ants and aphids
ants herding aphids.
"The mutualism shared between ants and aphids is extraordinary because of the 'intelligence' that the ants demonstrate in preserving their relationship with the aphids, rather than killing them for food. This behavior could have evolved over millions of years, or over a much shorter period of time. Aphids obviously pose no threat to an ant colony. So why do ants not simply attack the aphid colonies that they encounter to obtain nourishment? Sometime in the past, it became evident to various species of ants that maintaining an aphid colony would be much more beneficial to them than simply eating these aphids. An aphid repeatedly produces honeydew throughout its life; therefore, it is better for the ants to protect an aphid colony and reap the benefits of its nutritious honeydew, than to kill an aphid and obtain the one-time benefit.
It is also hypothesized that another mutual benefit has arisen from this symbiosis. In the winter time, aphids can serve very little purpose to the ants because of the scarcity of edible plant materials. During this season, ants bring their aphids within a chamber in the hive to protect them from the cold. However, this may also benefit the ant colony by providing extra body heat to the colony through the presence of the aphids."