The Morphology of Dolphin Retractor Penis Muscle
The retractor penis muscle (RPM) is a common feature among some mammalian species. It holds the penis inside the penile silt during resting phase, relaxes as the penis extends outside the male’s body to form an erection, and contracts again after copulation to bring the penis back in. We are particularly interested in dolphin penile morphology and adaptations because they mate underwater, where the penis is subject to additional drag forces not present on terrestrial animals. Through this research, we want to 1) investigate whether dolphin RPM is smooth or skeletal muscle; 2) examine the correlation between the volume of dolphin RPM (potential for force production) and the volume of the penis it connects with; 3) examine changes in thickness and density of RPM muscle fibers. We determined that dolphin RPM is smooth muscle. The volume of dolphin RPM has tighter positive linear relationship with the volume of the penis base than with the penis tip. RPM fiber thickness shows significant difference across species and dolphin RPM contains more connective tissue. We propose that morphological variation is due to phylogenic factors and different forces the penis receives. A larger study including more species would be needed to test these hypotheses.