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Animal sexual behavior takes many different forms, even within the same species and the motivations for and implications of their behaviors have yet to be fully understood. Bagemihl's research shows that homosexual behavior, not necessarily sexual activity, has been documented in about 500 species as of 1999, ranging from primates to gut worms.
Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft).
More than 99 percent of all species, amounting to over five billion species, [7] that ever lived on Earth are estimated to be extinct. [8] [9] Estimates on the number of Earth's current species range from 10 million to 14 million, [10] of which about 1.2 million have been documented and over 86 percent have not yet been described. [11]
A number of factors determine how quickly any changes may occur in a species, but there is not always a desire to improve a species from its wild form. Domestication is a gradual process, so there is no precise moment in the history of a given species when it can be considered to have become fully domesticated.
The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms.This number, along with the visual appearance of the chromosome, is known as the karyotype, [1] [2] [3] and can be found by looking at the chromosomes through a microscope.
In several species, females and juveniles exhibit a light-coloured coat, while those of males darken with age. As in the wildebeest, the coat may be marked with prominent or faint stripes. In some species such as the addax, the coat colour can vary by the season. [33] Scent glands and sebaceous glands are often present. [26]
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One species, the scimitar oryx, was once extinct in the wild, though populations are now recovering. The bluebuck went extinct in the last 200 years, and the aurochs went extinct 400 years ago. A third extinct species, the red gazelle, potentially never existed, [2] and the kouprey is potentially extinct, with no sightings since 1969. [3]