Context-specific signal plasticity of two common bottlenose dolphin ecotypes (Tursiops truncatus) in Far North waters, New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Conservation Biology at Massey University, Albany, New Zealand
Loading...
Date
2018
DOI
Open Access Location
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Massey University
Rights
The Author
Abstract
Common bottlenose dolphin (Tursiops truncatus, referred to hereafter as bottlenose dolphin)
fission-fusion groupings are temporary in nature, lasting from minutes to hours, necessitating
efficient signal exchange. The selective pressures and contexts acting on signal exchange, such
as ecotype variation, are not well understood. The objectives of the current study are three-fold
to: 1) quantify the density, distribution and abundance of bottlenose dolphin ecotypes and
identify the nature and areas of spatial overlap between the two in Far North waters; 2) examine
aspects of mechanical signal exchange based on biotic and abiotic factors; and 3) estimate the
effect of key ecotype specific contexts on group multimodal signal exchange. This thesis
applies a holistic approach to the assessment of signal exchange in ecotypes using the highly
social bottlenose dolphin as a model genus. Additionally, this body of work provides the first
comprehensive assessment of oceanic bottlenose dolphin distribution, abundance, and
behaviour ecology and the first ecotype spatial and behavioural overlap within New Zealand
waters.
Knowledge of population size, social behaviour, threats, and ability to integrate new
individuals is required to define management units. Although it has long been recognised that
the nationally endangered coastal bottlenose dolphin is not resident in the Bay of Islands but
genetically part of a North East coast population, no studies have quantified bottlenose dolphin
distribution in Far North waters outside of the Bay of Islands. This study provides the first
systematic analysis of detectability, distribution, and spatial overlap of both the coastal
bottlenose dolphin and the previously unquantified oceanic bottlenose dolphin in Far North
waters. Results suggest Far North waters are important for the coastal bottlenose dolphin,
supported by a higher average density (0.620 individuals/kilometre) than that reported for the
coastal bottlenose dolphin in other areas of the North East coast population. The importance of
assessment outside areas of commercial interest is further reinforced in this study. The Bay of
Islands local abundance is not reflective of the coastal bottlenose dolphin in the wider area, as
indicated by the higher Far North waters estimates of 212.8% (Austral Summer) and 196.1%
(Austral Winter). With no previous density or abundance estimates for oceanic bottlenose
dolphin in New Zealand, no comparisons can be drawn with other studies or historic research.
However, the distance sampling-based population estimate of 3,634 (SE = 152) indicates
oceanic bottlenose dolphin abundance is much higher than the 389 (SE = 108) coastal
bottlenose dolphin abundance estimate in Far North waters, even though their distribution is seasonal with detection only in austral Summer and Autumn. Kernel density also indicates the
representative ranges (95 % kernel range) of the coastal bottlenose dolphin are smaller than the
oceanic bottlenose dolphin, extending over a total area of ~794 km2 and ~1,003 km2,
respectively. This study further suggests these ecotypes should be described as largely
parapatric and non-resident in nature, with a minimal spatial overlap of only 7.4 % of Far North
waters surveyed (~196 km2). This is further supported by no sightings of the two ecotypes
within the same survey zone on the same day (n = 372 bottlenose dolphin sightings) during the
present study.
Behaviour can further add insight into the partitioning and variation of parapatric units within
a species. In Far North waters, whilst research on surface behaviour has been previously
conducted in the Bay of Islands, behavioural assessment in wider areas and based on a holistic
assessment of multiple signal forms is lacking in the literature. This study, however, applies a
systematic sampling technique and integrated analysis to identify trends in signal exchange
use. This is done by examining multiple behavioural modes (states and events, surface and
subsurface) concurrently. This ultimately provides an additional method for quantifying group
behavioural plasticity as a result of covariates acting on parapatrically occurring bottlenose
dolphin groups. In one assessment, cues were taken from avian acoustic research to accurately
quantify and analyse ecotype variation in call repertoire. In support of the parapatric definition,
call repertoire was correctly assigned to ecotype, with an 89.4% success rate (n = 31,432 calls).
