A young adult whale performing a "head lunge" shows off his ventral grooves (or "pleats"). They run from the lower jaw to the umbilicus about halfway along the underside of the whale. These grooves are less numerous (usually 14–22) than in other rorqual ("furrowed" in Norwegian) whales, but they are wider. When feeding, the whale takes in large quantities of seawater and the pleats allow expansion of the mouth cavity. Note the orangish-yellow "beard" of barnacles clinging to the lower jaw.
Ma'alaea Bay, Maui -- 4 January 2015