Ants in your Plants: Mutualism benefits both myrmecophyte and insect

What is an “ant plant”? Because we are striving to portray a “real” tropical rainforest, we have several and plants at the Cockrell Butterfly Center, but what makes them so special? Technically called myrmecophytes (from the Greek myrmeco – “ant”, and phyte – “plant”), these plants have a very special relationship with ants, one that is beneficial to both parties. Such mutually beneficial partnerships are known as symbioses or mutualisms, and they are fascinating to evolutionary biologists, ecologists, and lay people alike. So-called “ant plants” typically provide shelter, and sometimes food as well, for ants, and the ants taking advantage of these resources in turn defend the plant against herbivores or other threatening animals, and sometimes even against competing plants. In some cases the ants may provide their host plant with nutrients.

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Ants take shelter inside the passages within the swollen stem of this  Hyndophytum formicarum.

Examples of ant plants abound, particularly in the tropics. They occur in many different families of plants, and involve many different species of ant. Some plant-ant combinations are specific – i.e., the plant has co-evolved with a specific species of ant. In such cases this ant is usually found nowhere else: it has an obligate mutualism with its plant host.  Often the plant has difficulty surviving on its own as well.

More common are facultative mutualisms, where plants provide a resource (usually food) that a variety of ant species may visit. Simply by virtue of their presence on the plant, these ants discourage (or attack) herbivores or other organisms that might harm the plant.

Hundreds of plants bear extrafloral nectaries. These nectar-secreting glands on structures outside of the flower (usually on leaves or the petioles of leaves) are typically most active on new growth, which most needs defense against hungry herbivores. Ants gather at the nectaries to collect the sugary fluid they exude, and kill or chase off insects that try to eat the leaf.

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Ants taking nutrition from extrafloral nectaries on an Inga leaf.

Less common, but still with plenty of examples especially in tropical areas, are plants that provide shelter for ants. These domatia may be hollow stems, swollen petioles, or other hollow spaces on the plant that ants can use as living space.

A few neotropical shrubs in the large genus Piper (black pepper family) have evolved large, envelope-like petioles that house a species of Pheidole ant. Tiny pearl bodies (lipid-containing food bodies) are produced on the inside surface of these domatia – at least when the ants are present. These tiny ants are not aggressive and would not seem to be very effective defenders of their host; however, they have been observed removing insect eggs and small larvae. The real benefit to the host plant appears to be the extra nitrogen that the plants absorb from waste (feces, dead bodies, and other debris) left by the ants inside the petioles.

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Ants taking shelter within the domatia of a Miconia petiole.

Two shrubs in the coffee family, Hydnophytum and Myrmecodia, have very large and complicated domatia. The stems of these epiphytes from the mangrove forests of tropical Southeast Asia and Australia are lumpy and swollen, and almost look diseased. These bizarre-looking structures are riddled on the inside with hollow chambers, much like a Swiss cheese.  If you cut one of the tubers in half, you will find that some of the chambers have smooth surfaces, while others have darker, rough surfaces. The inhabiting ants use the smooth chambers for living space and the rough chambers for dumping their trash. Biologists have observed that the rough-walled chambers are able to absorb nitrogen and other nutrients from the decomposing wastes deposited in them by the ants, while the smooth-walled chambers are not absorbent. Since it is always a challenge for plants without roots in the ground to get enough nutrients high up in the treetops, this is a great adaptation to enhance these epiphytes’ survival. And, not only do the ants provide the plant with extra nutrition – their presence serves to deter things that might eat it.

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Enlarged and swollen stipules on an ant acacia offer shelter for a colony of Pseudomyrmex, an ant species found nowhere else but this plant.

The ant-plant mutualisms that really capture our imagination are those obligate mutualisms where the plant provides both food and shelter for the ants, and the ants are particularly fierce defenders of their plant host.  One of the most famous ant-plants is the bull-thorn acacia from the dry forests of Central America. Acacia is a very large genus of plants in the legume family, especially abundant in dry tropical areas of the world. All Acacia species are characterized by having their paired stipules (little flaps of tissue at the base of each leaf, found throughout the legume family) modified into spines or thorns. In the Acacia species associated with ants, these thorns have become very large and swollen, resembling a pair of bull’s horns. A specific species of stinging ants in the genus Pseudomyrmex has evolved as an obligate mutualist of these acacias.  Worker ants make a small hole at the tip of one of the horns, hollowing out an interior, and the colony lives inside these chambers. These ants are found nowhere else.

