6 pages/≈1650 words
The Biological Comparison between Dolphins and Whales (Essay Sample)
Biological Comparison between Dolphins and Whales Student’s Name Institutional Affiliation Introduction About seventy million years ago, the terrestrial ancestors of dolphins and whales re-entered the marine environment where life originally started. This had a profound effect on their locomotion, diet social behavior and reproduction; for example the breathing system in the waters rendered useless the olfaction of the cetaceans. In water, both animals were met with vision challenge, since light doesn’t travel very well in water; they had to adapt. They started to communicate through low frequency sounds. Dolphins have evolved echolocation similar to sonars and bio sonar communication method used by bats. Large baleen whales also evolved low frequency calls suited for long distance communication: they produce and hear the low-frequency calls(Tyack & Clark, 2000). A comparison of the dolphins and whales is given in the discussion below. Culture The ethnographic evidence for cetacean culture is remarkably strong, given the substantial difficulties of studying whales and dolphins in the wild. In only four (of 80) species of Cetacea have more than a handful of papers on behavior been published (Mann 1999): the bottlenose dolphin (Tursiops spp.), the killer whale (Orcinus orca), the sperm whale (Physeter macrocephalus) and the humpback whale (Megaptera novaeangliae). These four species span a wide range. For instance, their sizes range from 2m (bottlenose dolphins) to 16m (sperm whales), their habitat from protected coastal lagoons (bottlenose dolphins) to deep oceanic waters (sperm whales), and trophic levels from partial planktivores (humpback whales) to top predators (killer whales). The four species have diverse social systems: humpback whales live in loose fission- fusion societies (Clapham 1993); both sexes of killer whale generally remain within their natal matrilineal groups from which males disperse to lead quite solitary adult lives (Whitehead and Weilgart 2000). From the ethnographic perspective, cultural transmission is deduced from spatial, temporal or social patterns of variation in behavior that are not consistent with genetic or environmental determination or individual learning (Rendell and Whitehead 2001). Reproduction in Dolphins and small whales of the family Delphinidae Many populations of dolphins and small whales are exploited directly or incidentally (IWC, 1976) and must be assessed and managed. Gestation period is one of the least variable reproductive parameters on a within-species basis in delphinids and in mammals in general (Kiltie, 1982). The published estimates of gestation period range from about 10 months to over 16 months. There appears to be a relationship between length of gestation, fetal growth rate and birth size or brain weight at birth, such that larger delphinids (e.g. Orcinus and Globicephala) have longer gestation periods than do the smaller forms (Stenella or Lagenorhynchus); (Perin and Reilly). The estimates of age at attainment of sexual maturity are available for 11 of roughly 32 species of delphinids. They range from 3 (Delphinus delphis) to 16 years (Orcinus orca), (Perrin and Myrick 1980). The ovaries of cetaceans are unusual in that scars (corpora albicantia, or CAs) resulting from ovarian events persist for years and probably indefinitely in at least some species e.g. Globicephala mocrorhynchus (Marsh and Kasuya, 1984) and perhaps G.melaena (Sergeant, 1962). Although ovarian scars do not provide a reliable record of fecundity, they are at least for younger animals, a relative index of ovarian activity, i.e. estrus and ovulation, and thus potentially are still valuable in characterizing the structures, breeding systems and dynamics of populations. Therefore, some delphinids (e.g. Globicephala melaena, Pseudorca crassidens, and perhaps Stenella spp. and Lagenorhynchus obliquidens) are reflex ovulators, i.e. require copulation and/or presence of a mature male to trigger ovulation (Harrison, 1969 and Harrison et al. 1972). Adaptation Coastal waters of the California current support a rich and diverse marine fauna, including at least 30 species of cetaceans (Leatherwood et al. 1982a). It has long been known that the abundance of some cetacean species changes on both seasonal and inter-annual time scales (Norris and Prescott 1961, Leatherwood and Walker 1979). In 1975- 1978and 1980- 1983, year- round aerial surveys were conducted in southern and north-central California, respectively, to investigate seasonal patterns in cetacean