All adaptations have a downside : horse legs are great for running on grass , but they ca n't scratch their backs ; mammals ' hair helps temperature , but offers a niche for ectoparasites ; the only flying penguins do is under water .
All adaptations have a downside : horse legs are great for running on grass , but they can not scratch their backs ; mammals ' hair helps temperature regulation , but offers a niche for ectoparasites .