Spinner dolphins, S. l. longirostris. These are the most common form and are found in most areas of the world, except for some parts of tropical Asia and the eastern tropical Pacific. Eastern spinner dolphins, S. l. orientalis, are found only in the waters of the far-eastern tropical Pacific Ocean, predominantly offshore. Central American spinner dolphins, S. l. centroamericana are found only in the coastal waters of the eastern tropical Pacific near Central America. Dwarf spinner dolphins, S. l. roseiventris are common in the waters of Southeast Asia and northern Australia. Whitebelly spinner dolphins (a hybrid between Gray's spinner dolphins and eastern spinner dolphins) are found in the eastern tropical Pacific. There is some overlap in the distribution of this morph and Gray's spinner dolphins. Adaptations to specific environmental conditions in different parts of the range are believed to underlie the emergence of subspecies and associated differences in physical appearance, mating system, and more. ("Spinner Dolphin", 2013; Norris, et al., 1994; Perrin and Mesnick, 2003; Perrin, et al., 2007; Perrin, et al., 1977; "Arkive", 2003), are found throughout the tropical and subtropical regions of the ocean, and may appear near the shores of continents, islands, and reefs. It ranges between 40°N and 40°S latitudes. There are 5 different geographic morphs: Gray's spinner dolphins, also known as Hawaiian spinner dolphins,
Spinner dolphins are most often found in warm, surface waters. Some populations are predominantly offshore, especially those in the eastern tropical Pacific. Other populations rest in shallow, coastal waters and during the day stay in sandy-bottomed bays or coral atolls. Although these dolphins tend to remain near the surface, their distribution often relates to the topography of the oceanic floor far below them; they are much more common where the bottom is rough than where there is a flat, abyssal plain. Similar to other ocean species, spinner dolphins often congregate near divergence zones at current margins and at current ridges because there is a high concentration of food organisms in these areas. They may go as deep as 400 m in search of prey. ("Spinner Dolphin", 2013; Dolar, et al., 2003; Norris, et al., 1994; "Arkive", 2003)
Compared to other cetaceans, this species is relatively small and slim. The body is torpedo-shaped but irregular. The dorsal fin is triangular in shape, but the tip can vary so that it is directed anterior (canted), posterior (falcate), or straight up (erect), depending on the geographic morph. Usually, these dolphins are tripartite, having a three-layered skin pattern with a dark grey back (dorsal), light grey sides, and a white belly (ventral), but the physical appearance is highly dependent on the geographic range of the subspecies, as described below. In Gray's spinner dolphins, adult females range from 1.39 to 2.04 m and adult males, from 1.60 to 2.08 m. They exhibit the tripartite pattern and have a falcate dorsal fin, a small or non-existent post-anal hump, and relatively small dorsal fin and flippers. In eastern spinner dolphins, adult females are between 1.52 and 1.93 m and adult males are from 1.60 to 1.99 m. They are a monotone steel grey color and have white patches around the genitals and axillary line. They have a relatively long beak and are highly sexually dimorphic, especially when compared to the other subspecies. Males have a strongly forward-canted dorsal fin, a medium to large post-anal hump, upturned fluke tips, and smaller testis compared to the males of other subspecies. In Central American spinner dolphins, adult females are between 1.75 and 2.11 m and adult males can reach over 2.16 m, making this the largest subspecies of spinner dolphin. Like eastern spinner dolphins, they are a monotone grey color, but they lack the white patches and have an even longer beak than eastern spinner dolphins. They also have a pronounced sexual dimorphism similar to eastern spinners, that is, a strongly canted dorsal fin, a large post-anal hump, and upturned fluke tips. In dwarf spinner dolphins, adult females range between 1.38 and 1.45 m and adult males are between 1.29 and 1.58 m, making this the smallest subspecies. They have a tripartite color pattern, an erect or falcate dorsal fin, and relatively large flippers and dorsal fin. Sexual dimorphism in this subspecies is not marked. Whitebelly spinner dolphins appear to be a hybrid between Gray's spinner dolphins and eastern spinner dolphins and it is a morphological intermediate between the two. The dorsal fin is either falcate or erect, and males have large testis. Sexual dimorphism is low. There are several differences in physical appearance between adult female and male spinner dolphins. In females, the posterior portion of the body is longer, thus allowing room for the calf to grow and develop; the girth is smaller, especially near the anus; and both the dorsal fin height and fluke span are smaller. It is believed that the degree of sexual dimorphism in this species is correlated with mating system. ("Spinner Dolphin", 2013; Norris, et al., 1994; Perrin, et al., 2007; Perrin, 2009; "Arkive", 2003; Yousuf, et al., 2010)
The mating system varies among subspecies and is related to the degree of sexual dimorphism, which in turn reflects geographical range. Male testes size and the degree of sexual dimorphism in the shape of the dorsal fin are good indicators of the mating system.
In eastern spinner dolphins, which have higher sexual dimorphism and smaller testis than other subspecies, pre-mating competition for access to females is intense, which on the population level leads to fewer males mating, and what is probably a polygynous mating system.
In contrast, whitebelly spinner dolphins have much lower sexual dimorphism and much larger testis relative to body size. A larger proportion of males mate, which makes sperm and post-mating competition much more important. The result is a polygynandrous, or promiscuous, mating system.
