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. 2013 Aug 21;280(1769):20131384.
doi: 10.1098/rspb.2013.1384. Print 2013 Oct 22.

Anthropogenic environments exert variable selection on cranial capacity in mammals

Emilie C Snell-Rood  1 , Naomi Wick
Affiliations

Anthropogenic environments exert variable selection on cranial capacity in mammals

Emilie C Snell-Rood et al. Proc Biol Sci. .

Abstract

It is thought that behaviourally flexible species will be able to cope with novel and rapidly changing environments associated with human activity. However, it is unclear whether such environments are selecting for increases in behavioural plasticity, and whether some species show more pronounced evolutionary changes in plasticity. To test whether anthropogenic environments are selecting for increased behavioural plasticity within species, we measured variation in relative cranial capacity over time and space in 10 species of mammals. We predicted that urban populations would show greater cranial capacity than rural populations and that cranial capacity would increase over time in urban populations. Based on relevant theory, we also predicted that species capable of rapid population growth would show more pronounced evolutionary responses. We found that urban populations of two small mammal species had significantly greater cranial capacity than rural populations. In addition, species with higher fecundity showed more pronounced differentiation between urban and rural populations. Contrary to expectations, we found no increases in cranial capacity over time in urban populations-indeed, two species tended to have a decrease in cranial capacity over time in urban populations. Furthermore, rural populations of all insectivorous species measured showed significant increases in relative cranial capacity over time. Our results provide partial support for the hypothesis that urban environments select for increased behavioural plasticity, although this selection may be most pronounced early during the urban colonization process. Furthermore, these data also suggest that behavioural plasticity may be simultaneously favoured in rural environments, which are also changing because of human activity.

Keywords: cognition; cranial capacity; fecundity; rodents; urban evolution.

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Figures

Figure 1.
Figure 1.
Variation between urban (light grey) and rural (dark grey) populations in cranial capacity. Shown are least-squares means from an ANOVA that included body size (the first principal component of a PCA on total body length, tail length and hindfoot length, run separately for each species), year, population (urban or rural) and a population-by-year interaction, run separately for each species (table 1).
Figure 2.
Figure 2.
Variation in fecundity is correlated with degree of difference between rural and urban populations. Shown are the results of a model testing for effects of litter size and number on the relative difference in cranial capacity between rural and urban populations of a species. More positive values of the dependent variable indicate that urban populations have a greater cranial capacity than rural populations of the same species (based on least-squares means from a model that controlled for body size and year—see table 1). PGLS analysis indicated that phylogeny had little impact on the data, so data uncorrected for phylogenetic relationships are plotted.
Figure 3.
Figure 3.
Changes in cranial capacity over time in rural populations. Shown are leverage plots from ANOVAs that included body size (the first principal component of a PCA on total body length, tail length and hindfoot length, run separately for each species) and year, run separately for each species and population (table 2).
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