Surely, the question “What do frogs eat?” is moot, especially to anyone who has studied basic biology. If you recollect, our school textbooks often have an oversimplified drawing of what is called a food web showing grass being eaten by a grasshopper. The grasshopper is eaten by a frog. The frog is eaten by a snake. The snake is eaten by a bird. This process of energy flow is dubbed ‘the food chain’ and a more complex version is called ‘the food web’. So, it is not surprising that people can intuitively guess that frogs eat insects. But saying that would be a textbook example of oversimplification.
Who cares what frogs eat?
The answer to this is slightly more complicated. Amphibians are among the most threatened vertebrates that are rapidly heading towards extinction globally. To conserve them, a careful understanding of their ecology is necessary. This could be their reproductive behavior, their habitat dependence, or simply, their diet. Food is an important resource and in nature, there is usually a competition between individuals of either the same or different species for access. One way to overcome this competition is by exclusion. Perhaps the famous Darwin’s finches are a great example of this competitive exclusion where closely related species have evolved to feed on different types of seeds. Over the years, this strategy has resulted in birds occupying specific areas of the habitat and evolution of exclusionary body form that enables a species to thrive while depending on a particular source. Another way of avoiding competition is to come out and feed at different times. Again, there are examples of this among different groups of animals. In both cases, there is a cost to being specialised and dependent on one type of food.
While amphibians eat insects, they also have varying levels of specialisation. The poison dart frogs of South America are known to eat ants and assimilate the ingested chemicals to bear toxic secretions from their skin, making them unpalatable. Perhaps this strategy has led them to become active during the day, unlike most other frogs. Closer to home, we have several anurans (frogs and toads) of which we know next to nothing about. A few studies in the past have documented the then widespread Bullfrog (Hoplobatrachus tigerinus) to be feeding on other frogs and poultry chicks. In another study, the Pond frog (Euphlyctis hexadactylus) was documented to consume more plants than insects. It was in this background that we stepped into waterlogged paddy fields in a remote village in Tamil Nadu.
So, do you sit and look at frogs and make a list of things they eat?
We began by documenting the frog species found in rice paddy fields. The Ornate Narrow-mouthed frog (Microhyla ornata), and the Common Cricket frog (Minervarya caperata) were the most abundant species. We decided to explore their diet patterns. Of course, waiting around for a frog to make a meal would be painfully slow and instead, people would collect the frogs, kill them, and look inside their stomachs.
But these days there are techniques to examine the frogs’ diet without having to kill them. Just like how doctors wash our stomachs if we ingest poison, one can do the same on frogs. A small cannula is inserted into their stomach from their mouth and clean water is pumped in. The water, when gushing out, brings all the stomach contents out which are filtered and sorted under a microscope. While this process sounds gory, it is considered more ethical than simply killing a frog. Night after night, we caught frogs and flushed their stomachs before releasing them back to the paddy fields. Then came the onerous task of looking through a microscope to identify the stomach contents and sort out the diet.
You are what you eat.
Both species of frogs ate a lot of insects, spiders, and some had eaten small stones as well. We managed to examine the stomach contents of 57 individuals of two species and found arthropods belonging to eight orders and 56 distinct taxonomic units. Taxonomic units are a way of classifying species when the specimen is damaged to identify with certainty. The two species, the Ornate Narrow-mouthed frog and the Common Cricket frog consumed a similar number of prey items and volume of prey despite the difference in size. The key differences were in terms of the diversity of prey items consumed by either species with little overlap in the orders and hardly any overlap in the taxonomic units.
For example, the Ornate Narrow-mouthed frogs almost always consumed ants, but the cricket frogs ate a lot of leafhoppers and occasionally, spiders. From this, we could conclude that although the Ornate Narrow-mouthed frog is smaller than the Common Cricket frog, they appear to eat similar volumes of very different prey items. Thus, the strategies to catch the arthropods must have been different. Do they lay in wait for the prey to pass by or do they actively go in search of them? We do not know for sure.
If frogs go extinct, so will we.
Historically, frogs must have been the primary means of pest control in rice paddy fields and other agricultural activities globally. With the invention of pesticides, it is easy to protect crops by saturating the fields with pesticides that indiscriminately kill insects. We do not know how frogs persist in these depleted habitats. Surely the diversity of insects that are present in paddy fields must be lower than those found in lakes or reservoirs. This needs to be studied in the future.
However, frogs seem to continue helping the farmers by eating whatever little pests they can find. Many of the leafhoppers are known to a damage rice paddy and are also vectors for crop diseases. Under the cover of the dark, the frogs appear to be making our life easy by eating these insects. If one frog ate about 150 insects in one night, imagine how many insects an army of frogs could consume every night! With both amphibians as well as insects on the decline globally, we must document the processes that enable the seamless functioning of our fragile ecosystem. That process begins by being aware that frogs are helping us without our knowledge, and they do not seek any reward. Perhaps the only reward we can bestow upon them is to let them hop around, maybe a little closer to us.