Otters Turn Insect Hunters in Chiapas River Where Fish Run Scarce

On a tropical river in southern Chiapas, Mexico, a population of Neotropical otters has rewritten the rulebook on what these aquatic carnivores are supposed to eat. A year-long study of the El Despoblado River near Villa Comaltitlán found that insects made up more than half of the diet of the local otters, Lontra annectens, a finding that stands in stark contrast to the fish-dominated menus recorded across the rest of the species’ range. The dominant prey was not just any insect but the aquatic larva of the dobsonfly, known to anglers and ecologists as the hellgrammite, a large, armored underwater predator that had never before been documented as a major food source for any otter population.

The research team, publishing in Neotropical Biology and Conservation, surveyed roughly 16 kilometers of riverbank in monthly visits from April 2018 to April 2019, collecting 63 otter scats identified by their size, shape, color, and odor. Back in the laboratory, each sample was washed in soap and water to separate the contents and neutralize bacteria, then examined under a stereomicroscope. Food remains were identified to the lowest possible taxonomic level using reference guides and by direct comparison with fish, crustaceans, mollusks, and insects captured in the same river near the scat sites. The team calculated the percentage of occurrence, the frequency of occurrence, and the relative biomass contributed by each prey type, then compared the results against 21 published diet studies of Lontra longicaudis and L. annectens from throughout the Americas.

The numbers were unambiguous. Dobsonfly larvae of the genus Corydalus appeared in 51 of the 63 scats, with a frequency of occurrence of 91.3 percent and a percentage of occurrence of 55.3 percent. Of the scats containing insects, 52 percent included only hellgrammites, while 48 percent also contained fish and crustaceans. Individual samples held between 4 and 13 larvae, averaging eight per scat. Fish, the presumed staple of the species, ranked second with a percentage of occurrence of 34.2 percent, represented by characins, catfishes, and silversides. Crayfish of the family Pseudothelphusidae came third at 10.5 percent. Biomass analysis reinforced the picture: dobsonfly larvae accounted for about 42.9 percent of the total biomass consumed, with crayfish at 29.2 percent and fish at 27.9 percent. In other words, these otters derived roughly half of their nutritional intake from insect larvae.

Across the Neotropics, insects typically rank fourth in the otter diet, behind fish, crustaceans, and mollusks, and the average contribution of insects across the 21 comparative studies was just 11.3 percent. The highest previously recorded values for L. annectens were 32.4 percent in the Marabasco River of Jalisco, 29.9 percent in the Bavispe-Yaqui River of Sonora, and 22.8 percent in the Santiago River, also in Jalisco. For the South American species L. longicaudis, the peak was 23.3 percent in El Rey National Park, Argentina. The Chiapas population’s 55.3 percent is nearly double the previous record and almost five times the continental average. In some studies, insects were entirely absent from otter diets, and researchers have suggested that insect remains in scats may sometimes be indirect, swallowed inside the gut contents of consumed fish. That explanation cannot apply here, where larvae were recovered in the majority of samples and often as the sole prey type.

The authors attribute this dietary inversion to the scarce availability of fish and crustaceans in this stretch of Chiapas. Neotropical otters are facultative predators specialized in aquatic prey, but they are also classic opportunists, showing wide geographic and seasonal plasticity. When preferred prey declines in abundance, alternative prey tends to increase in the diet. What makes the El Despoblado population unusual is that insects did not merely supplement a reduced fish intake; they became the main food source outright, exceeding both fish and crustaceans even during the rainy season, when most insect-containing scats were collected in June. Hellgrammites are abundant in the river, easy to catch, and nutritionally valuable, making them an ideal substitute when vertebrate prey runs thin.

The biology of dobsonflies helps explain why their larvae suit otter foraging so well. Adult dobsonflies are flying insects, but their larvae are fully aquatic and can live underwater for one to three years before pupating, spending most of their time sheltered under rocks. That sedentary, slow-moving behavior aligns with a general pattern in otter predation: Neotropical otters preferentially take slow-moving prey with limited escape abilities. Although capturing hellgrammites probably requires considerable time and energy, since the larvae must be extracted from beneath submerged stones, the nutritional payoff is high. Insects supply otters with abundant amino acids, proteins, and vitamins, along with a balanced content of essential macronutrients.

Larvae may be even more valuable than adult insects. Larval tissues carry high fat content, and the larval exoskeleton contains less chitin than that of adults, making larval structures easier to digest. For a small carnivore with a high metabolic rate, a prey item that combines high fat, digestible protein, and predictable availability under rocks represents an efficient energy source, particularly in a river where fish are scarce. In tropical regions such as Chiapas, hellgrammites are probably most abundant during the rainy season, and the otters fed on them preferentially at that time, contrary to the pattern in most other populations where insects never become the primary food regardless of season.

The study also carries taxonomic weight. A 2024 phylogenomic analysis of ultraconserved genomic elements demonstrated that the Neotropical otter is actually two species: L. longicaudis, occurring throughout South America from the cis-Andean region of south-central Colombia through Brazil to northern Argentina, and L. annectens, occurring in Mexico, Central America, and the trans-Andean portion of Colombia, Venezuela, and Ecuador. In Mexico, L. annectens inhabits tropical areas from southern Tamaulipas to the Yucatán Peninsula on the Gulf side and from Sonora to Chiapas on the Pacific side. The split means that many older references citing L. longicaudis actually describe L. annectens, complicating efforts to understand the ecology of each species individually and making locally grounded studies like this one all the more important.

Neotropical otters are top predators in freshwater ecosystems and are considered umbrella species, whose protection benefits a wide variety of other organisms. Their presence in a waterbody signals good conservation status of the aquatic environment, and they serve as bioindicators of the fish, crustaceans, mollusks, and insects on which they feed. The El Despoblado River, bordered by riparian vegetation including figs and rubber trees and surrounded by remnants of high tropical forest amid agricultural land, evidently supports a prey community in which megalopteran larvae dominate. Whether the otters’ insect-heavy diet reflects seasonality, an isolated population exploiting the most abundant resource, or a persistent local adaptation remains an open question that the authors say requires further study on this river.

What the findings establish is that L. annectens is an opportunistic carnivore with a flexible diet, consuming food items according to their availability, abundance, and behavior. When main prey is absent or at low density, otters display opportunistic behavior and consume prey that is not normally common or that usually occurs in low abundance. The Chiapas otters push that flexibility to an extreme never before quantified, demonstrating that a predator long defined by its taste for fish can thrive on a diet of aquatic insects when the river demands it. For conservation planners, the lesson is that protecting otters means protecting the full prey spectrum of a river, down to the larvae hiding under its rocks.

Subject of Research: Ecology

Article Title: Unusual importance of insectivory in the diet of the Neotropical otter, Lontra annectens (Major, 1897) ( Mammalia , Carnivora )

Article References: Pozos-López, L. Á., González-Ruiz, N., & Ramírez-Pulido, J. (2024). Unusual importance of insectivory in the diet of the Neotropical otter, Lontra annectens (Major, 1897) (Mammalia, Carnivora). Neotropical Biology and Conservation, 19(4), 505-516. https://doi.org/10.3897/neotropical.19.e131700

Image Credits: AI Generated

DOI: 10.3897/neotropical.19.e131700

Keywords: Neotropical otter, Lontra annectens, hellgrammites, dobsonfly larvae, diet analysis, Chiapas, insectivory, freshwater ecology, opportunistic predation, conservation, Unusual, importance