Experimental apparatus (IMAGE) Elsevier Caption The “subject” calf in the middle has access to two side pens through a push gate: one with a social partner, and one empty pen. Weight attached to the push gate is increased every time the subject accesses the pen. Credit Illustration by Ann Sanderson, made using Image to Sketch Usage Restrictions Please include attribution License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.