Oceanic Dolphin Societies: Diversity, Complexity, and Conservation

Abstract

Sociality—collective living—confers multiple advantages to oceanic dolphins, including enhanced foraging, predator avoidance, and alloparental care and may be particularly important in oceanic environments where prey is patchy and refuge nonexistent. This chapter covers broad aspects of the social lives of the delphinid community that inhabits the vast eastern tropical Pacific Ocean (ETP). Our approach is socio-ecological: the chapter ties dolphin social structure and mating systems to environmental factors, including oceanographic patterns, distribution of prey, and risk of predation that shape behavior. By merging a top-down look at schools distributed over a variable environment, with a bottom-up look from the perspective of subgroups that comprise schools, a picture of fission–fusion societies emerges. We also consider impacts of the tuna purse seine fishery on the socio-ecology of affected dolphins and discuss likely effects on behavior, learning, social bonds, and population dynamics.

ETP dolphin societies are diverse, spatially and compositionally fluid (pure or mixed species), yet socially complex and structured. They have distinct schooling, reproductive, and sexual characteristics, different patterns of association with other species, and differing degrees of interaction with the tuna purse seine fishery. Individuals may have distinct roles (older, experienced, and post-reproductive females), form stable or at least semi-stable subgroups (female/young, adult male, juvenile), and leave or join the company of others in response to a variety of social and ecological factors, including distribution of prey and risk of predation. In some taxa, individuals school with a small number of companions who may be related and recognize one another (common bottlenose, Tursiops truncatus; Risso’s, Grampus griseus; rough-toothed, Steno bredanensis; and striped dolphins, Stenella coeruleoalba), while in other species school size is larger, membership is fluid, and unrelated individuals abound (pantropical spotted, Stenella attenuata; spinner, Stenella longirostris; and common dolphins,Delphinus delphis). Mating systems are variable among species and sometimes within species, likely reflecting differences in habitat productivity. In some taxa, e.g., eastern spinners (S. l. orientalis), a few sexually mature males may be responsible for most mating, while in other taxa, e.g., “whitebelly” spinners, large relative testes suggest a more “open” mating system where many males in the school engage in copulation.

For pantropical spotted and spinner dolphins in the ETP, the behavior of schooling with large tuna that has led to their ecological success and abundance has also led to their depletion by making them a target of purse seiners. Schooling and sociality, normally adaptive traits, have caused ETP dolphins to become collateral damage in the tuna fishery. Yet dolphins have learned some things from their experiences with purse seiners. Some individuals know how to evade capture or, alternatively, how to await a lowering of the net (“backdown”) to escape. But, behavior that helps to avoid capture can cause high stress, exertion, or social separation and disruption, and these could be factors slowing or inhibiting population recovery. Survival and reproductive success of oceanic dolphins likely depends largely on social and behavioral factors that may also help determine their ability to recover from severe depletion caused by human activities.

Appendices

Appendices

1.1 Appendix 1

Delphinid species recorded in the eastern tropical Pacific. Adapted from Ballance et al. (2006). Endemic taxa are noted. Naming convention follows the Committee on Taxonomy (2018) (taxonomy) and Dizon et al. (1994) (management stocks).

• Delphinus delphis — Common dolphin , saddleback dolphin

  • D. d. delphis — Common dolphin

    • Recognized ETP management stocks:

    •   Northern short-beaked common dolphin

    •   Central short-beaked common dolphin

    •   Southern short-beaked common dolphin

  • D. d. bairdii — Eastern North Pacific long-beaked common dolphin

• Feresa attenuata — Pygmy killer whale

• Globicephala macrorhynchus — Short-finned pilot whale

• Grampus griseus — Risso’s dolphin , grampus

• Lagenodelphis hosei — Fraser’s dolphin

• Lagenorhynchus obliquidens — Pacific white-sided dolphin

• Lagenorhynchus obscurus — Dusky dolphin

• Orcinus orca — Killer whale, orca

• Peponocephala electra — Melon-headed whale, Electra dolphin

• Pseudorca crassidens — False killer whale

• Stenella attenuata — Pantropical spotted dolphin

  • S. a. attenuata — Offshore pantropical spotted dolphin

    • Recognized ETP management stocks:

    •   Northeastern offshore pantropical dolphin

    •   Western-southern offshore pantropical dolphin

  • S. a. graffmani — Coastal pantropical spotted dolphin — Endemic

• Stenella coeruleoalba — Striped dolphin

• Stenella longirostris — Spinner dolphin

  • S. l. centroamericana — Central American spinner dolphin — Endemic

  • S. l. longirostris — Gray’s spinner dolphin

    • Whitebelly spinner (S. l. orientalis × S. l. longirostris)

  • S. l. orientalis — Eastern spinner dolphin — Endemic

• Steno bredanensis — Rough-toothed dolphin

• Tursiops truncatus — Common bottlenose dolphin

• T. t. truncatus — Common bottlenose dolphin

1.2 Appendix 2: Basic Oceanography of the Eastern Tropical Pacific

Major geographic and oceanographic features of the eastern tropical Pacific (ETP), including surface currents and water masses. Background shading is annual mean surface temperature (°C, 1980–2015); adapted from Fiedler et al. (2017)