distribution and abundance (Dohl et al. 1980, 1983, 1986). Gray whales (Eschrichtius robustus), short- finned pilot whales (Globicephala macrorhynchus) and a northern right whale (Eubalaena glacialis) were observed only during winter period. Striped dolphins (Stenella coeruleoalba) and Baird’s beaked whales (Berardius bairdii) were seen only during the summer period (Forney and Barlow 1998). However, no significant seasonal difference in abundance were identified for five cetacean species: offshore bottlenose (Tursiops truncatus), sperm whales (Physeter macrocephalus), Dall’s porpoises (Phocoenoides dalii), Killer whales (Orcinus orca), and humpback whales (Megaptra novaeangliae). All of the remaining delphinid species are substantially more abundant during winter than during summer (Forney and Barlow 1998). This included the cool-temperate species, pacific white- sided dolphins (Lagenorhynchus obliquidens) and northern right whale dolphins (Lissodelphis borealis) as well as the warm-temperate to tropical common dolphins (Delphinus spp) and Risso’s dolphins (Grampus griseus). With the exception of Risso’s dolphin, all of these delphinids also exhibit significant differences in distribution between seasons. It has been reported that the population of blue whales in west coast of iceland(Bigg, Olesiuk, Ellis, Ford, & Balcomb, 1990), they have also been sighted in in a few other places such as western north Atlantic, southern coast of newfoundland and Gulf of Maine (Bigg et al., 1990) Feeding During the past several years, stomachs of stranded, harpooned, or incidentally captured to purse- seine and gillnet fisheries cetaceans have been routinely examined and have saved otoliths, cephalopod beaks and other identifiable remains that have been encountered (Fitch 1968). The stomachs of seven species (pygym sperm whale, pacific spotted dolphin, spinner dolphin, white- sided dolphin, common dolphin, northern right whale dolphin, and Gulf of California harbor porpoise) have yielded otoliths and all but Gulf of California harbor porpoise also contain cephalopod beaks (Fitch 1968). There has been much speculation on whether the otoliths found in the folds of cetacean stomachs represent the remains of a single meal, or are an accumulation from several days of feeding. The researcher concludes that not even 15,191 otoliths (representing more than 7596 fishes) reported by Schmidt (1923) represented a ‘full’ meal for the Delphinus that they come from (Fitch 1968). This conclusion was based on the fact that it takes from 7000 to 10000 adult Benthosema panamense to fill a gallon jar, which is much smaller volume than a distended dolphin stomach. Over the years, anecdotal observations and circumstantial evidence have accumulated indicating that killer whales feed upon a variety of marine mammals and fish (Ford et al. 1998). Early reports of killer whale predation mentioned the harbor seal (Phoca vitulina), Steller sea lion (Eumetopias jubatus), Dall’s porpoise (Phocoenoides dalii), gray whale (Eschrichtius robustus), and minke whale (Balaenoptera acutorostrata) (Sheffer and Slipp 1948; Hancock 1965; Pike and MacAskie 1969). Behavior Cetaceans are highly predated upon by killer whales. They usually hunt in packs, which makes them very effective predators, to a point they can even hunt: dolphins, porpoises and large whales (blue and sperm whales) (Ballance, 2006). Other predators according to Ballance (2006) include: large sharks, false killer, pygmy killer and pilot whales. Large whales frequently show signs of killer whale tooth rake mars on their: flippers, fins and flukes. About a one-third of the bottlenose dolphins from eastern Australia have shark bite scars; which suggest that they regularly meet predators. It has been thought that large whales migrate annually to reach their calving grounds: areas where killer whale densities are low. The prey species are thought to have evolved schooling behavior as a defense mechanism: where they swim together in large groups (Balance, 2006). These behavioral adaptations according to Balance (2006) have highly affected their: distribution, abundance, social structure, timing and reproduction modes, foraging strategies and speciation patterns. Feeding cetaceans provide feeding opportunities for seabirds by driving prey to the surface sometimes injuring or disorienting it. In one study up to 87% of all feeding individuals from four seabird species in the Bearing Sea associated with gray whale mud plumes (Obst and Hunt, 1990). Killer whales strongly believe in the idea of gro...
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