The bull-thorn acacia also provides its ant inhabitants with food. Like many legumes, acacias have extrafloral nectaries. In this case, they take the form of little slits along the main rachis of the compound leaf. Ant-acacias don’t just feed their ants sugar, however; they also produce small, protein-rich food bodies for the ants on the tips of the new leaflets. The ants thus have all their needs for shelter and food provided by their host plant. In return, the ants vigorously defend their home against anything that threatens it – leaf-feeding insects or other animals – swarming out of the thorns and stinging the intruder. Because these ants also bite and sting any plant that touches or grows too close to their home, they reduce competition to their host from other plants as well, and because tropical dry forests often burn, the vegetation-free circles around the ant-acacia may be particularly important as a fire brake.  Evidence for how much bull-thorn acacias rely on their fierce little ant defenders can be discerned by tasting the leaves. While most Acacia species are full of bitter compounds, ant-acacias have little need to maintain these chemical defenses, and no longer produce them. Their leaves are not bitter. But if the ant-acacia should lose its ant colony, it is liable to be hit hard by hungry herbivores.

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A species of Azteca ants takes shelter inside the hollow stems of Cecropia plants like this one.

The famous British naturalist Thomas Belt first described this amazing symbiosis, which he observed during his sojourn in Nicaragua in the mid 1800s. Today the food bodies borne at the tips of the new leaflets are called “Beltian bodies” in his honor.

A similar scenario, also from from the Neotropics, is seen in the relationship between several species of Cecropia (a fast-growing, early successional tree) and a species of Azteca ants. Cecropia has hollow stems – with special thin skinned “dimples” in the stem opposite each leaf.  A queen Azteca who finds a young Cecropia not yet colonized bores her way through this thin spot (called a prostoma).  The eggs she lays inside the stem hatch into worker ants, who tunnel through the membranes that divide the stem at each node, and open up more of the prostomata, giving them access to the outside. As the tree grows, the colony moves upwards, eventually making the whole tree a long, hollow shelter for the colony. Cecropia trees also provide food bodies for the ants; these “Mullerian bodies” (named for another 18th century European biologist, Fritz Muller) are produced on special spongy structures at the base of each leaf petiole. The occupying ants also bring small homopterans (aphid relatives) inside the stems of the Cecropia, where they feed on the plant’s sap in a protected environment.  The ants milk these small insects for their sugary exudate (called honeydew) just like farmers milk cows. Azteca ants do not sting, but they can bite, and the workers swarm out in huge numbers to attack any animal that touches their plant.

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Cutting into a hollow stem shows just how extensive the ant colony runs inside this plant.

A relationship very similar to the New World Cecropia-Azteca mutualism is found in the Old World tree Macaranga, also an early successional species, but in the spurge family, a completely different plant family from Cecropia. The ants inhabiting these plants are members of the ant genus Crematogaster (unrelated to Azteca). This is a great example of what is called convergence in evolutionary terms, whereby similar situations in different locations give rise to the same adaptive solutions among unrelated organisms.

Ants are amazing little creatures, and their complicated social behaviors, which often seem to mimic ours, make them particularly good partners for plants in the fight for survival.

Look for the bull-thorn Acacia (sans ants, unfortunately) next time you visit the Cockrell Butterfly Center. We recently received a specimen of Hydnophytum and of Mymecodia from a generous donor. These will eventually be placed in the rainforest. Our Cecropia tree, alas, got too big and died a few years ago. We hope one day to replace it!

 

Behind the Scenes: HMNS Greenhouses [12 Days of HMNS]

Today is the Eighth Day of HMNS! In the spirit of the classic holiday carol, we’re taking 12 days to feature 12 different videos that preview or go behind-the-scenes of a holiday museum activity, here on the blog (or, you can get a sneak peek at all the videos on 12days.hmns.org – we won’t tell).

For most people, the beauty of the Cockrell Butterfly Center is, well – the butterflies. Thousands flit and flutter around each visitor to this butterfly conservatory – and they are stunning. But have you ever wondered about the exotic plants they land on?

The botanists and volunteers at the Butterfly Center spend much of their time nurturing the plants that the butterflies eat and live on – and much care is taken to ensure that their habitat is both healthy and representative of the rainforest environment to which the butterflies are native.

If you’ve visited the Butterfly Center, you know the plants there are healthy, hearty – and adult. So, where do they grow up? I am so glad you asked. In the video below, you can see for yourself – as Butterfly Center Director Nancy Greig takes you on a behind-the-scenes tour of the Museum’s greenhouses!

