Mate Boost

A Mate Boost is when a creature has at least one nearby allied creature of the same species and opposite sex, which gives the affected creatures a Mate Boost. Creatures that are Mate-Boosted will receive a bonus to their damage and resistance of about 33%, and wild creatures will be more willing to come to the aid of their mate. If a creature is Mate-Boosted, a pink heart will appear above the creature's name.

Eggs
Mate Boost allows female creatures to lay unfertilized Eggs by themselves.

Wild
Wild Mate-Boosted creatures will be more dangerous. A Male and Female Stegosaurus will be more dangerous than 2 Males or 2 Females of the same level. When hunting, be aware of whether the creature you are planning to attack is Mate-Boosted, or if there are any of its species nearby that could potentially add a Mate Boost if they got closer.

Domesticated
Domesticated creatures can still give each other the Mate Boost effects. Taming a Male and Female of the same species is beneficial for any creatures that you intend to use as fighters or hunters.