There's a question for the Consult-Ant this week. (The “Consult-Ant” started on the Leaping from the Box website, where I answered questions about ants and ant farms. From now on I will post the answers here, and when Karen has time she will also post the answers on her site.)
This comes from Ol Donyo Orok Hill in Southern Kenya (on the border with Tanzania) at an altitude of around 4500 feet. I found these ants all dead at the entrance of a hole (what I assumed to be their nest). This was far enough away from any human habitation, farming, or other activity (other than some goat herding) that I ruled out the use of insecticide. There were a few more dead, spread out of the the space of about 1 square foot, but the attached photo shows the vast majority of them in a small, dense pile. Some of their exoskeletons were coming apart (especially the tips of the abdomens), but it didn't look like ruptures from predation, rather just decay.
The stem of the plant in the photo is Solanum incanum (Poison/Sodom Apple), which is both indigenous to the area and poisonous to humans and domestic stock, but I doubt that ants would eat something familiar to them which if it was poisonous. I don't know what species of ant they are, but the closest I can get is some Pachycondyla sp. (though I'm really not sure).
Do you know what may have caused this seemingly sudden mass die-off?
Taking a look at Ants of Africa for ants in the genus Pachychondyla, your identification seems in the right ballpark. Are there any experts out there who can tell us more about what kind of ants these are?
As for what has happened, when I've seen similar piles after a couple of events. Most commonly, either the ant colony had a battle with a neighboring colony or there has been some adverse weather that has caused a mass die off.
If the battle was with a neighboring colony of ants, not all the causalities will be torn apart, particularly if the damage was caused by stings or chemical weapons.
Flooding of portions of the nest during heavy rains can also cause big die offs in species that nest underground.
Readers: Do you have any suggestions why these ants might have died?
Please leave your thoughts in the comments.