Forest monitor lizard

A unique prey for a unique predator

In Photo: One of the first photographs in life of the newly discovered (Welton et al. 2010) forest monitor lizard, Varanus bitatawa from Barangay Dibuluan, San Mariano, Isabela

‘You are what you eat.” Besides applying to humans, this saying is also true with the iconic Philippine eagle, a unique species and prolific hunter with a distinct diet.

A critically endangered species, with only about 400 pairs left in the wild, the Philippine eagle continues to survive and thrive in the wild despite the continuing destruction of their natural habitats.

This underscores the importance of protecting the country’s remaining virgin forests, rehabilitating open, degraded and denuded forest, to allow the country’s rich biodiversity to thrive and take care of its own, including the Philippine eagle, the country’s National Bird.

Verified report

Experts studying the iconic Philippine eagle, under the auspices of the Philippine Eagle Foundation (PEF), recently discovered the existence of possibly a thriving population of forest monitor lizards in Apayao province in the Cordillera Administrative Region in Luzon.

The accidental discovery of the Varanus bitatawa, a fruit-eating forest monitor lizard, came after a Philippine eagle brought home its prey to its nest in the forest of Apayao province.

Scientists confirmed the prey to be the carcass of the unique monitor lizard, or the Northern Sierra Madre forest monitor lizard.

The Philippine eagle nest in Apayao is some 80 kilometers away from the Northern Sierra Madre ranges in Cagayan, the closes part, where the monitor lizard is known only to exist and nowhere else in the region.

This report appeared in the paper, authored by Tatiana Abano-Sarigumba, Jayson Ibanez, Rafe M. Brown and Luke J. Welton which was published in the Herpetological Review, the official bulletin of the Society for the Study of Amphibians and Reptiles in June 2018.

Unique prey

“As the country’s top carnivore of the forest canopy, it [Philippine eagle] is an important component of the Philippine forest food web. Indeed, our national bird maintains a complex relationship with the equally unique animals that it shares its forest habitat with. And through patient eagle research, we are discovering the occurrences of other mysterious forest creatures,” Dennis I. Salvador, PEF executive director, said in an e-mailed message to the BusinessMirror on July 31.

The forest monitor lizard was first detected in the Cordillera in 2015 during another diet-behavior study of a nesting pair of Philippine eagles in the mountain of Calanasan, according to the study.

“One of the adults delivered a disarticulated limb of a varanid lizard to the eaglet on the nest,” the report said. Besides the forest monitor lizard, the adult eagles have brought, on six different occasions, Philippine water monitor lizards, the report added.

The second encounter of the forest monitor lizard was in 2016 during monthly forest patrols by local “Green Guard” volunteers organized by the PEF and the municipality of Calanasan.

“Finding the bitatawa and the nesting of the Philippine eagles in the range, both species of which are forest specialists, indicate that the forest ecosystem is thriving well and that it is able to support the needs both of the prey and predator populations,” said Abano-Sarigumba, the senior biologist for the Luzon Conservation Program of PEF at the time of the study.

Abano-Sarigumba documented the prey during nest observations and later had the prey photos examined by expert herpetologists.

Definitive evidence

Welton, the collection manager of the Kansas University Biodiversity Institute and Natural History Museum, said the paper documented the first definitive evidence of the presence of frugivorous monitor lizards in northwestern Luzon.

Welton, who also wrote scientific papers extensively about the lizards in the Philippines, including the discovery of the new species Varanus bitatawa, said the discovery highlights the uniqueness and conservation priority of the Cordillera faunal region.

“Forest obligates, like frugivorous monitors and Philippine eagles, require large tracts of intact and healthy forest, and so the presence of both in the northern Cordillera is a good sign about the health of that ecosystem. I think the biggest question that remains is whether this occurrence of Varanus bitatawa represents an isolated population in the northern Cordillera, or if the species, in fact, occurs throughout the entire Cordillera Mountain Range. Hopefully, additional surveys and biodiversity monitoring will be able to answer these questions,” he said.

