feeding strategy

Guest blog post: Unique feeding behaviour of Asian kukri snakes gutting frogs and toads

Guest blog post by Henrik Bringsøe

In September 2020, we reported the first evidence for a newly discovered behaviour in snakes, as we provided extensive photographic documentation, demonstrating a macabre feeding strategy of Asian kukri snakes of the species Oligodon fasciolatus, the Small-banded Kukri Snake: a snake cutting open the abdomen of a toad, inserting its head and pulling out the toad’s organs which are then swallowed.

A Small-banded Kukri Snake attacking a Painted Burrowing Frog, which is inflating its lungs. The snake makes rotations about its own longitudinal body axis (“death rolls”), as it is biting and holding the belly of the frog. Video by Navapol Komanasin.

This is done while the toad is alive and it may take several hours before it dies! We have now provided new evidence that two other species of kukri snakes also exhibit this highly unusual behaviour: Oligodon formosanus, the Taiwanese Kukri Snake, and Oligodon ocellatus, the Ocellated Kukri Snake. These three species are closely-related and belong to the same species group in the genus Oligodon.

On two occasions in Hong Kong, a Taiwanese Kukri Snake was observed eviscerating frogs of the species Kaloula pulchra, the Painted Burrowing Frog or Banded Bullfrog. In one case, the snake had cut open the belly of the frog and inserted its head deep into the frog’s abdomen. In this position, the snake performed repeated rotations about its own longitudinal body axis, also called “death rolls”! We believe that the purpose of these death rolls was to tear out organs to be subsequently swallowed. In the other case, the organs of the frog had been forced out of its abdomen.

A Taiwanese Kukri Snake with its head buried deep into the abdomen of a Painted Burrowing Frog. Initially, the frog moves its long fourth toe of the left hind foot up and down 21 times. During the subsequent active struggle, the snake makes three “death rolls”. Video by Jonathan Rotbart.

A Small-banded Kukri Snake was also observed eating a Painted Burrowing Frog in Northeast Thailand, but it swallowed the frog whole. That snake also performed death rolls, although we have never before seen that behaviour in this species of kukri snake (this species was treated in our 2020 paper). This frog is not considered toxic and is also eaten by other snakes. We believe that prey size is crucial in determining whether the gape width allows large prey to be swallowed whole by kukri snakes. If the prey is too large, the snake may eviscerate a frog or toad, in order to swallow the organs. Afterwards, the snake will perhaps be able to swallow the rest of the frog or toad.

In another new paper, we describe and illustrate the Ocellated Kukri Snake eating the toxic toad Asian Black-spotted Toad (Duttaphrynus melanostictus) in Vietnam. Initially, the large snake’s head was buried past its eyes into the abdomen of the toad, but eventually the snake swallowed the toad whole despite its toxicity. We interpret this behaviour that kukri snakes are in fact resistant to the toads’ cardiac glycoside toxins. Furthermore, toads are only eviscerated if they prove too large to be swallowed whole.

An Ocellated Kukri Snake first pierced this poisonous Asian common toad and buried its head deeply into the abdomen of the amphibian, as it was probably eating the organs. However, as seen in the photo, the kukri snake proceeded to swallow the toad whole.
Photo by James Holden.

We suggest that the unique behaviour of eviscerating frogs and toads and eating their organs may have evolved specifically in a group of kukri snakes named the Oligodon cyclurus group or clade because it has now been recorded in three of its species, namely Oligodon fasciolatus, Oligodon formosanus and Oligodon ocellatus. We hope that future observations may uncover additional aspects of the fascinating feeding habits of kukri snakes though we may indeed call them gruesome.

See more video recordings of the snakes’ unique, even if quite gruesome, behaviours provided as supplementary files to one of the discussed research papers.

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Research papers:

Bringsøe H, Suthanthangjai M, Suthanthangjai W, Lodder J, Komanasin N (2021) Gruesome twosome kukri rippers: Oligodon formosanus (Günther, 1872) and O. fasciolatus (Günther, 1864) eat Kaloula pulchra Gray, 1831 either by eviscerating or swallowing whole. Herpetozoa 34: 49-55. https://doi.org/10.3897/herpetozoa.34.e62688

Bringsøe H, Holden J (2021) Yet another kukri snake piercing an anuran abdomen: Oligodon ocellatus (Morice, 1875) eats Duttaphrynus melanostictus (Schneider, 1799) in Vietnam. Herpetozoa 34: 57-59. https://doi.org/10.3897/herpetozoa.34.e62689

How bears bulk up ahead of the summer: A study into the Asiatic black bear’s spring diet

Much like gym enthusiasts, every year Asiatic black bears seem to be on the lookout for protein-rich food ahead of the summer, so that they can bulk up on lean muscle mass in place of the fat tissue formed last year prior to hibernation. This was concluded in a study by Dr. Shino Furusaka, Tokyo University of Agriculture and Technology and his team, based on direct observations on bears living across an area of about 60 km2 in Japan. The study is published in the open access journal ZooKeys.

