Acoustic analysis of slow click function and foraging in sperm whales (Physeter macrocephalus) off Kaikoura, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Sexually immature male sperm whales (Physeter macrocephalus) disperse from their natal areas and move to higher latitude male-only foraging grounds, such as those off New Zealand and Norway. In these areas they are found in aggregations, in which a relatively high concentration of animals congregate in a specific area. Males within aggregations continuously forage, yet seemingly dive in a solitary manner. The Kaikoura submarine canyon off New Zealand is an area where male sperm whales aggregate. This canyon is among the most productive deep sea regions in the world, and has been used for foraging both by individual male sperm whales over years, as well by transient animals which have only been seen once.
Slow clicks are vocalisations only used by males. Typically displayed in bouts with inter-click intervals of 3-9 s, they consist of low frequency (2-4 kHz) sharp clicks with a strong reverberation and apparent source levels around 201 dB peak re 1 μPa at 1 m. Slow clicks have much lower directionality than regular echolocating clicks, resulting in their characteristic strong and lasting seafloor echo. At breeding grounds, slow clicks seem to be displayed in long sequences while males are close to, or at the surface, and these have been suggested to function as vocal display used in competition and/or to attract females. In contrast, at male-only foraging grounds, slow clicks are heard in shorter sequences at the end of foraging dives (and also, at times, during surfacing), with these being heard in almost half of all foraging dives recorded in this study. The diving-phase pattern associated with slow clicks appears consistent across male aggregations recorded here, off Kaikoura, and those found off Norway1. While the function of slow clicks is still unclear, the different social and slow-clicking patterns observed suggests a context-dependent function in communication.
I used a towed hydrophone array and photo-identification data from individually-tracked whales during complete foraging dives to further understand the function of slow clicks in the Kaikoura foraging ground. I investigated the presence and number of slow clicks as a function of other acoustically-detected whales in the area, which indicated an increased rate of slow clicking with increasing number of ‘neighbours’. I examined slow click structure, including centre frequency, waveform, and, as body size-related information is encoded in regular echolocating clicks, the occurrence of multiple pulses within clicks, to examine the relationship between this vocalisation and body length. The centre frequency of slow clicks differed between individuals and their waveform revealed a multi-pulsed structure in seventy percent of clicks analysed. However, neither the inter-pulse interval (time between pulses within a click) of slow clicks, nor their centre frequency, correlated with body size. Additionally, the analysis of the pulse structure and amplitude of slow clicks within bouts suggests that slow clicks display a broad but somewhat defined directionality, which may allow slow clicking whales to target goals by adjusting their body posture. Additionally, I explored the use of slow clicks and codas, which are stereotyped patterns of clicks used for communication, produced by a pair of males synchronising their surfacing time and dives. Over a complete dive cycle and surfacing time I examined the spectral characteristics and waveform of codas (click sequences) and slow clicks, as well as coda duration. Codas made using clicks with spectral and waveform characteristics of slow clicks were termed “slow click codas”. These were longer, and showed lower centre frequency and higher relative amplitude than traditional codas and may suggest some form of communication between whales in synchrony. Finally, I looked at the feeding vocalisation (creak) rate, duration, and the relationship between creak presence, bathymetry and the commercial fisheries within the underwater canyon off Kaikoura as a proxy for areas with an abundance for potential prey. Results suggest that males in this area may be targeting fewer but larger and/or more nutritious prey (i.e. fewer but longer creaks) compared to other studied areas.
In conclusion, the diving phase-related use of slow clicks, their acoustic properties, including the centre frequency and waveform related information, coupled with the field observations analysed and the history of male bachelor group composition, suggests that slow clicks may function as a contact call, which could be used for individual and/or group recognition.