Seasonal variation in the size of the gular lobe of a Male Musk Duck Biziura lobata

The Musk Duck Biziura lobata possesses elaborate secondary sexual traits that are not present among other ducks and include a leathery lobe attached at the base of the lower mandible of the male, which is thought to become larger at certain times of the year. To establish the extent of this seasonal variation, a captive male Musk Duck was photographed each month for a period of 16 months. Quantitative analysis of the photographs demonstrated seasonal variation in the area of the lobe, with an almost threefold difference between the smallest and largest measurement. Reasons for this variation are discussed.