Mating Systems

Sexual dimorphism is generally measured as the size difference between males and females, and is often calculated as the size difference in body size (BSSDI). Sexual dimorphism index (SDI) is used to determine the degree of male-male contest competition that has evolved, which gives us information on the species’ mating system in the absence of behavioral data.

Sexual Dimorphism Index (SDI) is calculated using the following formula:

SDI = male size / female size

SDI reflects how much larger a male is than a female, e.g. a SDI of 1.5 means the male is 1.5x the size of the female. The larger the SDI, the more male-male contest competition has evolved. If the SDI is below 1, the female is larger than the male, suggesting female-female competition is stronger.

Males also compete post-mating via sperm competition. The degree of sperm competition is associated with the relative testis size, or testis volume divided by body size. The larger the relative testis size, the more sperm competition. Sperm competition evolves when females mate with multiple males.