Bottlenose dolphins live in highly fluid societies where individuals routinely encounter other bottlenose dolphins, and where many interactions involve acoustic signals that transmit well through water. However, life is not always peaceful during such encounters. Pairs of males roam the coastal waters of Sarasota looking for potential mates. Some females that are encountered may be escorted by males for days to weeks, whereas other encounters end more quickly, often in brief but fast chases of females or female-calf pairs. While these encounters are often easy to observe from the surface, it is much less clear what acoustic signals are used by dolphins during these aggressive interactions.
During the past 4 years, we have been deploying suction-cup-mounted short-term digital archival tags, known as DTAGs, on paired animals upon release at the end of their health assessments, focusing primarily on mother-calf pairs and male alliances. The tags allow us to record the sound and motion of tagged individuals, and we combine these data with vessel-based observations of behavior and social interactions to understand the social lives and acoustic communication of free-ranging bottlenose dolphins.
These tags now allow us to understand how acoustic signals are used during aggressive encounters of wild dolphins. During interactions between males and females, bottlenose dolphins emit a suite of acoustic signals ranging from short, low-frequency quack-like signals to sudden, intense jaw claps and extended bursts of high-amplitude clicks. It is increasingly clear that most of these sounds, especially jaw claps and low-frequency narrowband signals, are used nearly exclusively by allied males during such interactions. Some of these signals, such as the low-frequency quack-like sounds, are low-amplitude, highly repeated signals that seem to be used for coordinating the behavior or movement of highly synchronized males and might also serve as an early warning signal for the nearby female. In contrast, jaw claps and burst pulses are more intense and appear to escalate conflicts. These sometimes precede more aggressive surface behaviors such as leaps and fast chases that can be observed during focal follows and measured by the inertial sensors of the tags. Many of the same sound types are also observed with dolphin under human care as well as in more extreme cases of aggression, including attempted infanticide between a pair of bottlenose dolphin males and a newborn calf in Savannah, Georgia, confirming their general importance for aggressive interactions with conspecifics.
The use of acoustic signals varies greatly with context. To see what role echolocation plays in wild bottlenose dolphins, we also analysed the use of echolocation clicks with DTAGs. Bethany Roberts analysed click rates from 15 animals tagged over the last few years. Not surprisingly, click rates were highest during foraging bouts, but what we did not expect were long periods without any clicking when animals were travelling. This suggests that dolphins often rely on other senses or their memory for orientation. We also found that click rates differ greatly between individuals, suggesting that animals may rely on echolocation to different extents or that some of the click trains recorded have a communication rather than an echolocation function.
This article appeared on page 10 in the December 2015 issue of Nicks n Notches.