Black rhinos are quadrupedal (they walk on four legs) and walk on their toes, with their feet supported by a fleshy pad. They run quickly and are capable of reaching speeds up to 35 mph. They are generally solitary animals. Females with recent offspring, however, continue to associate with their young until shortly before the birth of new offspring. Black rhinos do not generally form organized social groups, though they sometimes congregate in small groups for short periods of time. They are inactive during the hottest hours of the day, rest in the shade, and frequently wallow in shallow mud pools to reduce their body temperature. Though generally quiet, black rhinos use a variety of vocalizations, including a long snort for anger, a short snort for alarm, a lion-like growl for fighting, a variety of squeaks for different emotions, and a puffing snort for greetings between males and females. They display little aggression toward each other, though males fight occasionally. It is not uncommon for black rhinos to "bluff and bluster," which is when one rhino facing another rhino threateningly snorts and swings its head from side to side. Black rhinos also use their tail and ears to communicate. An upturned tail can mean the rhino is alarmed or curious; straight-up ears can mean the rhino is interested or curious; flat ears can mean anger.