[3] P. ferruginea is an obligate plant ant that occupies at least five species of acacia (A. chiapensis, A. collinsii, A. cornigera, A. hindsii and A. sphaerocephala).

[2] The symbiotic relationship begins when a newly mated queen is attracted by the odour from the tree and starts nesting inside the large hollow acacia thorns.

When the ants perceive one of these compounds, trans-2-hexenal, they aggressively swarm to the damaged leaf and attack creatures of all sizes attracted by the acacia leaves, killing insects such as crickets and stinging the mouths and tongues of mammals such as goats.

However, not all is mutually beneficial: the ants relish the sweet honeydew produced by scale insects which suck the sap of the acacia and therefore protect them as well, effectively providing entry to diseases.

[2] The development of myrmecophytism ("ant symbiosis") and spininess in African and New World acacia species was an adaptation to the presence of large faunas of effective browsing mammals.

As the number of ants reaches 50–100, workers start patrolling the open plant surface next to their home thorn, and as the population reaches 200–400, workers become more aggressive and attack other smaller nearby colonies, ward off phytophagous insects that make landing attempts near the thorn more effectively.

[4] In old colonies the queen is physogastric (i.e. has a swollen, membranous abdomen), heavily attended by workers, and accompanied by hundreds of eggs and young larvae.