The Mexican agave and the bat: a dance of evolution
Imagine yourself dying of sexual exhaustion. No, it wasn’t the tragic –and probably enjoyable- outcome of a crazy night. You have been preparing for this your entire life, growing, building up nutrients, waiting for the year in which everything is going perfect so you can put up a flashy add, up to ten meters high, that says “Hey! You! Come here, I’m ready”. That’s life for the agaves –a fleshy leaf plant that looks like an aloe vera- whom, after living several years, grow a stem that is many times taller than the whole plant, a signal for the wingmen, or in this case wing-mammals, the bats. These flying matchmakers visit the flowers at the point of the agave’s stem.
The histories of bats and agaves are constantly crossing each other’s paths. The latter needs the former for reproduction, and the former needs the latter for food. But it isn’t just any pollinator and it isn’t just any food. Bats can cover great distances and visit a lot of plants each night, while carrying a big load of pollen, making their pollinating activities very effective. The nectar and the pollen of the agaves contain proteins with many essential amino acids, allowing the bats to feed exclusively from this food.
But this precious nectar isn’t an exclusive treasure for bats. For some agave species, the ones without ramifications in the flowers, it was thought until recently that the main pollinators are morning visitors –like hummingbirds and insects-, and not the flying mammals.
While studying agaves in the state of Hidalgo, in Mexico, researcher Luis Eguiarte from the Instituto de Ecología of the National Autonomous University of Mexico and his team, were wondering who was pollinating this kind of agaves and how frequently they did it. Roberto Trejo, one of the students in the group, tells us about this research “we recorded the flowers for 24 hours in a row and counted how many diurnal and nocturnal visitors arrived. We already knew, from previous research, that the peak of nectar production occurred during the night. This counting allowed us to prove that the bats were the true main pollinators of the agaves”. What Eguiarte and his team found not only meant that the nectar eating bats –that account to at least 30 species- were the main pollinators of the agaves, it was a clue of how special the relationship is between the flying mammals and the plants.
There are other clues that put us on the same track. For instance, some adaptations that came along and enabled bats to make the best of their relationship with the agaves. Roberto describes them like this: “There are two kinds of nectarivorous bats: the ones that have grooves across their tongues, which, with peristaltic movements, move the nectar up;” –this kind of tongue is unique among animals- “and the ones that fold their tongue and stick it in and out of the flowers in order to get the nectar”.
The hour hands of evolution
To get to know just how special, Dr. Eguiarte’s team went deep into the evolutionary history of agaves and bats. They knew that the agaves were a genus with 208 species, which means they are very diverse, accounting for 72% of the species in their family. There are two ways of making sense of these numbers, either the agaves are an ancient genus with enough time for adding single species one by one over millions of years, or they are a young genus from which almost all of the species surged at the same time.
In order to solve this riddle, the research team went into the DNA sequence of different agave species. But DNA can be read in many ways, it is not only a collection of genes that are translated to proteins, it can also be read as a clock. The mutations that occur in different places of the genome appear at an almost constant rate, each new mutation ticks the evolution watch several thousands or millions of years, depending on the organism and the sequence being analyzed. You just need to calibrate your clock so that, when you are counting mutations, you are also telling time.
After running their analyses, Luis Eguiarte’s team found out that the agave’s clock started ticking just over a dozen million years ago. Now, this may sound like a lot of time, but when you are talking about forming new species, this is really short. The scientists that study evolution call this phenomenon –many new species over a short period– an adaptive radiation, and it is a telltale sign that the species, in this case the agaves, found a niche that wasn’t being used by anyone, and without competition, they were free to experiment on many ways of using this to their advantage, resulting in many species.
What was this precious niche? Did it had anything to do with the bats? It was now time to look into the evolutionary history of the nectarivorous bats. Without as many species to show off as the agaves, these bats also had a considerable amount of variation. So after applying the same clockwork analysis on the bats’ DNA, Dr. Eguiarte and his team could tell that the nectarivorous bats’ clock had been ticking for about 12.3 million years. Almost the same time as the clock of the agaves. And it was also an adaptive radiation! The evolutionary stories mirrored each other.
The adaptive radiation told that the bats also found a new niche in which they could thrive, and at about the same time as the agaves did. So, what was it? What was the big advantage that allowed them to build so much variation? Well, they found each other. Agaves discovered bats as pollinators, and bats discovered agaves as a food source.
My greatest advantage is you
How did this happened? The ancestor of the agaves was pollinated by insects. That meant there were usually insects around these plants. The ancestor of the nectarivorous bats used to eat insects. Do you see where this is going? The ancient bats started visiting the ancestor of the agaves to eat the insects that came near the flowers, getting closer and closer to the flowers. Until they started eating the nectar as well. But it wasn’t the nectar of just one plant, they visited many plants in one night, dragging pollen along with them, becoming a pollinator.
The bats could carry more pollen than the insects and also take it further, so those plants that could attract more bats than insects were going to have an advantage. How to allure bats? The ancestors of the agaves tried out different adaptations that were to be selected by the bats. The smell of the flowers turned more musky, the flowers got higher from the ground –so the bats didn’t have to dive in mid-flight-, and they managed to produce a nectar rich enough so the bats would stop eating insects.
Night after night, the bats began to visit more and more agave flowers. This may sound easy for the agaves, but it came with a huge effort. Let’s go back to the sexual exhaustion. As an agave you don’t just have to grow a stem several meters high, you also have to provide flowers with a nectar that is supposed to be the only energy source of a flying mammal. The agaves allocate so many resources to growing this tall signal and to attract as many bats as they can to assure their own reproduction, that they die afterwards.
The history of the pollinating bats and the agaves started at the same time and they’ve been depending on each other for millions of years. If we lose one of them, we’ll lose the other. Bat population numbers have been declining due to habitat loss and the growing mezcal industry has also been taking a toll on the natural populations of agaves. If we are not careful we might lose both. The work of Luis Eguiarte and other researchers in Mexico plays a key part into understanding these two beings and provides us with the necessary information to protect them in the best way possible.