Walking Up Walls: How Ants Defy Gravity

Every Wednesday after I eat my own lunch, I go to feed the ants. The process sounds simple: prepare the
food trays and then replace the old ones. However, in reality, this can be a bit complicated because the ants

that we work with are tree ants. Due to their arboreal habitat, they are good at climbing and, by extension,

escaping their containers. The ants’ ability to climb up the walls and ceilings of the containers is especially 

impressive because we coat them with a substance called fluon which is designed to create a surface that is

slippery for the ants. In particular, the ants from some colonies like to hang out on the roof of their container

which means that it can be a bit tricky to open. So, in order to get food in without the ants escaping, it is 

necessary to try and coax the ants off of the ceiling.

Ants crawling on the roof of one of their containers. [1]

But, how do the ants stick to the smooth walls and ceiling of the container? I had been thinking about this 

question for a while. So, I decided to see if I could find the answer.

Tree ants climb by using adhesive pads that secrete liquid. Many animals with climbing abilities use adhesive 

structures like these. And one especially interesting feature is that the adhesive structures  attach only when 

needed depending on what direction the leg is being pulled or pushed in. So, they can easily detach when 

they pull their feet in a certain way, but they can also make sure that they stick by not moving their feet in 

the direction needed to detach (which is different from the direction that gravity is pulling their feet in).  

This ensures that the animal is able to move freely when it does not need the adhesive. There are two general

types of adhesive pad. One is hairy while the other is smooth. These structures are very common, and some

animals have multiple pads of both types.

The climbing style of weaver ants (Oecophylla), as described by Endlein and Federle, is particularly interesting

and involves both types of adhesive structures. These ants have only a smooth adhesive “toe” pad, but the 

“heel” area is covered with very fine hair that is also used as an adhesive structure. These hairs are slightly 

flattened at the end and leave small droplets of the secreted fluid on the surface after they are detached. 

They point away from the ant’s body and produce friction forces that help with climbing. The hair array 

also contributes to a higher adhesive force on smooth surfaces. As the ants push harder against the climbing 

surface, more of these hairs make contact with it and produce more of the forces needed for climbing.

Endlein and Federle have also found that in weaver ants, generally, the smooth “toe” pads resist pulling 

forces while the hair arrays create pushing forces. So, tree ants use different parts of their feet depending

on what they are trying to do. They can also use their feet in different ways for different legs. This can help 

them with activities such as climbing vertical surfaces. For weaver ants, this is decided by where the legs are 

in respect to the center of mass (CoM) of the ant. For example, when they are climbing vertical surfaces, 

only the upper legs are above their CoM. So, they use the smooth pads on the upper legs to pull while the lower

legs push using the hair arrays on the “heel”.

Some research has even been conducted on Cephalotes (turtle ants), which is the genus of ants that we work 

with. Since they generally live in tropical regions, Stark and Yanoviak studied how increasing wetness affected
adhesion.  When they did this, they found that adhesion decreases with wetness and that the ants’ running

speeds are slower on wet surfaces. Walking on wet surfaces is likely even a concern for C. rohweri which is 

found in the Sonoran desert. They may be desert dwelling, but they even they experience two rainy seasons

per year with short, but heavy rains.

The ability of tree ants and other insects to stick to surfaces is not only super interesting, but it has also been

an inspiration to researchers. Recently, the hairy adhesive structures used by ants and other animals have 

inspired research into making adhesive structures using microfibers. One example of a manmade structure

that mimics the hairy adhesive structures found in animals is a type of tape that was recently created by Gorb et al. 

This tape’s “hairy” structure means that it is better at making contact with different types of surfaces. As more 

pressure is applied to the tape, more of the hairs touch the surface so the sticking ability improves.

When I first noticed the ants in the lab climbing on to the roofs of their containers I mainly felt inconvenienced 

due to the extra step it added in the feeding routine. However, I was not expecting their system for sticking 

to be as complex as I found it to be with multiple kinds of structures and functions depending on their specific

needs. Although their ability to stick to surfaces can be a nuisance in the lab, it is likely incredibly important

for their survival in trees. Due to this, they have evolved an impressive strategy for climbing and adhering

to many different types of surfaces. It is fun to see how their strategy inspired scientists, and makes me wonder

what else we can learn from them.

Further Reading

Bullock JM, Drechsler P, Federle W (2008) Comparison of smooth and hairy attachment pads in insects: 

friction, adhesion and mechanisms for direction-dependence. J. Exp. Biol. 211, 3333–3343.
doi:10.1242/jeb.020941

Endlein T, Federle W (2015) On Heels and Toes: How Ants Climb with Adhesive Pads and Tarsal

Friction Hair Arrays. PLoS ONE 10(11): e0141269. doi:10.1371/journal.pone.0141269

Gorb S et al. (2007) Insects did it first: a micropatterned adhesive tape for robotic applications. Bioinspiration 

& Biomimetics 2, S117-S125. doi:10.1088/1748-3182/2/4/S01

Stark A, Yanoviak S (2018) Adhesion and running speed of a tropical arboreal ant (Cephalotes atratus) on wet
substrates. R. Soc. Open Sci. 2018 5 181540; DOI: 10.1098/rsos.181540

Media Credits:

1. Photo by Elena Romero