Fruit-eating lizard

The Varanus bitatawa, Welton said, is unique because it is a fruit-eating lizard.

He added that from the few studies that have been done on fecal samples and gut contents, it is known that the primary fruiting trees consumed by frugivorous monitors are Pandanus and Canarium.

“During the description of bitatawa, we examined the stomach contents of the adult male holotype from Casiguran. This individual had partially digested husks of Pandanus fruits, as well as shell fragments from what looked like a giant land snail,” Welton said.

He said Filipino colleagues from University of the Philippines-Los Baños found similar contents in the stomachs of animals in Isabela, as well.

“To date, there hasn’t been any specific study on the dietary preferences of bitatawa. Apparently, the third species of frugivore, Varanus mabitang from Panay, has a higher preference for fruit than either of the other two species,” he said.

Elusive bird

Asked through e-mail if there was an official record of a Philippine eagle nest in Northern Sierra Madre, Jayson Ibanez, PEF director for research and conservation, said there is an abandoned or inactive eagle nest in San Mariano, Isabela, that was discovered in the 1990s.

A juvenile Philippine eagle and its parents were also spotted somewhere in the Northern Sierra Madre Natural Park in Isabela between 2003 and 2004, leading to the conclusion that Philippine eagles are naturally found within the Sierra Madres.

However, he said no active nest—which means with an egg, chick or young—were ever found.

“So they are there, but their nests are difficult to find,” he said.

Elusive lizard

The forest monitor lizard, on the other hand, is equally elusive. No official record of this species have ever been made outside its known habitat.

Before the discovery, its population is previously thought to be restricted in that part of the Sierra Madre. According to the paper, titled “Records of Northern Sierra Madre Forest Monitor Varanus bitatawa from Northern Cordillera Mountain Range of Luzon Island, Philippines,” the specimen of this lizard have been obtained only from the Sierra Madre range, particularly within the provinces of Cagayan, Isabela and Aurora.

In 2012 there was a scratch mark of what believed to be of the same species found on a Pandanus tree in Pagudpud, Ilocos Norte, indicating its presence beyond its documented range. Because of lack of sufficient data, the conservation status of the forest monitor lizard remained unknown.

Prolific hunter

The Philippine eagle is a prolific hunter. Ibañez said that it is possible that the bitatawa was caught by the eagle while the lizard was warming itself under the sun.

“Since lizards are cold-blooded, they needed to sunbathe to warm themselves during early morning or after prolonged cold rains. Exposed, the lizard was an easy prey,” he explained.

According to Ibañez, the eagle pair was observed to feed on 12 types of animals, and the bitatawa was one of them.

“The availability of the bitatawa as eagle prey helps fulfill the food needs of a growing eaglet. As an alternate prey, the eagles can shift to hunting bitatawa when there are more of them and less of the other prey species. Its existence, therefore, ensures the survival of both the adult eagles and their growing young,” he added.

Thriving secretly

According to Ibañez, forest monitor lizards, particularly the Varanus bitatawa, may have been thriving in Northern Cordillera all along.

“Apparently, it has been there all along. The locals, or indigenous Isnag of Apayao, have a local name for it. They call it lopi. The fact that it has a specific local name also means it has been there since time immemorial,” he said.

Moreover, he said the regular bayawak (monitor lizard), commonly seen on the ground, on the other hand, is locally called banyas—another proof that locals recognize the tree-dwelling bitatawa is very different from the mostly ground-dwelling banyas.

According to Ibanez, the only way a Philippine eagle can bring home a prey is when they exist or occur within the eagles’ range or territory.

“We don’t think it [bitatawa] migrated. Again, apparently, you have a resident population of the bitatawa in Apayao, and most probably also in Pagudpud,” he said.

He said it is also not possible for the eagles to have gone beyond its range to hunt to bring home prey to feed its young.

“This is not possible. Adult eagles do not migrate. They maintain a territory of their own. The Calanasan, Apayao, eagle pair, therefore, only hunts within its territory in Calanasan,” he said.

