Ostriches share symbiotic relationships with gazelles based on each animal's ability to detect predators. This relationship is referred to in biology as mutualism, defined as the way two organisms that are members of separate species exist in a mutually beneficial relationship.
Ostriches and gazelles are each capable of identifying threats the other would not notice in time. Ostriches have acute eyesight, which helps make up for their poor hearing and sense of smell, and their height enables them to spot predators from afar.
Gazelles also have keen eyesight. However, since they're much shorter than ostriches, gazelles cannot see predators over tall grasses and shrubs. Gazelles hold up their end of their symbiotic relationships with ostriches by employing their keen senses of smell and hearing to detect threats. When an ostrich becomes alarmed after seeing a predator over the foliage, gazelles know they need to be on the lookout as well. Likewise, when an ostrich sees a gazelle fleeing after smelling or hearing a predator, the ostrich knows to flee in the same direction.
This behavior is not limited to ostriches and gazelles. Ostriches have been known to share the same relationship with other prey animals such as zebras and antelopes.