pieces of sponge and wrapping them around their rostra, presumably to prevent abrasions and facilitate digging. Communication Whale song is the sounds made by whales and which is used for different kinds of communication. Dolphins emit two distinct kinds of acoustic signals, which are called whistles and clicks: Clicksquick broadband burst pulsesare used for echolocation, although some lower-frequency broadband vocalizations may serve a non-echolocative purpose such as communication (for example, the pulsed calls of orcas). Pulses in a click train are emitted at intervals of ≈35–50 milliseconds, and in general these inter-click intervals are slightly greater than the round-trip time of sound to the target. Whistlesnarrow-band frequency modulated (FM) signalsare used for communicative purposes, such as contact calls, the pod-specific dialects of resident orcas, or the signature whistle of bottlenose dolphins. There is strong evidence that some specific whistles, called signature whistles, are used by dolphins to identify and/or call each other; dolphins have been observed emitting both other specimens' signature whistles, and their own. A unique signature whistle develops quite early in a dolphin's life, and it appears to be created in imitation of the signature whistle of the dolphin's mother. Imitation of the signature whistle seems to occur only among the mother and its young, and among befriended adult males. Xitco reported the ability of dolphins to eavesdrop passively on the active echolocative inspection of an object by another dolphin. Herman calls this effect the "acoustic flashlight" hypothesis, and may be related to findings by both Herman and Xitco on the comprehension of variations on the pointing gesture, including human pointing, dolphin postural pointing, and human gaze, in the sense of a redirection of another individual's attention, an ability which may require theory of mind. The environment where dolphins live makes experiments much more expensive and complicated than for many other species; additionally, the fact that cetaceans can emit and hear sounds (which are believed to be their main means of communication) in a range of frequencies much wider than humans can means that sophisticated equipment, which was scarcely available in the past, is needed to record and analyse them. For example, clicks can contain significant energy in frequencies greater than 110 kHz (for comparison, it is unusual for a human to be able to hear sounds above 20 kHz), requiring that equipment have a sampling rates of at least 220 kHz; MHz-capable hardware is often used. In