In order to determine the bears’ food preferences and habits, the scientists followed a large number of animals in the Ashio area of the Ashio-Nikko Mountains in Japan from April to July in both 2013 and 2014. To avoid unnecessary intrusion, they stayed at a distance of at least 200 metres using video cameras with telescopic lenses to document the sightings. Having documented the plant species the bears consumed, the researchers studied their nutritional content and made conclusions about the nutrients needed for the species after hibernation.

While heavily dependent on food availability, generally the bears were noted to prefer food which is high in protein, but poor in fibre — likely because their stomachs and intestines were unable to efficiently digest the latter. Furthermore, the protein-rich diet ensures that the muscle mass is rebuilt to replace the lost winter fat.

Interestingly, the bears were observed to change their food preferences as spring progressed and that seemed to be linked to the shifts in the nutritional value of the available food.

Starting with their observations at the beginning of April, the scientists did not record any feeding behaviour until the end of the month. As leaf flush was yet to occur, the animals were active and feeding on overwintered grass. However, in early May, the bears began consuming newly emerged leaves, grass and, later in the month, they added flowers to their menu.

A shift in behaviour occurred in the following months. In June and July, the bears were seen to feed mainly on ants, with a small portion of their food intake consisting of grasses, sika deer carcasses and bees. Curiously, when the scientists looked into the nutritional content of the same plants which the animals sought only a few weeks ago, they found out that now they were significantly poorer in protein and richer in fibre.

Another finding showed that the calories in the different items were not related to the choice of food which likely proves that the key factor is none other than the amount of protein, provided that the fibre value is low enough for good digestibility.

Understanding the food preferences and habits of animals, as well as the reasons behind them, is essential for the development and revision of habitat management plans. However, previous knowledge of the feeding behaviour of Asiatic black bears has been based solely on faecal analyses which has not provided sufficient details on which nutritional factors influence the use of particular foods.

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Original source:

Furusaka S, Kozakai C, Nemoto Y, Umemura Y, Naganuma T, Yamazaki K, Koike S (2017) The selection by the Asiatic black bear (Ursus thibetanus) of spring plant food items according to their nutritional values. ZooKeys 672: 121-133. https://doi.org/10.3897/zookeys.672.10078

Feeding strategies in competing hummingbird species observed in a small area in Brazil

Being the vertebrates with the highest metabolic rate thanks to their rapid wing flaps, the hummingbirds have evolved various types of feeding behaviour. While the nectar-feeders tend to go for food high in energy, strong competition affects greatly their preferences and behaviour towards either dominance, subordination, a strategy known as trapline and a fourth one named hide-and-wait, conclude the Brazilian scientists Lucas L. Lanna, Cristiano S. de Azevedo, Ricardo M. Claudino, Reisla Oliveira and Yasmine Antonini of Universidade Federal de Ouro Preto. Their conclusions following six months of observations in an Atlantic Forest remnant in southeastern Brazil are published in the open access journal Zoologia.

To test previous assumptions as well as their own hypotheses, the researchers placed artificial bird feeders filled with variable in concentration sugar-water solutions across four separate patches at the core of the forest fragment.

The scientists sought to find out whether the birds would show clear preference for the most sugary food source; whether larger size and heavier weight would guarantee better access to the most nutritious feeders; what strategies would be adopted by each species; and which ones would prove the dominant and most aggressive.

As expected, the scientists concluded that the birds prefer the most sugar-dense solutions. However, when subordinate species, such as the white-throated hummingbird and the versicoloured emerald, confronted dominant species guarding the most nutritious food sources, they would be either frightened or expelled following a short chase. Subsequently, these hummingbirds would resort to the feeders with low-sugar solutions.

In their turn, the Brazilian ruby and the violet-capped woodnymph proved to be the dominant and most aggressive species in the studied area. Upon seeing an ‘intruder’ in their territory, which might be either another species, or belonging to their own, they would vocalise their threats, alert them by perching by the feeder, or expel them following a short pursuit. However, they would only try to limit the access for subordinate hummingbirds if the energy that could be gained from the feeder exceeded the energy loss of the chase.

Contrary to another initial hypothesis, it was not the largest and heaviest species that were the dominant ones. There were two species of hermit hummingbirds which were the largest and the heaviest, however, they expressed no territorial or aggressive behaviour. Instead, they were recorded intruding in the territory of the two dominant hummingbird species. In their turn, the Brazilian ruby and the violet-capped woodnymph would often frighten them. Nevertheless, rather than fleeing, the ‘castaways’ were seen hiding in the shrubs, remaining quiet, and returning to the feeder as soon as the dominant bird was gone. This behaviour strategy, named hide-and-wait, has not been reported in hermit hummingbirds prior to this study, according to the authors.

Having reported all feeding strategies in their study, the scientists conclude that the dominant territorial species and the trapliners feed most frequently and most sufficiently, as they use the most sugary sources.

However, the authors note that the high abundance of food, as well as the presence of aggressive territorial species might have affected the hummingbirds’ behaviour and preferences.

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Original source:

Lanna LL, de Azevedo CS, Claudino RM, Oliveira R, Antonini Y (2017) Feeding behavior by hummingbirds (Aves: Trochilidae) in artificial food patches in an Atlantic Forest remnant in southeastern Brazil. Zoologia 34: 1-9. https://doi.org/10.3897/zoologia.34.e13228