The Dangerously Cold Life of Acorn Ants

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While photographing acorns yesterday in a patch of melting snow, I wondered about the fate of the small Temnothorax that live in the acorns. Most temperate ants survive the season by tunneling through the soil to hibernate below the freezing zone. But Temnothorax don’t. They stay at the soil surface, holed up in walnuts, acorns, twigs, and other small cavities that freeze solid for several months. That can’t be the easy winter survival option.

An active acorn nest of Temnothorax curvispinosus during warmer times (Urbana, Illinois)

I moved on to ponder other things- namely, a warm cup of coffee- but today I was surprised to see a timely new study on just this problem:

Abstract: Most species of ants inhabiting the temperate zone overwinter underground, whereas those of the genus Temnothorax remain in nests aboveground. I studied the cost of aboveground overwintering. Workers of Temnothorax crassispinus survived in higher numbers (median = 88%) in artificial nests experimentally buried at a depth of 5 cm than those in nests on the surface (48%) of the soil. The results support the hypothesis that overwintering aboveground could be a consequence of a limited supply of nests and/or the advantage of being able to respond quickly to warm temperatures in spring.

Slawomir Mitrus buried 18 equal-sized nests of a European acorn ant under the soil and placed 18 control nests on the surface. The worker survival data between the two treatments were stark:


Most workers in the buried nests survived the winter while about half of the surface ants perished. With such a steep cost for overwintering above ground (see also Joan Herber’s earlier studies), remaining on the surface must provide some non-trivial benefits to the ants. The author suggests the surface-nesters are holding valuable nesting sites against usurpers, or perhaps the ants are able to jump-start their spring growth. These are plausible explanations, and I suspect that a deep-soil migration may also prove more costly in terms of fungal infection or exposure to predators than remaining  in their nests. We still don’t know a whole lot about the natural history of even common species.

My own overwintering strategy consists of travelling to sunny Belize, of course.

source: Mitrus, S. 2013. Cost to the cavity-nest ant Temnothorax crassispinus (Hymenoptera: Formicidae) of overwintering aboveground. Eur. J. Entomol. 110: 177–179.

14 thoughts on “The Dangerously Cold Life of Acorn Ants”

  1. The large spread of the above-ground mortality makes me wonder about microclimates. Buried acorns are all about the same, but on the surface the disposition of the acorn could matter a great deal. Exposed, covered by leaves or snow, sheltered by large roots at the base of a tree, etc. Finding just the right acorn in the optimal location could make a significant difference to a colony’s future. Not sure about Mitrus (don’t have access), but checking Herber’s PDF his method involved reducing all surface acorns to the same common denominator of being completely exposed on bare soil.

  2. Hi Alex,

    This post got me wondering. Do you have any special way of keeping up to date with new papers on subjects that interest you? Or do you just do searches every now and then?

    Perhaps you have some sort of saved search that alerts you to changes, or some sort of RSS method? Please tell 🙂


    1. That’s a good question- I don’t have any particular method.

      Google Scholar is helpful. When I am logged into Google it knows my publication record and suggests relevant new studies. I also monitor Web of Science and check relevant journal web pages. I sometimes get a scoop by maintaining correspondence with a number of ant researchers. This sometimes gives advance warning for big new papers, although I’ve been a little slow on these recently.

    2. You can also sign up for alerts from some journals/publishers. You get a lot of material you are not interested in as well, but at least the titles to new papers and links appear in your inbox on a regular basis and you can choose which ones you want to read.

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  4. What a lovely result. And a lovely photo.
    Ants in acorns aboveground experience the warming rays of the sun perhaps a month earlier than those belowground. It’s risky, but if the queen is kept in the middle, preventing her from frost-death, this likely means higher fitness in many years.

  5. I suspect that what we are seeing here with ant overwintering are the results of human intervention in forest floor organic matter (leaf litter, dead wood, etc) accumulation.

    Virtually all of our deciduous forests are relatively young second and third growth still accumulating organic matter which declines precipitously following cutting in temperate zone forests. Leaf litter and dead wood accumulate continuously over long periods. One of the most prominent features of old growth temperate zone forests is the massive amounts of accumulated dead wood, leaves, and the organic soil horizon layers. I would assume the ants evolved in a microhabitat that likely would have been much better insulated for more of the colonies.

  6. BioBob makes an interesting point, but it may not be so relevant to acorn-inhabiting Temnothorax, which seem to prefer more open woodlands, and is missing or nearly so in closed-canopy forest with deep, cool litter (better for other ants).
    Human intervention (i.e., humans as a part of nature) figures importantly in another way for ants and litter organisms generally, in that modern prescribed burning (and centuries of American Indian burning practices before that), for management of vegetation structure, oxidizes the upper portion of the litter layer and through top-killing of the woody understory, allows more light and air flow into vegetation, resulting in considerable temperature fluctuation in the litter-layer nesting habitat. In non-burn years, these and other acorn and thermophilic litter ants such as Tapinoma & Nylanderia, thrive in the warmer environment of the relatively exposed and thinner litter layer.
    I’d be interested to know the natural frequency of nesting by these species in buried acorns or other subterranean microhabitats, since these are little or not at all heated by a passing fire, and would serve as important fire (or severe drought or cold-snap) refugia for the organisms that inhabit them.

    1. I suppose we would need to know which ant species occupy which species of oak acorns at which frequency. Adaptation to natural fire is more relevant in lodgepole, longleaf and ponderosa pine woodlands with oaks, if any, and forests intermixed with grasslands than in the beech, birch, maple, white pine, oak forests of New Hampshire, for instance. Then also species like chestnut were virtually eliminated.

      At any rate, we will not know about ‘natural’ oak woodlands anytime soon since we have manipulated all of these present day environments and continue to do so.

  7. Such a timely article! I just arrived in New Jersey, and after working with desert seed-harvesters for the last six years, I’ve found myself acorn-ant-nest-hunting on the forest floor in the middle of the winter! I’ve found Temnothorax, Myrmica, Pyramica, Pachycondyla, Aphaenogaster, and Lasius, all huddled up in acorns, all of the acorns clearly above-ground, but under a relatively thick layer of leaves.

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