How to interpret a phenotypic frequency density plot
Judging from the first section of the Results part of the paper, it looks like the x-axis represents a continuum between two divergent phenotypes.  At one end is parental learning, and at the other end is social learning.  The density (y axis) represents the proportion of the population that displays a given balance between the two extremes.

Generally, density plots are a normalised way of presenting observations, such that the overall area underneath each density plot is equal, regardless of the number of actual observations making up any given plot.