Reticulated giraffes are sexually dimorphic (with two distinct gender forms); males are taller and have a slightly different facial appearance. The males have protruding median ossicones (the hornlike knobs on their head) and can develop calcium deposits that form bumps on their skull as they age. The ground color for giraffes is white to buff. Spots are chestnut brown to almost black and vary in size and shape. They are unique to each individual. Spot patterns cover most of their body, but giraffes' underparts are lighter and more faintly spotted. The ground color appears as a network of lines between the spots from which they get their name: reticulated. The coat pattern not only serves as camouflage but also serves as "thermal windows": sites for complex blood vessel systems and large sweat glands. Their skin secretes up to 11 chemicals that produce a strong and unique scent that repels parasites and is suspected to have a sexual function. Giraffes have a very long neck, necessitating elastic blood vessels and valves to compensate for the sudden increase in blood pressure when the head is lowered. They have a long, gray prehensile (capable of grasping) tongue and flexible upper lips. Both sexes have a pair of short frontal ossicones. These are horns but made of ossified calcium and covered skin and hair. They are unique to giraffes and okapi. The nostrils have muscular openings, which giraffes can open and close to protect themselves against sandstorms and ants that inhabit trees they feed on. They have long eyelashes, and their forelimbs are slightly longer than the hindlimbs.