Class Mammalia
Order Cetartidoactyla
Family Delphinidae 

Delphinidae is the cetacean family with the largest number of species, a total of 38 (refer to table below), and they are well known for their high levels of intelligence. While most of the members of this family are referred to as dolphins, a group of members referred to as the blackfish are called whales (the term whale is used to imply a large size and not a particular family of species) (1) (2). According to the IUCN Red List, 7 species of dolphins are threatened, while some lack sufficient data to be assessed.

The larger dolphins, collectively referred to as the blackfish are usually found in deep oceanic waters, and do not follow long migratory routes. Rather, their movement patterns are governed by food or climate conditions.

The remaining members of the Delphinidae family, are more coastal, with some even found in estuarine waters (1). A dolphin’s diet consists mainly of fish and squid.

The largest dolphin is the Killer Whale (Oricnus orca), a widely distributed species across the globe. Adult male killer whales can grow up to 31.2 feet (3).

Bottlenosed dolphins absorb 80% of the oxygen in one breath (humans only absorb 20%) and store it for their deep dives (4). They are known to dive up to a maximum depth of 650 m (1). See “Cetaceans” Section under “Marine Mammals” to learn more about the adaptations of dolphins underwater.

Species under the Delphinidae family and their status on the IUCN Red List of Threatened Species

Common name Species IUCN Red List Status
1 Killer Whale Orcinus orca DD
FOUND IN SRI LANKAN WATERS
2 Common Bottlenose Tursiops truncatus LC
3 Risso’s Dolphin Grampus griseus LC
4 Spinner Dolphin Stenella longirostris LC
5 Pantrophical Spotted Dolphin Stenella attenuata LC
6 Striped Dolphin Stenella coeruleoalba LC
7 Melon-headed Whale Peponocephala electra LC
8 Pygmy Killer Whale Feresa attenuata LC
9 Short-finned Pilot Whale Globicephala macrorhynchus LC
10 Indo-Pacific Bottlenose Dolphin Tursiops aduncus DD
11 Indo-Pacific Humpback Dolphin Sousa chinensis VU
12 Long-beaked Common Dolphin Delphinus capensis DD
13 False Killer Whale Pseudorca crassidens NT
14 Fraser’s Dolphin Lagenodelphis hosei LC
15 Rough-toothed Dolphin Steno bredanensis LC
16 White-beaked dolphin Lagenorhynchus albirostris LC
17 Commerson’s Dolphin Cephalorhynchus commersonii LC
18 Heaviside’s Dolphon Cephalorhynchus heavisidii NT
19 Hector’s Dolphin Cephalorhynchus hectori EN
20 Long-finned Pilot Whale Globicephala melas LC
21 Short-beaked Common Dolphin Delphinus delphis LC
22 Irrawaddy Dolphin Orcaella brevirostris EN
23 Northern Right Whale Dolphin Lissodelphis borealis LC
24 Northern Right Whale Dolphin Lissodelphis borealis LC
25 25 Indian Ocean Humpback Dolphin Sousa plumbea EN Sousa plumbea EN
26 Clymene Dolphin Stenella clymene LC
27 Atlantic Humpack Dolphin Sousa teuszii CR
28 Southern Right Whale Dolphin Lissodelphis peronii LC
29 Australian Snubfin Dolphin Orcaella heinsohni VU
30 Hourglass Dolphin Lagenorhynchus cruciger LC
31 Hourglass Dolphin Lagenorhynchus cruciger LC
32 Dusky Dolphin Lagenorhychus obscurus DD
33 Atlantic White-sided Dolphin Lagenorhychus acutus LC
34 Tucuxi Sotalia fluviatilis DD
35 Guiana Dolphin Sotalia guianensis NT
36 Australian Humpback Dolphin Sousa sahulensis Sousa sahulensis VU
37 Pacific White-sided Dolphin Lagenorhynchus obliquidens LC
38 Chilean Dolphin Cephalorhynchus eutropia NT

Communication and behaviour

Most dolphins demonstrate friendly behaviour even in the wild. Many instances have been reported where dolphins acted to protect humans at sea either from attacks from predators or drowning. They have also assisted boats in rocky conditions (3).

Dolphins, similar to other cetaceans, have an excellent sense of hearing and communicate using sound waves. Using echolocation and high frequency calls (whistles and clicks), dolphins navigate their waters and communicate with others. Their whistles can be at high, medium or low frequencies. Low frequency whistles transmit in all directions, however when frequencies increase they become more directional. This can be inferred as a form of language. Furthermore, while blowing air out of their blowholes, dolphins let out an unique whistle, which acts as their form of identity. The frequencies at which dolphins often communicate are not within (are beyond) the human hearing range. Other forms of communication dolphins display include slapping of their flippers or snapping of their jaws (1).

