During the middle of estrus, female elephants look for males in musth to guard them. This experience indicates that older bulls suppress musth and aggressiveness of younger bulls. In elephants, temporin is secreted by temporal glands during the period of musth. Bulls in musth emit a distinctive, low-frequency pulsated rumble nicknamed the "motorcycle". Bulls in musth are more successful at obtaining mating opportunities than non-musth bulls. Bulls in musth often chase each other and are aggressive towards other bulls in musth. Males sire few offspring in periods when they are not in musth. Adult males enter a state of increased testosterone known as musth. Females search for bulls in musth, listen for their calls and follow their urine trails. Bulls begin to experience musth by the age of 24 years. Most observed matings are by males in musth over 35 years of age. This is especially true of males in musth and of females with young. Another behavior affiliated with musth is urination.