June
27
NeurosciANTS
You probably walk past thousands of ants every day and give them no more than a passing glance, but the ecological impact of an entire ant colony is comparable to that of enormous animals like elephants. Together in colonies that can surpass 300 million individuals, ants have a profound effect on the landscapes they colonize, affecting soil temperature, nutrient recycling, and the behavior of other animals. Leafcutter ants, for example, can strip an entire tree bare of leaves in a single day and are thought to be responsible for harvesting 10% of all leaf production in tropical forests[1]. There are even specialized neotropical birds that have adapted to attend army ant raids to exploit the stirring up of easy game[2].
They’re more integrated into our human societies than we might expect. For instance,one species (Azteca sericeasur) is considered a keystone member of coffee farms, protecting our precious crop of caffeine from the sinister coffee berry borer[3]. Many aspects of human society are recapitulated, although in a much less primate and more insect way, in ant colonies. There’s a division of labor between different workers, age-related task transitions, and collective decision making[5][6]. Not only that, but researchers are writing algorithms to capture efficient ant organization and path covering strategies to optimize production in modern warehouses[4]. Ants like their cities to have many of the same amenities we enjoy, with dumps, highways, nurseries, and areas dedicated for cultivating crops of delicious and nutritious fungus. Ants also emulate some of the darker aspects of human societies, with one species, the “slaver ant”, that regularly raids other colonies for larval workers which they steal and bring back to their own burrows to raise as loyal workers[7]. Some even argue that ants have domesticated other animals themselves, tending to and benefitting from the sweet sweet secretions of aphids and other sucking insects[8].
Doing a lot with a little
So what do ants have to work with to exert such a complex effect on the environment? The average ant brain only contains about 250,000 neurons. Ant brains show all of the characteristic components one would expect to find in an insect brain including optic lobes, medulla and lobula, olfactory centers, the central body and the mushroom body[9].
One of the finest tools in the ant brain arsenal has proven to be olfaction. Ants rely heavily on olfaction for communication and orientation. Correspondingly, ants have experienced a dramatic expansion in the number of genes encoding different odorant receptors to ~400, possibly the highest number in any insect[10]. They use this precise sense of smell to great effect, coordinating trail following, alertness, recruitment, and reproductive state. The antennal lobes of insects are the business end of their sense of smell, and ants have some prodigious lobes, containing >400 glomeruli (nerve clusters), compared to 160 in honey bees and just 43 in fruit flies[11].
Ants also communicate through a variety of mechanical cues. When an ant comes across a difficult task, like dragging an entire jolly rancher back to the colony, they can go to nestmates and perform a series of jerking behaviors that induce the nestmate to curl up into a little ant ball so the recruiter can carry it to the target area[12]. Wouldn’t getting help at work be so much easier if our coworkers were so cooperative?
Although an individual ant isn’t much of a thinker, together their colonies are capable of impressive feats of collective decision making. If you consider the colony as a whole, a nest of 400,000 ants has the same number of neurons as a human. Much like how the collective activity of billions of human neurons gives rise to the emergent property of consciousness, social ant colonies as a whole are more than the sum of their individuals. Without any orders or direction, ant colonies can coordinate diverse group actions, while each individual ant remains almost entirely ignorant to the activity of the whole colony. These collective performances are on display during a variety of complex activities, including nest construction and maintenance, colony emigration, foraging, colony defense, and division of labor.
Given that the purported purpose of a brain is to generate and control behavior, ant brain size has an interesting relationship with the size and sociality of the colony. Generally, the behavioral repertoire of different castes of ant workers relates to the size of their brain[11]. Basal, solitary species of ants, which must produce the full set of ant behaviors including foraging, mating, etc, have larger brains than social ants which can afford to rely on other members of the colony to fill those behavioral roles[17]. This is in stark contrast to our understanding of primate sociality, where a prevailing theory is that we have evolved large brains help us understand and coordinate complex social behavior. Although the exact relationship between sociality and ant brain size is still a subject under debate, it is possible that sociality in the insect context can have a much different effect than in mammals.
Not all ants who wander are lost
As ant colonies grow, they must forage farther from the nest to find the necessary resources. This raises the question of how pioneering foragers on the outer ranges of the colony find their way back home to report on the location of some bits of discarded cookie. Instead of retaking the arduous, meandering path back to the nest that an ant took on the way out to forage, desert ants returning to the nest come back in a straight line. Interestingly, they seem to do this by performing a form of path integration using direction gleaned from a compass on top of their head that can measure the angle of the sun[13]. This information seems unnecessary for measuring distance however, as even in complete darkness ants are capable of correctly assessing travel distance[14]. In 2006, a hypothesis from 1904 was finally tested and provided evidence that ants are keeping distance by maintaining a “pedometer” to simply count the number of steps they had taken to calculate how far they needed to travel to return[15]. To test this hypothesis, ants were trained to walk 10 m across a straight track to a feeder. After training, ants caught at the feeding site were transferred to a testing track and would attempt to return to the colony. To increase stride length, stilts in the form of pig bristles were glued to their legs, or their legs were shortened by severing the middle of the tibia segment. Stilted ants significantly overshot their return trip, and shortened ants undershot, indicating a mismatch between their pedometer and the distance required to travel on the return journey.
The cost of simplicity
Ants’ enormous success has come with downsides, unfortunately for them. The price of being numerous with a very simple brain has left them vulnerable to a unique genus of fungal parasites: Cordyceps. Sometimes called the “zombie fungus”, there are several species of cordyceps that can infect ant colonies with rather creepy results. The infection is characterized by an invasion of the fungus into the ant’s brain, causing behavioral changes and driving ants to leave their normal habitat to take up residence in warm, moist areas suitable for fungal growth. They’ll then affix their mandibles to the underside of a leaf in a death grip before finally perishing and sprouting fruiting bodies from their head, spreading fungal spores to start the process anew in another unsuspecting ant. This affliction can be so deadly that ants have evolved to recognize the pheromone signals of infection and will quarantine infected members away from the nest[16]. Despite this challenge, ants persist as one of the most successful animal species and are a shining example of strength in numbers.
References:
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2. Willis, E. O. & Oniki, Y. BIRDS AND ARMY ANTS Further ANNUAL REVIEWS. Ann. Rev. Ecal. Sysi 9, (1978).
3. Morris, J. R., Vandermeer, J. & Perfecto, I. A Keystone Ant Species Provides Robust Biological Control of the Coffee Berry Borer Under Varying Pest Densities. PLoS One 10, e0142850 (2015).
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14. Thiélin-Bescond, M. & Beugnon, G. Vision-independent odometry in the ant Cataglyphis cursor. Naturwissenschaften 92, 193–197 (2005).
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16. Evans, H. C. & Samson, R. A. Cordyceps species and their anamorphs pathogenic on ants (Formicidae) in tropical forest ecosystems I. The Cephalotes (Myrmicinae) complex. Trans. Br. Mycol. Soc. 79, 431–453 (1982).
17. Amador-Vargas, S., Gronenberg, W., Wcislo, W. T. & Mueller, U. Specialization and group size: brain and behavioural correlates of colony size in ants lacking morphological castes. Proc. R. Soc. B Biol. Sci. 282, 20142502–20142502 (2015).
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