Incubating an empire: new prospects for the Bee Lab

Twelve test tubes sit in an incubator, whirring softly in a dark room. A dim red light allows one to navigate around the tables and boxes, and as I open the door of the insulated box, I peer at the thermometer behind the secondary glass door. 35 degrees. They like to be warm. Hot even.

This summer in the HMC Social Insect Behavior Laboratory, an exciting new project is hatching, a project that started stooped over a mound of seed husks around a nest entrance of a local species of seed harvesting ant, Pogonomyrmex californicus. Professor Donaldson and I had been monitoring these nests for a few weeks. We were watching the well-trafficked entryway of each nest, looking for winged ants lingering near the surface.

These winged ants aren’t workers; they are actually some of the few individuals in an ant colony who can reproduce. These are young queens and young drones, waiting for the annual mating flight of their species; and for this event, timing is critical. The flight must be synchronized for all colonies of the same species to maximize the chances for the young queens to find a mate (or mates). So the princes and princesses remain close to the entrance, waiting for whatever information is required for this particular species to fly. For other species of this genus, mating flights are generally spurred by summer rains, making this massive coordinated mating event easy to anticipate for the relatives of this harvester ant.

Only, here in Claremont at the Bernard Field Station there are no predictable summer rains. In places like Arizona, summer monsoons occur annually, and Pogonomyrmex can synchronize with the predictable weather pattern of the area. Here, P. californicus must rely on a different cue to trigger their flight, lest they fail to mate altogether.

So this project for the first two weeks was mostly surveillance; meandering through the field station, locating nests, looking for winged reproductives (called alates), and waiting. We didn’t know what would trigger the flight, only that it would happen soon.

Then, something happened on the night of June 9^th.  Rain. A modest amount of moisture from an evening storm appeared enough to spur the event we had been waiting for. The rains had saturated the sandy loose soil, allowing it to be dug into, balled up, and molded. Newly fertilized queens were frantically seeking refuge; excavating the first shallow burrows that, with luck, would form the nucleus of a mature colony.

This discovery was somewhat unexpected given some previous reading about the reproductive habits of P. californicus, but not altogether surprising considering the local environment. Although summer rains do not occur regularly or frequently in Claremont, P. californicus appears to have retained the ability to take advantage when it does. So, although they don’t need the rain to announce a flight, they’ll sure take the opportunity of moistened soil when it happens.

Armed with a trowel and a few plastic containers, I set out to collect some of these queens, looking for the characteristic piles of newly disturbed soil. A helpful tip when hunting for ant queens is to know what soil type the species you are looking for likes best. This is especially useful for harvester ants, which may avoid unnecessary competition with other ants by staying picky about their substrate. Competition is an important limiting factor for large desert ant colonies like the harvester ant. Collectively, hundreds of workers sum to a substantial metabolic load on the ecosystem, and two massive adjacent colonies pillaging the same ground spells depletion and disaster for both. After a few years of seeing harvester ant populations near my home in Colorado, on trips to New Mexico and Utah, and now seeing the Pogonomyrmex here in Southern California, I’ve gotten a bit of a feel for the spacing of these colonies.

Why then, are harvester ant mating flights so massive? Why do colonies invest so much energy in producing potential competitors? If all of the queens born in a colony were to be fertilized and founded colonies nearby, surely the mother colony would be negatively affected by being surrounded by fertile daughters and marauding columns of workers. Actually, there are a few ways that this problem is mitigated and the proper distribution of colonies is maintained. Firstly, newly mated queens are vulnerable and suffer high mortality. Juicy ants swollen with energy reserves ample for egg laying make tempting prey for birds, lizards, and other arthropods –including other ants. One of the queens I collected required no digging at all actually: I found it squirming between the jaws of an ant worker of an already established colony. The funny part? They were of the same species –both P. californicus. It’s not entirely inconceivable that the worker may even belong to the very same colony that the queen had just flown from hours before. This violent reaction by the worker to a queen of its own species illustrates one of the rules of survival for the resource-limited landscape that the harvesters populate. As soon as a queen is fertilized, she has turned potential competitor, enemy and food source to her parent colony.

