Vital seed dispersers
In the context of forests, the fruit-eating habit of these primates makes them ecologically important as seed dispersers. For that matter, about one-third of fruit eating species in forests are primates. After eating fruits, primates have the ability to carry the seeds far and wide. They can thus bring about forests in new areas, and help in the continued survival of plant species, and forests as a whole.
A fruit feast
Studying what kind of fruits primates prefer can reveal their importance in forest dynamics — the processes that shape and change a forest ecosystem over long timescales. Two researchers, Asmita Sengupta and Sindhu Radhakrishna from the School of Natural and Engineering Sciences at the National Institute of Advanced Studies, Bengaluru undertook a study at the Buxa Tiger Reserve in West Bengal to explore this further.
The researchers chose a group of macaques near the Checko Timber Depot in the buffer zone of the reserve. The study group comprised 18 males (9 adults and 9 juveniles), 21 females (11 adults and 10 juveniles) and two infants. These monkeys were completely dependent on natural resources. “The broader aims of this project were to look at how seasonal fruit availability, fruit characteristic preference of the macaques themselves and human interference in the form of providing food subsidies to the macaques influenced seed dispersal effectiveness of Rhesus macaques,” said Asmita.
The study was carried out between July 2012 and June 2013. Individuals were observed for snapshots of 30 minutes, to note various aspects of their feeding behaviour — like which parts of the fruits they fed on, and whether the seeds were eaten or not. For each month, the researchers calculated the amount of fruit available, and the physical characteristics of the available fruits. The monkeys ate 43 out of 80 observed species of fruits. The period from May to September was estimated to be the months during which fruit availability was high.
Researchers working on primates in other countries have shown that primates prefer sweet, juicy fruits with few seeds — very similar to what we as humans like, too! When it comes to species like macaques that eat whatever they find — from garbage thrown by humans and smaller animals to insects and fruits — it is hard to predict what kind of fruits they may prefer, making this study interesting. On a trip to Buxa Tiger Reserve in 2013, Asmita saw troops of Rhesus macaques exclusively eating fruits, and decided to study their fruit preferences further.
At Buxa, there were enough macaque groups that were completely dependent on the forest. This is a rarity, because human interference in the form of providing food, accidentally or on purpose, is common in many parts of the country. This trait of the region made comparison between forest areas disturbed by humans and undisturbed areas easier. “Buxa was also logistically convenient. A number of researchers had worked there before us; so setting up a base was rather convenient and we had highly experienced field assistants also,” she added.
Macaques seemed to eat all kinds of fruits, which varied in terms of pulp, seed and hardness. However, they seemed to prefer large fruits with good fruit protection and medium-large seeds. Chaplash (Artocarpus chaplasha) fruit was the most preferred throughout the year. They also showed preference to fruits with juicy edible tissue that could be pierced with a fingernail, and did not often eat dry fruits with little or no pulp.
“I observed that usually juveniles avoided feeding on fruits with hard covers such as Elephant apple (Dillenia indica). For Artocarpus chaplasha, I observed that even within the same tree, the juveniles usually fed on smaller fruits as compared to adult individuals. I have not recorded any sex based variations,” explains Asmita.
Within primate troops, there are usually hierarchies, with dominant individuals that exert more ‘control’ over others and subordinate individuals. “While the dominant individuals fed on fruits for a longer period of time sitting on the same tree, I believe subordinate individuals may be more important for long distance dispersal as they would store larger number of fruits in their cheek pouches, move away from dominant individuals and spit out the seeds away from the parent trees,” she opined.
The Rhesus macaque shows more tolerance towards human disturbances than many species of birds or bats, who are also known to be important seed dispersers. Thanks to this and the fact that these monkeys preferentially disperse seeds of large fruits, the researchers concluded that they may have a significant role to play in seed dispersal in forests. This is in opposition to the common view that macaques in general tend to destroy seeds. At Buxa, the Rhesus macaques destroyed seeds of only six species, and acted as dispersers for 41, out of the total 80 species.
Are these results specific to these monkeys in Buxa Tiger Reserve? “Fruit trait preference is measured in terms of fruit availability. At sites with varying plant community compositions, fruit availability and hence, preference measures might differ. But I do think that the results will remain similar with respect to implications for seed dispersal i.e., juicy fruits with easily removable pulps would be dispersed whereas those without any discernible pulp or dry pulp would be destroyed,” concludes Asmita.
(The author is with Gubbi Labs, a Bengaluru-based research collective)