Based on this, we can infer that Gray's spinner dolphins are most likely polygynandrous as well because their behavior and reproduction is very similar to that of whitebelly spinner dolphins. Similarly, one can hypothesize that the Central American spinner dolphin, which has a high degree of sexual dimorphism, is probably polygynous, and the Dwarf spinner dolphin, which has reduced sexual dimorphism, is probably polygynandrous.
Seasonality in spinner dolphin breeding does exist, but it varies geographically. For example, northern whitebelly spinners are best prepared for breeding in February and from July to August, whereas eastern spinners are most prepared between June and July. The calving interval is roughly every 3 years. The average length at birth is 77.0 cm and the gestation period is approximately 10.6 months. It normally takes between 10.1 and 17.5 months to wean the calves. The ages at which females and males reach sexual maturity are different. In females, it takes approximately 5.0 years on average, and for males, from 6.0 to 11.5 years. It appears that the pregnancy rate per individual decreases after 11.5 years of age on average. Furthermore, it seems that older females have fewer calves, but their calves are nursed for a longer period of time. (Larese and Chivers, 2009; Norris and Dohl, 1980; Perrin, et al., 1977)
Young calves are usually always near their mother or another adult. The social bond between mother and offspring continues throughout life. It appears that males provide no parental investment. (Larese and Chivers, 2009; Norris and Dohl, 1980; Perrin, et al., 1977)
Spinner dolphins are estimated to live about 20 years on average, but the maximum age recorded in the wild was 26 years. ("Age structure of female eastern spinner dolphins (Stenella longirostris orientalis) incidentally killed in the Eastern tropical Pacific tuna purse-seine fishery", 2002)
Spinner dolphins are well known for their aerial behaviors, which can be divided into different actions, including spins, leaps, tail-over-head leaps, backslaps, headslaps, noseouts, tailslaps and any combination of these. These behaviors are good predictors of daily activity patterns, can reveal the activity state of a school, and play an essential role in communication (see Communication). This is a social species and individuals form schools, which are ever-changing and can vary in size. Physical contact between individuals while swimming is common. Some populations of Eastern spinner dolphins are even known to school with populations of spotted dolphins. It is believed that this provides protection during resting, since spotted dolphins feed in the daytime and spinner dolphins feed at night. This resting strategy works for eastern spinner dolphins because they occur primarily offshore. However, Gray’s spinner dolphins, which spend much more time near the coast, often spend the day in “rest areas,” which are shallow areas generally less than 50 m deep in which dolphins can enter into what is known as a ‘quiescent period.’ These rest areas are relatively close to drop-off’s of very deep water so that the animal can be ready to hunt for prey at nightfall. ("Spinner Dolphin", 2013; Norris and Dohl, 1980; Norris, et al., 1994; "Arkive", 2003)
The distinct aerial behaviors for which spinner dolphins are known function primarily to make noise and thus communicate with other individuals within a school, specifically when individuals are not within each other’s sight. Most, if not all of the different behaviors make a noise which is believed to travel in all directions. These dolphins also have a sound generation and beaming apparatus that sends vocal signals via clicks (a type of echolocation) in a single direction. (Norris and Dohl, 1980)
Spinner dolphins feed primarily on a wide variety of mesopelagic fish, especially lanternfish (Order Myctophida). They also eat squid (Nototodarus and Mastigoteuthis) and even some crustaceans that migrate every night from the ocean depths to the surface in search of their own food. Spinner dolphins may go as deep as 400 m in search of prey. (Dolar, et al., 2003)
Spinner dolphins are often attacked by sharks. The wound and scar marks found on some dolphins indicate attack by the small squaloid shark and other larger sharks. Spinner dolphins are also threatened by the chase and capture techniques used by commercial fisherman to catch yellowfin tuna. (Edwards, 2007; Norris and Dohl, 1980)
Yellowfin tuna follow spinner dolphins in search of food. This relationship is probably mutualistic. There are some known parasites in other members of genus Stenella (specifically in striped dolphins), but no studies have specifically focused on spinner dolphins. These parasitic organisms include Strobilocephalus triangularis, Phyllobothrium delphini, Tetrabothrius forsteri, Sarcocystis sp., Anisakis simplex, Halocercus lagenorhynchi, Crassicauda sp., Monorygma grimaldii and Oschmarinella rochebruni. (Dailey and Stroud, 1978; Edwards, 2007; Gibson, et al., 1998; Norris and Dohl, 1980)
Spinner dolphins, along with spotted dolphins, are often associated with yellowfin tuna because the tuna follow them in search of food. As a result, tuna purse-seining fishermen get a good indication of where to find the highly valued tuna by locating schools of these spinner dolphins, specifically those that inhabit the eastern tropical Pacific Ocean. These dolphins are also an important in ecotourism. ("Spinner Dolphin", 2013; Edwards, 2007; Norris and Dohl, 1980; Norris, et al., 1994; "Arkive", 2003)
There are no known adverse effects ofon humans.
Spinner dolphin populations are affected as by-catch in the yellowfin tuna purse-seine fisheries. Although the number of direct dolphin mortalities has decreased since the 1960s, from hundreds of thousands per year to 5000 dolphins annually in the 1990s, the population has not recovered as expected. ("The IUCN Redlist of Threatened Species", 2013)
Cassandra Mac (author), Michigan State University, Barbara Lundrigan (editor), Michigan State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
generally wanders from place to place, usually within a well-defined range.
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
an animal that mainly eats fish
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
having more than one female as a mate at one time
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.
mainly lives in oceans, seas, or other bodies of salt water.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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