Comparative socioecology of the dusky dolphin (Lagenorhynchus obscurus) in New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Manawatu, New Zealand
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Date
2012
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Massey University
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Abstract
Behaviour and social groupings of animals are often closely tied to habitat structure.
Ecological factors (e.g., predation pressure, resource distribution) and social pressures
profoundly influence behaviour and social organization. This study examined the
socioecology of dusky dolphins (Lagenorhynchus obscurus) off Kaikoura, one of the most
productive submarine canyon habitats in the world, supporting marine mammal populations
and a thriving tourism industry. Behavioural data were collected during 332 dusky dolphin
group follows from a small research vessel and onboard 174 dolphin tours during 2006-2009.
Continuous and interval sampling were used to document behavioural states, behavioural
events, and changes in group composition. Data from small groups (n=197) were compared
with similar data collected from 67 small group encounters with Hector’s dolphins
(Cephalorhynchus hectori) in the same habitat. In winter, dusky dolphins formed larger
groups that ranged more widely and small groups that primarily rested during the day near
shore. Although resting, small groups increased dispersion and swam in parallel formation in
winter. In summer, dolphins formed small groups for calf-rearing, mating and resting. Groups
with calves were the largest and most stable of these small groups, and were found closest to
shore, in relatively shallow water, often near river mouths. In mating groups, rapid chases
decreased intromission rates as competition among males increased. Sexual selection may
favour traits improving male success in behavioural contests as well as sperm competition in
this species. Apparent mechanisms for female choice included ‘escape’ strategies that
prolonged chases. Hector’s dolphins formed smaller groups with higher fission-fusion rates,
less cohesion, and fewer social bond reinforcement behaviours than dusky dolphins.
Interactions with tour vessels had short-term effects on the behavioural state, heading,
dispersion, and leaping rate of large dusky dolphin groups. Resting was disrupted during tour
interactions and dolphins increased travel away from approaching tour vessels following tour
interactions. Assertive approach methods reduced the duration of swimmer-dolphin
interactions, particularly in summer when dolphins may be sensitised to high levels of tourism.
These large-brained, highly social mammals form intricate societies that are responsive to
changes in habitat, predation pressure and resource distribution. Understanding these
responses is vital for protection of dolphins in the wild.
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Keywords
Dusky dolphin, Lagenorynchus obscurus, Kaikoura, Dolphin behaviour, Dolphin encounters, Hector's dolphin, Dolphin tours