Sloth bears may be doing much more than foraging across Rajasthan’s semi-arid deciduous forests. A new study from the Aravalli hills suggests they may also help shape the forests by dispersing viable seeds.

“A relatively small number of careful studies have been conducted on sloth bear diet ecology, and an even smaller number on their role in enabling seed dispersal,” says K.S. Gopi Sundar, independent scientist and co-author of the study. “Since we studied both of these aspects, we are able to show that bears are influencing a number of plant species in germinating more and faster than if they were left to germinate naturally. Clearly, dry deciduous forests’ plant compositions are a function of sloth bear presence, movement, and dietary habits,” he says.

Studying bears in the Aravallis

The research was conducted in the Kumbhalgarh and Todgarh-Raoli wildlife sanctuaries in Rajasthan’s south-central Aravalli range. Together, the protected areas cover about 1,100 square kilometres of dry deciduous forest, scrubland and grassland.

“The two sanctuaries formed a unique ecotone between the drier western Aravalli slopes and the much greener eastern slopes,” says co-author Utkarsh Prajapati, a senior project associate at the Wildlife Institute of India. “This zone of transition, we thought, likely holds a greater diversity of resources for omnivores such as the sloth bear.” Prajapati conducted the research while at the Wildlife Research Laboratory at the Department of Zoology, University College of Science, Mohanlal Sukhadia University (MLSU), Udaipur.

Researchers surveyed the area between November 2021 and July 2022, during winter and summer seasons. They divided the landscape into 5×5 km grids and monitored forest tracks used by sloth bears. Researchers collected 180 scat samples — 90 in winter and 90 in summer to understand what the bears were eating. Sampling during the monsoon was not possible because heavy rains washed away scats and made many areas inaccessible.

Later, in the laboratory, the scats were washed, dried and sorted to identify undigested remains. Researchers recorded plant and animal material and identified food items to the lowest taxonomic level possible. They also surveyed fruiting trees and shrubs along the sampling routes to compare food availability with what the bears actually consumed.

Summer and winter diets

The analysis identified 38 dietary items. Overall, plant material dominated the winter diet, while animal matter became more important during summer. Even so, ants were the most frequently occurring food item in winter, appearing in more than 80% of scats. In summer, termites were the most frequently occurring food item, found in over half the scats.

Among fruits, Grewia flavescens emerged as the most important species during both the seasons. Other commonly consumed fruits included Ziziphus nummularia during winter and Cassia fistula during summer.

“The strong shift in diet was of great surprise. Previous studies on sloth bears in semi-arid regions have shown that they consume a lot of fruits during the summer. During our study, however, the bears shifted to a largely insect diet during the summer, while focusing on fruits (mostly Ziziphus and Grewia) during the winter,” says Prajapati. The authors suggest the unusual weather during the study period may have contributed to this pattern. A wetter-than-normal summer associated with La Niña likely increased soil moisture, making termites and ants easier to access.

“The changing food habits seasonally showed that bears were highly adaptable, switching to foods that were more available in a particular season. This of course would require bears to change their behaviours — dig more for ants and termites in summer, and seek fruits more during the winter,” says co-author Vijay Kumar Koli, head and assistant professor, Department of Zoology, MLSU, Udaipur.

The study also showed that sloth bears did not simply consume fruits in proportion to their availability. Instead, they showed a preference for certain species during summer, including Cassia fistula, Diospyros melanoxylon, Ficus religiosa and Grewia flavescens.

Testing whether bears disperse seeds

To examine the bears’ role as seed dispersers, the researchers selected the eight most important plant species in their diet and compared seeds recovered from bear scats with those collected directly from fruits on trees. For each species, they planted 25 scat-derived seeds and 25 tree-derived seeds and monitored them for up to 180 days, tracking both germination and seedling survival.

The results showed that seeds of six plant species remained viable after passing through a bear’s digestive tract. Germination success varied among species, but many seeds were still capable of germinating after gut passage. In some cases, seeds recovered from bear scats germinated earlier than those collected directly from fruits. For example, Ziziphus nummularia seeds from scats germinated about 50 days after sowing, while seeds collected directly from fruits germinated after around 100 days.

The researchers also found that survival rates of seedlings grown from scat-derived seeds were broadly similar to those grown from tree-collected seeds for several species. However, differences emerged for some plants. Seedlings of Diospyros melanoxylon originating from bear-dispersed seeds survived better, while Grewia flavescens seedlings from tree-collected seeds showed higher survival.

“Other taxa such as insects, birds, and other mammals such as civets also likely carry out similar functions. The next interesting step would be to measure and determine the relative roles of each of these taxa in structuring the forests of the dry deciduous forests. Only then will we truly realise the individual role of the sloth bears,” says Sundar.

The spread of invasives

One of the study’s most striking findings involved Lantana camara, one of India’s most widespread invasive plants. In the germination experiment, lantana seeds germinated only after passing through a sloth bear’s digestive system, while seeds collected directly from the plant’s fruits did not germinate.

“I was surprised that lantana germinated only after the sloth bear had eaten it. Because sloth bears have relatively large home ranges, it is likely that they are dispersing lantana across large landscapes,” says Prajapati.

The findings suggest that sloth bears may contribute to the spread of the invasive shrub by dispersing its seeds over long distances. However, the researchers caution that the effect observed in the experiment was modest. Only three of the 25 Lantana seeds recovered from bear scats germinated.

“Lantana is known to benefit from being eaten by birds such as bulbuls. This study shows that large omnivores such as sloth bears are also helping disperse it and improve its germination. But birds are likely still performing the largest proportion of germination support for this plant species,” says Sundar.

Read more: Biocontrol agents as a solution for invasive species

Rethinking the role of omnivores in forests

The findings highlight an often-overlooked ecological role of large omnivores in semi-arid deciduous forests. “Omnivores are rarely studied in Indian ecosystems, with most effort and resources going to predators and herbivores. But being an omnivore gives species like bears a lot of dexterity to survive in harsh locations where most species would find it hard to live throughout the year,” says Sundar.

The researchers say the findings underscore the need to better understand the ecological roles of species beyond the predators and herbivores that dominate conservation efforts. There are plenty of tiger-centric sanctuaries, but not too many sloth bear-centric ones, he points out. “An additional important takeaway is the necessity to have a wide diversity of species supported in our ecosystems so that the benefits are enjoyed by a variety of species, like the sloth bear in this instance,” he adds.

The study also highlights the need for long-term ecological research. It remains unclear whether sloth bears have only recently begun eating lantana or whether they consume more of it in some regions or over more years than others. “Whether the same pattern occurs elsewhere remains an important question for future research,” says Sundar.

Banner image: A sloth bear in the Aravallis. Image by Utkarsh Prajapati.

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Priyanka Shankar Editor

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BiodiversityConservationEcologyEcosystem servicesEndangered speciesFoodForestsMammalsMountainsPositive EnvironmentalResearchWildlifeRajasthan

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