"porpoises and dolphins...are as different as horses and cows or dogs and cats."Porpoises are in the Family Phocoenidae, which contains 7 species. This is a separate family from that of dolphins, which are in the larger family Delphinidae, which contains 36 species. Porpoises are usually smaller than dolphins, and have a blunter snout, whereas dolphins usually have a pronounced "beak."
Porpoises are toothed whales.
Like dolphins, and some larger whales such as orcas and sperm whales, porpoises are toothed whales - also called odontocetes. Porpoises have flat or spade-shaped, rather than cone-shaped, teeth.
Porpoises look different from other cetaceans.Compared to many cetacean species, porpoises are small - no porpoise species grows larger than about 8 feet in length. These animals are stocky, and don't have a pointed rostrum. Porpoises also exhibit paedomorphosis in their skulls - this large word means that they retain juvenile characteristics even in an adult. So the skulls of adult porpoises look like juvenile skulls of other cetaceans. As mentioned above, porpoises also have spade-shaped teeth, an easy way (well, if you see one with its mouth open) to tell them apart from dolphins.
Porpoises have bumps on their back.All porpoises except for the Dall's porpoise have tubercles (small bumps) on their back, on the front edge of their dorsal fin or dorsal ridge. It is not known what the function of these tubercles is, although some have suggested that they have a function in hydrodynamics.
Porpoises grow quickly.
Porpoises grow quickly, and reach sexual maturity early. Some can reproduce when they are 3 years old (e.g., the vaquita and harbor porpoise) - you can compare that another toothed whale species, the sperm whale, who may not become sexually mature until its teens, and may not mate until it is at least 20 years old.
In addition to mating early, the reproductive cycle is relatively short, so porpoises may calve annually. So, it is possible for a female to be pregnant and lactating (nursing a calf) at the same time.
Unlike dolphins, porpoises don't usually gather in large groups.Porpoises don't seem to gather in large groups like dolphins - they tend to live individually or in small, unstable groups. They also don't strand in large groups like other toothed whales.
This might go in the "little-known facts about porpoises" category. To be reproductively secure, harbor porpoises need to mate with multiple females during the mating season. To do this successfully (i.e., produce a calf), they need lots of sperm. And to have lots of sperm, they need big testes. The testes of a male harbor porpoise may weigh 4-6% of the porpoise's body weight during the mating season. A male harbor porpoise's testes usually weigh about .5 pound, but may weigh more than 1.5 pounds during the mating season.
This use of lots of sperm - rather than physical competition between males for females - is known as sperm competition.