Giraffe In A Swimming Pool

The other main theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests to establish dominance and obtain access to sexually receptive females. In support of this theory, males have proportionally larger necks than females, and males with longer, bigger necks are more successful in dominance displays and courtship behavior. However, a major criticism of this theory is that it fails to adequately explain why female giraffes also have long necks.
Circulatory system
Modifications to the giraffe's structure have evolved, particularly to the circulatory system. A giraffe's heart, which can weigh up to 10 kg (22 lb) and measure about 60 cm (2 ft) long, must generate approximately double the normal blood pressure for an average large mammal to maintain blood flow to the brain. In the upper neck, a complex pressure-regulation system called the rete mirabile prevents excess blood flow to the brain when the giraffe lowers its head to drink.