Final answer:
African army ants are efficient predators that form large colonies capable of killing insects and small vertebrates. The largest animal they can kill is not definitively known, as it depends on various factors. Anecdotal reports exist of army ants killing trapped small vertebrates.
Step-by-step explanation:
Ants live together in large colonies and demonstrate remarkable social organization and collective hunting ability. African army ants, in particular, are known for their efficiency as predators, capable of taking down insects and small vertebrates such as rodents and birds. Typically, the sheer number of ants in a super-colony and their cooperative hunting behavior enable them to overcome much larger prey.
However, the exact size limit of animals army ants can kill is not well-established, as it largely depends on the circumstances, including the size of the ant colony, the health and mobility of the prey, and environmental factors.
There have been anecdotal reports, such as the one mentioned by David Attenborough, of army ants killing trapped snakes. This suggests they have the potential to kill small vertebrates when the conditions are favorable for the ants. Still, for large and mobile animals, it's unlikely for ants, even in large numbers, to be successful in such an endeavor as the animals would normally manage to escape.
It is ethically and scientifically inappropriate to test the limits by placing animals in harm's way, so observations tend to be serendipitous rather than experimental.