & Harvey 1988b
; Jablonski 2007
; Finarelli 2008a
). Neither neonatal mass nor litter size residuals correlate significantly with encephalization residuals prior to mean-centring. Individual carnivoran clades arrive at different specific relationships between these variables, and these among-clade adaptations shift clades within the residual space, obscuring relationships when analysed across Carnivora. However, conserved within-clade relationships are evident, revealed in the mean-centred residuals. Both musteloids and canids have higher encephalization residuals than herpestoids/viverrids, but both clades are characterized by higher litter size residuals (p
= 0.001 and p
< 0.001, respectively). This among-clade offset makes it appear as if no correlation exists across Carnivora, despite a consistent, negative within-clade correlation (figure S2 in the electronic supplementary material). Similarly, musteloids have significantly lower neonatal mass residuals (p
= 0.001), obscuring the positive correlation within-clade correlation with encephalization and neonatal mass.