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 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 m in search of prey. 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. 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. 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. 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. 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.
Hawaiian Spinner Dolphins off the coast of Maui and Lanai
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. 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.
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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.
Sexual interactions can occur in male-female pairs and male-male pairs. Perrin and Mesnick, ; Perrin, ; Yousuf, et al. 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 It normally takes between The ages at which females and males reach sexual maturity are different. In females, it takes approximately 5. It appears that the pregnancy rate per individual decreases after Furthermore, it seems that older females have fewer calves, but their calves are nursed for a longer period of time. Larese and Chivers, ; Norris and Dohl, ; Perrin, et al.
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. Spinner dolphins are estimated to live about 20 years on average, but the maximum age recorded in the wild was 26 years. 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.
10 Facts About Spinner Dolphins
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. The home range size of spinner dolphins is not known and probably differs among subspecies. 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, 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 m in search of prey. Dolar, et al. 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, ; Norris and Dohl, Yellowfin tuna follow spinner dolphins in search of food.
This relationship is probably mutualistic.
evuxavykypit.tk 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.
Dailey and Stroud, ; Edwards, ; Gibson, et al. Spinner dolphins, along with spotted dolphins , are often associated with yellowfin tuna because the tuna follow them in search of food.
Related The Hawaiian Spinner Dolphin
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