"Long before it's in the papers"
January 27, 2015


Ants make well-organized rafts with own bodies

Feb. 23, 2014
Courtesy of Public Library of Science
and World Science staff

When fac­ing a flood, ants build well-or­gan­ized rafts out of their own linked-to­geth­er bod­ies, us­ing their own brood as “flota­t­ion de­vices” and pro­tect­ing the queen spe­cif­ic­ally, ac­cord­ing to a stu­dy.

The study found that the pro­cess over­all is sur­pris­ingly safe—even most of the ants on the bot­tom of the raft sur­vive just as well as if they had been left alone on dry land.

The study was pub­lished Feb. 19 in the re­search jour­nal PLoS One by Jes­si­ca Pur­cell of Uni­vers­ity of Lau­sanne, Switz­er­land, and col­leagues. 

Ants building a raft. (Cre­dit: Jess­ica Pur­cell)

So­cial an­i­mals in­clud­ing in­sects of­ten work to­geth­er when put in har­m’s way. For in­stance, Jap­a­nese hon­ey­bees “will sur­round large pred­a­to­ry hor­nets and form an ‘oven,’ rais­ing the in­te­ri­or tem­per­a­ture to kill the in­trud­er,” they not­ed. 

Ants liv­ing on flood plains are known to link to to­geth­er to cre­ate rafts dur­ing floods, but lit­tle is known about the make­up, shape, and so­cial struc­ture, if any, of these rafts. The sci­en­tists col­lect­ed flood­plain-dwel­ling ants of the spe­cies For­mi­ca selysi in Switz­er­land. They brought the in­sects to a lab so they could in­duce flood­ing in ant popula­t­ions con­tain­ing dif­fer­ent com­bina­t­ions of work­er ants, queens, and broods—the new genera­t­ion, con­tain­ing ants in stages known as lar­vae and pu­pae.

Dur­ing the “flood­ing,” sci­en­tists watched where the work­ers, brood, and queens were po­si­tioned in the raft. The flood­ing al­so al­lowed them to ob­serve the buoy­an­cy and re­cov­ery abil­ity of the work­er ants and brood.

The work­er ants and brood turned out to be ex­tremely re­sist­ant to sub­mer­sion. The work­ers pro­tected the most vul­ner­a­ble and val­u­a­ble nest mate, the queen, by plac­ing her in the cen­ter of the raft. Both work­ers and brood showed high sur­viv­al rates af­ter they rafted, which sug­gests that oc­cu­py­ing the raft’s base is­n’t as deadly as sci­en­tists ex­pected, the in­ves­ti­ga­tors said.

“We were as­ton­ished to see the ants sys­tem­at­ic­ally place the youngest col­o­ny mem­bers in that po­si­tion,” Pur­cell said. “Fur­ther ex­pe­ri­ments re­vealed that the brood are the most buoy­ant mem­bers of the so­ci­e­ty and that raft­ing does not de­crease their sur­viv­al,” which was meas­ured at about 83 per­cent.

How­ev­er, ants are re­luc­tant to make rafts ex­cept in ex­treme situa­t­ions, Pur­cell and col­leagues wrote, sug­gest­ing “there may be oth­er costs or dan­gers not ac­counted for in our ex­pe­ri­ments.”

“Ob­vi­ous costs of raft­ing in­clude the risk of los­ing the nest, of col­o­ny frag­menta­t­ion, and of be­ing washed away to un­suit­a­ble hab­i­tat. More­o­ver, preda­t­ion by fish or ex­po­sure to tur­bu­lent wa­ters may cause high­er mor­tal­ity than meas­ured in lab­o­r­a­to­ry con­di­tions.”

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When facing a flood, ants build well-organized rafts out of their own linked-together bodies, using their own brood as “flotation devices” and protecting the queen specifically, according to a study. The study found that the process overall is surprisingly safe—even most of the ants on the bottom of the raft survive just as well as if they had been left alone on dry land. The study was published Feb. 19 in the research journal PLoS One by Jessica Purcell of University of Lausanne, Switzerland, and colleagues. Social animals including insects often work together when put in harm’s way. For instance, Japanese honeybees “will surround large predatory hornets and form an ‘oven,’ raising the interior temperature to kill the intruder,” they noted. Ants living on flood plains are known to link to together to create rafts during floods, but little is known about the makeup, shape, and social structure, if any, of these rafts. The scientists collected floodplain-dwelling ants of the species Formica selysi in Switzerland. They brought the insects to a lab so they could induce flooding in ant populations containing different combinations of worker ants, queens, and broods—the new generation, containing ants in stages known as larvae and pupae. During the “flooding,” scientists watched where the workers, brood, and queens were positioned in the raft. The flooding also allowed them to observe the buoyancy and recovery ability of the worker ants and brood. The worker ants and brood turned out to be extremely resistant to submersion. The workers protected the most vulnerable and valuable nest mate, the queen, by placing her in the center of the raft. Both workers and brood showed high survival rates after they rafted, which suggests that occupying the raft’s base isn’t as deadly as scientists expected, the investigators said. “We were astonished to see the ants systematically place the youngest colony members in that position,” Purcell said. “Further experiments revealed that the brood are the most buoyant members of the society and that rafting does not decrease their survival,” which was measured at about 83 percent. However, ants are reluctant to make rafts except in the most extreme situations, Purcell and colleagues wrote, suggesting “there may be other costs or dangers not accounted for in our experiments.” “Obvious costs of rafting include the risk of losing the nest, of colony fragmentation, and of being washed away to unsuitable habitat. Moreover, predation by fish or exposure to turbulent waters may cause higher mortality than measured in laboratory conditions.” bodies