Need to catch up?

The First Day of HMNS – Explore: Snow Science
The Second Day of HMNS – Preview: The Chronicles of Narnia Exhibition
The Third Day of HMNS – Preview: Disney’s A Christmas Carol
The Fourth Day of HMNS – Investigate: The Star of Bethlehem
The Fifth Day of HMNS – Shop: The Perfect Gift
The Sixth Day of HMNS – Marvel: Faberge
The Seventh Day of HMNS – Glimpse: Spirits & Headhunters

Get into the holiday spirit! Visit our 12 Days of HMNS web site to see the videos and get more information about each event, exhibit and film: 12days.hmns.org Happy Holidays!

Science Doesn’t Sleep (8.27.08)

MEC's green roof among others
Creative Commons License photo credit: 416style

So here’s what went down after you logged off.

There’s a reason that cowboys don’t make good anthropologists – and it has to do with Hobbits.

It’s aliiiiiiive! A green roof can reduce your heating bills and protect your waterproofing – plus, it’s pretty! Check out a how-to here.

Some ancient documents are taking their message high-tech: the Israel Antiquities Authority is putting all of the Dead Sea Scrolls – all 15,000 fragments – online.

Construction in London has unearthed thousands of human skeletons – and the oldest are soon going on display.

Recently developed: a wheelchair that walks for you by means of a “robotic exo-skeleton.” Check out the video here.

How fast can we go? Usain Bolt’s astonishing, record-setting Olympic races have forced scientists to reconsider.

A weekend in the land of Giants

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Feeling tiny, looking up from the
floor of the Redwood forest.

Last weekend I headed to the West coast for a visit to Fort Bragg, CA to see the College of the Redwoods Fine Woodworking program’s spring furniture show.

During my visit, I wanted to see some of the sites beyond the drive from San Francisco to Fort Bragg, including a couple of the fun tourist spots. We drove a couple of hours up the very VERY twisty coast highway (I would recommend keeping windows at least cracked – fresh air is key on that sort of drive) to Leggett, CA to see the Chandelier Tree.
The Chandelier Tree is in Drive Thru Tree Park on Drive Thru Tree Road and you’ll never guess what kind of tree it is — that’s right, it’s a Drive Thru Tree!! The opening at the base of the tree is 6 feet wide and 6 feet and 9 inches tall and seems to accomodate most cars driving through and still has enough root structure to stabilize and feed the top of the tree 315 feet up!

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 The winding road through the forest.

The path was cut through in the 1930’s and for five dollars you too can drive your car through the tree – it seems like a pretty good money-making scheme but it was worth every penny to see those amazing trees.

When I visit major metropolitan areas in the country I always think to myself that Houstonians are so lucky with all of the terrific trees we have lining our streets in the heart of the city when other cities have miles of lifeless concrete – but we have nothing on the overwhelming nature of the Pacific coast. It really looks like there were just a series of awkward roads carved out among the scenery but otherwise completely wild – the ferns in the redwood forest are so dense you’d think some sort of prehistoric creature would be around any corner.

 

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 Chandelier Drive Thru Tree, Leggett, CA. 

 In Fort Bragg, near the town hall, they have displayed a slice of a tree that was cut down in 1943 and at that time was the largest redwood tree known to have grown in Mendocino County. The information on the tree notes that it took a 22 foot saw and 60 man hours to fall the tree. Not a job I would sign up to do. 
Coastal Redwoods really only appear on the Pacific Northwest coast of the US in a pretty concentrated area. After talking with some woodworkers and observing some of the damaged trees along the road in the forest it is clear that while redwoods are very large they splinter easily and would be difficult to work with in terms of furniture, but they are incredibly rot resistant.
The ability to resist rotting – along the coast where salty wind is constantly hitting you – means that the redwoods are able to thrive and the rot resistant quality of the lumber makes it a great material for building. Many of the public buildings are built from redwood. In addition to the massive Redwoods, the Northern California coast has a great number of other impressive vegitation from a rainbow of rhododendrons (a relative of our native azalea), succulents of all varieties, as well as cypress trees with impossible looking twists in their branches.
It is the sort of place where you can really just feel like a tiny ant among a sea of nature. We also spent a few hours in the botanical gardens… but that’s an adventure for another post!

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Standing by a slice of history.
(I’m 5′8″- that was a BIG tree!)

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Red rhododendrons in full bloom for Mother’s day!