fermentation may also provide additional heat and is correlated with their low metabolic rate. Vibrissae Manatees have sensitive tactile hairs that cover their bodies and faces called whiskers or vibrissae. Each individual hair is a vibrissal apparatus called a follicle-sinus complex. Vibrissae are blood filled sinuses bound by a dense connective tissue capsule with sensitive nerve endings that provides haptic feedback to the manatee. Usually vibrissae are found on the facial regions of terrestrial and non-sirenian aquatic animals and are called whiskers. Manatees, however, have vibrissae all over their bodies. The vibrissae located in their facial region are roughly 30 times denser than the vibrissae on the rest of their body. Their mouth consists of very mobile prehensile lips which are used for grasping food and objects. The vibrissae on these lips are turned outward during grasping and are used in locating vegetation. Their oral disks also contain vibrissae which have been classified as bristle-like hairs that are used in nongrasping investigation of objects and food. Manatee vibrissae are so sensitive that they are able to perform active touch discrimination of textures. Manatees also use their vibrissae to navigate the turbid waterways of their environment. Research has indicated that they are able to use these vibrissae to detect hydrodynamic stimuli in the same way that fish use their lateral line system. Reproduction Male manatees reach sexual maturity at 3–4 years of age, while females reach sexual maturity at 3–5 years of age. Manatees appear capable of breeding throughout their entire adult life, although most females first breed successfully at ages 7–9. Breeding occurs in ephemeral mating herds, where several males aggregate around an estrous female and compete for access to her. Larger males, presumably older, have been observed to dominate mating herds and are likely responsible for most successful copulation events. The gestation period in manatees lasts for 12–14 months, after which they give birth to one calf at a time, or rarely twins. When a calf is born, it usually weighs and is long. Manatees do not form permanent pair bonds and the male contributes no parental care to the calf, which remains with the mother for up to two years prior to weaning. A single female can reproduce once every 2–3 years. Wild manatees have been documented producing offspring into their late 30s, and a female captive manatee has given birth in her 40s. Threats and conservation The