Boxplots of the average rate of yawning per minute during Baseline (white) and Stimulus (grey) for tongue protrusion Non-imitators (top) and Imitators (bottom).

<p>LPS = lipsmacking condition, TP = tongue protrusion condition, CTRL = control condition. Boxplots depict 1^st and 3^rd quartile, line depicts median, whiskers depict 1.5 times inter-quartile range (IQR). Circles represent mild outliers larger than Q3 + 1.5 * IQR, asterisks represent extreme outliers larger than Q3 + 3 * IQR.</p

Boxplots of the average rate of yawning per minute during Baseline (white) and Stimulus (grey) for lipsmacking Non-imitators (top) and Imitators (bottom).

<p>LPS = lipsmacking condition, TP = tongue protrusion condition, CTRL = control condition. Boxplots depict 1^st and 3^rd quartile, line depicts median, whiskers depict 1.5 times inter-quartile range (IQR). Circles represent mild outliers larger than Q3 + 1.5 * IQR, asterisks represent extreme outliers larger than Q3 + 3 * IQR.</p

Methodological details.

<p>Procedure (A), examples of lipsmacking, tongue protrusion, and control stimuli (B), and examples of infants' gestures (C). Human models portrayed here have given written informed consent, as outlined in the PLOS consent form, for publication of their photographs.</p

Scatterplots of the relationship between imitation strength and gesture rates during the Return period.

<p>Imitation strength is reported separately for lipsmacking (LPS; plots A and B) and for tongue protrusion (TP; plots C and D), and was calculated using the LPS and TP Imitation Index (see main text). Higher scores indicate stronger imitation. Only for LPS was there a stronger correlation when the return person was the same (A) compared to when the return person was different (B), <i>p</i><.001; for TP there was no difference in the correlation coefficients when the return person was the same (C) or different (D), <i>p</i> = .17.</p

Frequencies of Looks That the Infants Oriented at the Stimulus during the Baseline and the Stimulus Period

<p>Asterisks (*) indicate a significant increase in number of looks (stimulus versus baseline) for a specific stimulus (at least <i>p</i> < 0.05). Pound symbols (#) indicate that this effect is close to statistical significance (<i>p</i> < 0.07). Because data concerning looks at models' LPS, MO, and TP were very similar, they were averaged and pooled. Frequencies are ± standard error of the mean.</p

Two Examples of a Monkey's Response to the Stimuli Mouth Opening and Tongue Protrusion

<p>MO is shown on the left; TP on the right. Figures were taken from <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040302#pbio-0040302-sv001" target="_blank">Videos S1</a> and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040302#pbio-0040302-sv002" target="_blank">S2</a>. Frame A1 was taken 21.12 s after frame A, whereas frame B2 was taken 13.38 s after frame B.</p

Experimental Conditions

<p>Figures on the left represent stimuli during resting conditions and baseline. Figures on the right depict the stimuli when fully expressed. In the DISK condition, the disk was repeatedly rotated 90° clockwise and counterclockwise.</p

Association of Assertiveness and fWHR in adult males and females, split by alpha status.

<p>In both sexes a significant positive linear relationship between fWHR and Assertiveness is visible. This relationship held when examining non-alpha individuals only.</p

Linear effects of age and sex on fWHR.

<p>In males, fWHR increases significantly with age, suggesting developmental changes at puberty. In females, fWHR appears to decrease over the lifespan, although no significant change is observed when excluding animals older than 20 years.</p

Illustration of the facial width-to-height ratio: zygomatic width (distance between vertical lines) divided by upper face height (distance between horizontal lines).

<p>Illustration of the facial width-to-height ratio: zygomatic width (distance between vertical lines) divided by upper face height (distance between horizontal lines).</p