Including the rescued queen, I collected a total of twelve foundresses. Most I found digging their burrows, during which I would wait while they brought out a new load of soil to scoop them up into the collection tubes. The others I caught while they were out foraging for seeds- rich carbohydrate sources that they will use to fuel the production of their nascent brood. Twelve queens may sound like a lot, considering mature colonies can swell to thousands of workers. But in these micro-colonies a week after collection, the industrious queens are the sole active inhabitants; their nuclear families more than adequately contained in styrene tubes with a few inches of moist, sandy soil.

The test tube assembly line: queens were collected in the white plastic containers then placed in a clear styrene collection tube with moistened soil (in jars) and barley (the bag in the lower right corner). The labels list the date and queen ID.

The first tiny white eggs sit tucked away in dark caverns while their mother surfaces occasionally to forage, a behavioral quirk that is rare in this genus. Most other Pogo. queens remain underground during the founding process, metabolizing stored fat and spare muscle while their first workers develop. Why does this mother forage in the outside world when similar species are able to stay safely within a burrow? An analysis of the growth rate of P. californicus compared with the growth rate of other species in the same genus revealed a possible benefit to this added risk. The extra calories collected by the queens of this species during these risky foraging trips seem to contribute to an extra large first brood, meaning P. californicus will have functional, foraging workers sooner.

Queen 7 emerges from her burrow to forage.

Environmental pressures on P. californicus seems to have produced some curious reproductive behaviors in this species that tell a tale of scarce water, limited resources and defensive neighbors. This has made observation and collection quite fun as I essentially get acquainted with the behavior of this species.

And ultimately, that’s what we’re interested in here at the Social Insect Behavior Laboratory: “how” groups are organized and “why” they are organized the way they are. The “why” comes from ecology, analysis of what environmental circumstances the species has molded around and had to adapt to. The “how” comes from experiment, direct observation and careful manipulation of variables.

With an observable colony housed in the lab, we can observe how the collective interactions of workers serve to recruit and distribute individuals to the various discrete tasks of nest maintenance, from tasks like child rearing, to foraging, excavation, trash duty.

But these are still ambitions. First, we wait. As for the incubator, a high temperature and a dark, moist environment coupled with sprinklings of nutritious barley boost the cold-blooded queens into egg-laying overdrive. Check back here for an update post in the coming weeks as I anxiously await the emergence of the first workers, when these exciting new research questions will finally have some legs.

Queens 1-3 aligned in the incubator, digging away.

Further Reading:

Enzmann, BL et al. 2014. The cost of being queen: Investment across Pogonomyrmex harvester ant gynes that differ in degree of claustrality. Journal of Insect Physiology, 70, 134-142.

URL: http://www.sciencedirect.com/science/article/pii/S0022191014001565?np=y

Holbrook, CT et al. 2011. Division of labor increases with colony size in the harvester ant Pogonomyrmex californicus. Behavioral Ecology, 22, 960–966.

URL: http://beheco.oxfordjournals.org/content/early/2011/06/03/beheco.arr075.abstract

Johnson, R. 2002. Semi-claustral colony founding in the seed-harvester

ant Pogonomyrmex californicus: a comparative analysis

of colony founding strategies. Oecologia. 132, 60-67.

URL: http://www.asu.edu/clas/sirgtools/Oecologia%20-2002.pdf

Johnson, R. 2006. Biogeographical parallels between plants and ants in North American deserts. Mymecologische Nachrichten, 8, 202-218.

URL: http://www.asu.edu/clas/sirgtools/Myrmecological%20News%202006.pdf

Johnson, R. 2006. Capital and income breeding and the evolution of colony founding strategies in ants. Insectes Sociaux, 53, 316–322.

URL: http://link.springer.com/article/10.1007%2Fs00040-006-0874-9