Relationship between the age of males and the minimum formant dispersion of groans. Males produce groans with higher minimum formant dispersion, indicating shorter apparent vocal tract lengths when they are five and eight years old than when they are seven and eight years old (quadratic relationship). Dots represent mean values per year for each male and lines show repeated measures of the same individual across years.
Briefer et al. BMC Biology 2010 8:35 doi:10.1186/1741-7007-8-35