Significant discovery

According to Ibañez, the significance of the paper is its having found the evidence that the bitatawa and the Philippine eagles maintain an important ecological relationship.

“The eagle feeds on the bitatawa. Thus, it’s a necessary prey/food species for the eagles. If we are to conserve the rarest and most endangered eagle in the world, we must also conserve and protect the bitatawa. The eagle, in turn, is a predator/hunter of the bitatawa,” he explained.

He said the science of ecology tells us that predators feed mostly on the sickly and weaklings of its prey or food animals.

“Therefore, by feeding on the animals that are less fit, the eagles naturally cull those that are unhealthy or are genetic rejects. Doing so maintains a healthy and fit population of bitatawa,” he said.

Brown, curator-in-charge, Herpetology Division, Kansas University Biodiversity Institute and professor at the Department of Ecology and Evolution, Natural History Museum, said, “A healthy bitatawa population necessitates protected-area establishment, public education aimed at local understanding and appreciation of these unique lizards, enforcement of anti-poaching/smuggling for pet trade, legislation and heavy penalties for hunting bitatawa.”

According to Ibanez, it now appears that the bitatawa is a normal part of the diet of the Philippine eagle. “Apart from the bitatawa, Luzon eagles also eat the ground-dwelling bayawak. Thus, monitor lizards appear to be a normal part of their diet. Eagles on Mindanao island also feed on monitor lizards,” he said.

Lesson on interconnectivity

According to Brown, the study also demonstrated by example how biodiversity conservation must involve coordinated networks of people and protected areas, such that information-sharing, data and technology transcend single sites or individual national parks and protected areas.

“For example, to understand and appreciate the diet of eagle populations in the Cordillera, we may need to know something about herpetological species diversity in the Sierra Madre, where eagles may fly to hunt­—or even other islands. The bottom line: integrating fundamental baseline data across protected areas, islands or political boundaries is often the key to understanding predator-prey relations and biodiversity—because animals and plants don’t adhere to the artificial land use constructs of mankind,” he said.

Ibanez added that the forests of Calanasan, which is part of the bigger Apayao lowland forest Key Biodiversity Area, are an important habitat of the bitatawa.

“The bitatawa is also a highly unique species of monitor lizard, which is found so far only in Northern Luzon. Being restricted only in Northern Luzon means its population numbers are relatively low, making it vulnerable to extinction,” he said.

“It also proves that our endemic species maintain close and complex relationships, which is the very foundation of a healthy Philippine forest ecosystem. The predator-prey relationship between the eagles and the bitatawa is a concrete example of that profound ecological ties,” he said.

The authors of the study recommended that the public should support biodiversity research and monitoring, including collaborative researches between local or indigenous people communities and biologists.

“The monitoring work by local citizens like the indigenous Green Guards of Calanasan, for instance, are indispensable to conservation. The Calanasan Green Guards helped locate the nesting eagles in Calanasan and assisted in the documentation of their nesting behavior and food habits. Through their local knowledge of flora and fauna and patient monthly foot patrols of indigenous protected areas, they also found and photographed a live specimen of the bitatawa. The bitatawa case is just one example of the scientific contribution that local citizen scientists can make to research and conservation,” Ibanez explained.

“It’s also worth mentioning that the initial scientific description of Varanus bitatawa was only a ‘discovery’ to western scientists…. But, in fact, the species was well known to local indigenous peoples’ groups living in and around the Casiguran, Aurora, where it was first reported in 2010. Clearly, local knowledge in the forests of the northern Philippines has much to teach western science,” Ibañez added.

Image Credits: Wikimedia Commons

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Jonathan L. Mayuga is a journalist for more than 15 years. He is a product of the University of the East – Manila. An awardee of the J. G. Burgos Biotech Journalism Awards, BrightLeaf Agricultural Journalism Awards, Binhi Agricultural Journalism Awards, and Sarihay Environmental Journalism Awards.