Hello all,
This past summer I lifted up a mat that had been sitting on the ground and found that an ant colony had been storing larvae and pupae there in two separate aggregations. In the picture, pupae are at the top, larvae at the bottom.
I watched them for ~20 minutes and observed that the workers were rushing like mad to get the pupae underground and almost totally ignoring the larvae. There were a couple pupae in the larvae pile, and the workers took these too. Several hours later, all the pupae were gone and the larvae still hadn’t been touched. Could this be because pupae are farther along in development and they’re prioritized as a bigger sunk “investment” of the colony? Are larvae less vulnerable to being outside of the nest for a longer period? These are just my off-the-cuff hypotheses.
Furthermore, when taking pupae some of the workers walked around/over the pupae pile, seeming to evaluate the pupae, before selecting one. It seems to me that one pupa is as good as another, especially in a timely (for the ants) scenario like this, so what might be influencing workers’ selections?
Someone on BugGuide suggested the ants may have been Formica sp.
Any input on this from myrmecologically inclined folx would be appreciated!
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This situation happened to me once as well, and they put their larvae at the top chambers just under the mat. However, I checked the area later and all of them were moved further into the nest or otherwise disappeared. I’m not great at identifying ants, so it could have been one of the many common species in my area.
Again, I’m not a myrmecologist, but I don’t think the workers did this randomly. I agree it could be that the sunk costs are greater for pupae than larvae and there weren’t enough nanitics available to take care of them all after a large portion of them became exposed to the elements. It could also be that exposure to the outside air is certain death for them, but I know some more nomadic species that don’t use permanent nests are just fine carrying their immatures in the open air. There doesn’t seem to be any evolutionary pressure for nest-reliant ants to have thick outer membrane when they are larvae.
It’s very weird for them not to move all of the brood after you disturbed the nest because having more larvae now means the rebuilding process happens faster when this cohort of larvae emerge as adults. The colony could have a very fertile and healthy queen (or multiple queens, because some colonies are fine with that) to replace the brood they abandoned.
I think your sunk cost hypothesis is likely correct.
If the mat had been lifted by a hungry predator, they wouldn’t have had time to save all the young, and the pupae are much closer to becoming useful members of the colony. Since you were (presumably) just observing without gobbling any up, they should have started on the larvae once the pupae were safe, but perhaps by that time exposure had rendered the larvae unviable.
As far as walking around over the pile to select one, some of the pupae may have been closer to ready to emerge, and it would make sense to prioritize those, if they can tell the difference by chemical signals or feel.
Back in the 1960’s, an evolutionary biologist named Bill Hamilton furthered a concept called reproductive value in which an individual’s value to future generations changes over time as they age through the stages of development. So, yes, it’s conceivable that pupae have greater reproductive value based on having successfully completed most of their development. It’s also possible that adults recognize pupae as most vulnerable to things like desiccation or thermal stress, and so prioritize their movement.
Note [this](https://en.wikipedia.org/wiki/Reproductive_value_(population_genetics)) page attributes the origination of the idea to R.A. Fisher (which is true). Hamilton developed the idea further.
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