Of all parameters examined, 71.4% exhibited significant variation, with harmonics and contour
inflections used significantly more in oceanic bottlenose dolphin. Both Dynamic Time
Warping in Luscinia and Hidden Markov Models add reliable insight into the categorisation of
key signal parameters and important tools for the primary assessment of differences in
bottlenose dolphin behaviour within Far North waters. Including the full behavioural repertoire
of oceanic bottlenose dolphin and coastal bottlenose dolphin through Hidden Markov Models
adds additional insight to the possible drivers behind the divergence in the call parameters
noted. It is notable that signal parameters are not influenced by the same key drivers for both
ecotypes. This is an important finding in a species in which most communication exchanges
involve acoustic signals in some form.
The formation of interspecific groupings has the largest effect on social signal exchange in
oceanic bottlenose dolphin of all covariates considered. The response magnitude is associated
with group parameters, for example, the ratio of individuals (e.g. pilot whales (Globicephala sp.) to oceanic bottlenose dolphins) and the behaviour and/or overall size of the focal group.
These aspects in turn determine the effect of participation in interspecific groups on oceanic
bottlenose dolphin behaviour. Oceanic bottlenose dolphins in interspecific groups with pilot
whales display higher call plasticity, both in the time and frequency domain, utilising
significantly higher measures for 66.7% of parameters, including longer and more complex
calls (increased number of harmonics and inflects), than in intraspecific oceanic bottlenose
dolphin groups. Signal adaptation is more evident when oceanic bottlenose dolphins and pilot
whales are directly interacting with each other during social events. During socialising states,
the predominant subsurface event type observed is aggression (59.4%, n = 233). As the ratio
of pilot whales to oceanic bottlenose dolphins increases, the use of long-distance signal
exchange decreases. This suggests that the observed decreases in interspecific similarity of
whistle parameters during social interactions may act to increase the ability to differentiate
species-specific rather than whole group signal exchange cues. Although species-specific call
differentiation is likely heightened by variation in gross morphology, phylogeny and
geographical constraints, frequency domain characteristic overlap between oceanic bottlenose
dolphins and pilot whales. The detected adjustments in signal structure away from intermediate
values may suggest a decrease in the increment in this overlap. Support is given to the theory
that signal exchange might be modified not only as a result of group behaviour state but also
due to the signaller’s motivational state (i.e. stress). While the full biological effects of changes
in whistle rates and parameters remain uncertain, these changes add initial insights to the
dynamics of interspecific groupings.
Number of vessels had largest effects on coastal bottlenose dolphin social signal exchange,
with response magnitude significantly related to group composition (ex. with or without calves)
and use of other signal exchange behaviours (ex. tactile type and rate). The response is also
influenced by acoustic parameters considered (frequency or call rate). The highest coastal
bottlenose dolphin density area, the Bay of Islands, contains the highest proportion of groups
with calves and the highest level of vessel traffic in Far North waters. Groups with calves
favour sounds typically used for short-distance signal exchange, appearing to increase the use
of mechano-reception in the presence of vessels. Overall, coastal bottlenose dolphins are more
likely to: (1) leave the low-call-rate state in the presence of one to two vessels (within 300m);
(2) leave the high-call-rate state when three or more vessels (adults only), or two or more
vessels in the case of groups with calves, are within 300m; and (3) stay in a relatively silent
state when more than three vessels are present. Furthermore, coastal bottlenose dolphins are more likely to (1) leave the low-call-rate state when contact rate is low; (2) leave the high-callrate
state when contact rate is high; and (3) stay in a relatively silent state when contact rate is
high. This suggests an inverse relationship exists between call rate and contact rate, i.e. as
possible vessel effect increases, vocalisation-mediated coordination decreases, and mechanomediated
coordination increases with a lower threshold for groups with calves than groups
without. Several drivers of elevated signal exchange rates have been suggested in the presence
of vessels. These include an increased motivation for individuals to stay close together, a
changed group cohesion and amplified arousal. However, this study adds new insights with the
quantification of multi-modal signal exchange in the presence of vessel. This has not been the
focus of previous research in any of the bottlenose dolphin populations in New Zealand.
Distribution, density, and abundance is now available for both bottlenose dolphin ecotypes,
with multimodal group behaviour in ecotype specific contexts additionally quantified. As such,
supplementary monitoring and reviews of the coastal bottlenose dolphin and oceanic bottlenose
dolphin parapatric populations in New Zealand are essential. Pre-emptive rather than reactive
conservation is recommended to effectively manage both bottlenose dolphin ecotypes
separately and efficiently in New Zealand waters.
Description
Keywords
Bottlenose dolphin, Geographical distribution, New Zealand, Islands, Bay of, Vocalization, Behavior, Effect of human beings on, Dolphin sounds, Research