As with most other cetaceans, dolphins travel in pods which are to a certain extent managed by a few individual dolphins referred to as socialites.

Mating and reproduction

In his book, “Out of the Blue”, Martensytn (2013) describes how groups (gangs) of male dolphins work together to separate and tire out an ovulating female until she mates with one or more members of the gang. Marks and scars on Risso’s male dolphins also indicate fighting among males during mating periods.

Most small dolphins have a gestation period of 11-12 months and a lactation period of 1 month. Blackfish have a longer gestation period which lasts 12-17 months and a much longer lactation period of 24-36 months. Small dolphins demonstrate a reproduction rate of 2-3 years (1)(2). Female short-beaked common dolphins are said to reach sexual maturity around 7.9 years (5).

Dolphins in Sri Lanka

Although cetaceans have been recorded in Sri Lankan waters as far as the 14th century, it was not well known across the country until the 1980s (3). The table above identifies the 15 species of dolphins found in Sri Lankan waters. Out of these 15 species, other than the Orca, Short-finned pilot whale, and False killer whale who are nomadic blackfish, all other 10 members of the Delphinidae family are residents of Sri Lankan waters (1).

In Sri Lanka, the Fauna and Flora Protection Ordinance protects all 15 species under the Delphinidae family where the killing, injuring, possession of, sale of, purchase of or transport of them is punishing by fine (between LKR 20,000 to 50,000), imprisonment (between 2-5 years) or both. The Fisheries Act of 1996 also protects cetaceans of Sri Lanka.

Threats to dolphins

Large numbers of (small) whales and dolphins face death (as by-catch) when entangled in fishing nets. Although illegal in Sri Lanka, sometimes dolphins are also directly hunted using hand harpooning (3). In Sri Lanka, past studies (Dayaratne and Joseph, 1993; Ilangakoon, 1989; 1997; 2007; Ilangakoon et al., 2000b cited in Ilangakoon 2012) estimate that spinner dolphins encapsulate over 50% of the recorded cetacean bycatch (6). Dolphins have also been hunted, not just for food, but also to reduce competition for fishermen in some parts of the world (7).

Destructive fishing practices (blast fishing, cyanide fishing, mechanised bottom trawling) as well as pollution from land based activities and off-shore activities degrade and disrupt the habitats of these dolphins (6).”

Unsustainable and unregulated tourism practices also pose as a threat to these marine mammals (6) (7). Resting, feeding or socialising activities may be disrupted when tourist boats persistently follow marine mammals in large numbers (7). In Sri Lanka, where whale and dolphin watching industry has grown exponentially, this is a particular threat as there is minimal control over boat operators. In general, the lax implementation of the law (due to a lack of finance and human resources) in Sri Lanka worsens the threats imposed by above factors as monitoring is poor and punishment is not always enforced (6).

Offshore activities such as drilling and from heavy vessels travelling at sea add to underwater noise disrupt lives of these marine mammals by either masking their natural sounds or even cause damage to their sense of hearing. As species whose lives depend greatly on sonic communication, these noise sources have the ability to strongly disrupt the behaviours of these species (7). Shipping vessels can also pose as a threat to cetaceans, especially large whales, through ship strikes (6).

References

  1. Martenstyn, Howard. Out of the Blue – A . Colombo : s.n., 2013. 978-955-54534-0-0.
  2. IUCN Red List. The IUCN Red List of Threatened Species. [Online] [Cited: 28 February 2019.] https://www.iucnredlist.org/.
  3. Illangakoon, Anouk. Whales & Dolphins of Sri Lanka. Colombo : WHT Publications (Private) Limited, 2002. 955-9114-28-X.
  4. Savage, Stephen. Endangered Species – Dolphins & Whales . New Jersey : Chartwell Books, 1994. 1-55521-578-5.
  5. Growth and reproduction of female short-beaked common dolphins, Delphinus delphis, in the eastern tropical Pacific. Danil, K and Chivers, S J. s.l. : Canadian Journal of Zoology , 2007, Vol. 85.
  6. A review of cetacean research and conservation in Sri Lanka. Ilangakoon, Anoukchika D. 2, s.l. : Journal of Cetacean Research and Management, 2012, Vol. 12.
  7. Reeves, Randall R, et al. Dolphins, Whales and Porpoises: 2002-2010 Conservation Action Plan for World’s Cetaceans. Cambridge : IUCN, Gland, Switzerland and Cambridge, UK, 2003. 2-8317-0656-4.