Hector’s and Māui Dolphins: Small Shore-Living Delphinids with Disparate Social Structures

Abstract

Hector's dolphins (Cephalorhynchus hectori hectori) are a small ({\textasciitilde}1.5 m long) marine dolphin, primarily inhabiting turbid, coastal waters discontinuously around the South Island of New Zealand. The M{\={a}}ui dolphin (C. h. maui) is a critically endangered subspecies of Hector's dolphin, only found along a small part of their original range spanning the west coast of the North Island of New Zealand. Both subspecies have small alongshore home ranges of around 50 km, with high levels of site fidelity and low levels of gene flow. Despite this, some individuals have traveled distances of at least 400 km, interacting with local animals. Hector's dolphins exhibit seasonal movements linked to prey availability and social aggregation behaviors associated with the summer mating and calving period. They typically occur in small groups of 2--10, with high levels of fission-fusion and low levels of association among individuals. Sex segregation occurs in small groups (<5 individuals) of Hector's dolphins throughout the year, but this same pattern does not hold for larger groups. Mother-calf pairs are typically associated with other females, a common pattern for delphinids. M{\={a}}ui dolphins do not show the same pattern, with mixed-sex aggregations of dolphins independent of group size, perhaps an artifact of the extremely small population size. Hector's dolphins largely communicate with ultrasonic clicks, with different vocalizations among social groups and during feeding. Their echolocation clicks are important when foraging in their preferred habitat of low visibility. They forage on a wide range of benthic and demersal fishes and squids, with most prey <10 cm long and some regional differences in species composition, but overall similarities in prey preferences. Despite their distribution around New Zealand and variation in local population sizes, Hector's and M{\={a}}ui dolphins have broad similarities in behavior, association patterns, and habitat use. Where differences exist, the habitat, prey movements, and population size are potential explanatory factors. In New Zealand, a hot spot for cetacean diversity, these dolphins occupy a small and specific niche that is typical for Cephalorhynchus elsewhere in the Southern Hemisphere. Because they occur close to shore in waters affected by humans, they are vulnerable to anthropogenic disturbance. But with recognition of dangers and appropriate protections, the species should flourish in New Zealand's productive coastal waters.