<P> Dolphins engage in acts of aggression towards each other . The older a male dolphin is, the more likely his body is to be covered with bite scars . Male dolphins can get into disputes over companions and females . Acts of aggression can become so intense that targeted dolphins sometimes go into exile after losing a fight . </P> <P> Male bottlenose dolphins have been known to engage in infanticide . Dolphins have also been known to kill porpoises for reasons which are not fully understood, as porpoises generally do not share the same diet as dolphins and are therefore not competitors for food supplies . The Cornwall Wildlife Trust records about one such death a year . Possible explanations include misdirected infanticide, misdirected sexual aggression or play behaviour . </P> <P> Dolphin copulation happens belly to belly; though many species engage in lengthy foreplay, the actual act is usually brief, but may be repeated several times within a short timespan . The gestation period varies with species; for the small Tucuxi dolphin, this period is around 11 to 12 months, while for the orca, the gestation period is around 17 months . Typically dolphins give birth to a single calf, which is, unlike most other mammals, born tail first in most cases . They usually become sexually active at a young age, even before reaching sexual maturity . The age of sexual maturity varies by species and gender . </P> <P> Dolphins are known to display non-reproductive sexual behavior, engaging in masturbation, stimulation of the genital area of other individuals using the rostrum or flippers, and homosexual contact . </P>

How many dolphins are born at a time