Spotlight on: Black Tree Monitor

When it comes to seeing animals at zoos, guests often prefer to head for large, charismatic mammal species, like giraffes and rhinos or lions. But lizards, snakes, crocodiles, turtles, and other reptiles are fascinating animals in their own right—and they are important to their ecosystems as well as human health and culture!

As predators, reptiles help to control populations of small animals such as rodents and insects, some of which carry diseases to humans. As prey, they sustain larger animals and are an important part of the food cycle. Many species are indicator species, reflecting the health of their environment; monitoring how well they do in their native ecosystems can help scientists understand the state of the areas they live in. And the poisonous and venomous qualities of some reptiles have helped advance the field of medicine to treat certain conditions in humans.

Lincoln Park Zoo cares for 27 species of reptiles, most of which reside at Regenstein Small Mammal-Reptile House. These species include black tree monitors (Varanidae Varanus beccarii), which are distinctive because of their large adult size, their shiny black scales, and their curiosity and intelligence.

Magnificent Monitors

Black tree monitors are slender reptiles with a prehensile tail that makes up two-thirds of their almost 4-foot length. They have long claws at the end of each toe and long forked tongues. They hail from forests and swamps in the Aru Islands in Indonesia, off the coast of New Guinea. In the wild, these solitary animals eat small invertebrates, mammals, amphibians, and birds and eggs.

After mating, female black monitor lizards may lay up to seven eggs twice a year in a raised nest. Incubation lasts five to six months. Juveniles hatch with dark gray or yellowish-green dot markings, but as they mature, they turn completely black. When born, black tree monitors are around 8 inches in length, with the majority of that being their tail. They reach their adult size at 2 years of age.

Black Tree Monitors at Lincoln Park Zoo

In 2021, a male black tree monitor arrived at Lincoln Park Zoo from Brookfield Zoo Chicago. Two years later, a female came from Indiana’s Potawatami Zoo. In early 2024, the two were introduced, but the female hid in her habitat for the first couple of weeks. By February, however, the two were observed breeding, and in May of that year she laid her first clutch of six eggs. Five eggs were incubated artificially at between 84–86 degrees for six months.

Image courtesy of Animal Keeper Bryan Summerford

Since these two adults started producing offspring, Lincoln Park Zoo has hatched a total of nine young monitors—five in 2024 and four in 2025, and all now reside here at the zoo. The number of clutches incubated, as well as the matching of the male and female, is based on Black Tree Monitor Species Survival Plan® recommendations. The SSP will determine if the current pair will have more offspring, based on genetic representation of the pair in the AZA population, as well as current population size as it pertains to the sustainability of the species in AZA zoos.

As with other animals at the zoo, black tree monitors undergo training. This is especially helpful since baby monitors are extremely fast and somewhat delicate—and their bite can be quite harmful to humans. So, training reduces risks while allowing optimal care for the young lizards. “They are conditioned to enter a cylindrical pod or tube that is connected to their habitat,” explains Curator Dan Boehm. “A drop-down gate can be closed with the lizard inside so that keepers can get a weight, more closely examine the individual, or simply service their habitat without the lizards present. Motivated by food, the juvenile lizards enter pods voluntarily—growing monitors are hungry and motivated, so getting them to ‘pod,’ as we call it, is very easy.”

Right now, you can come to Lincoln Park Zoo and view the parent black tree monitors and one of their offspring in Regenstein Small Mammal-Reptile House.

Bringing Up Babies

At Lincoln Park Zoo, staff have developed a lot of experience caring for young tree monitors, thanks to two hatches in two years (and 17 total hatches from five separate clutches back in 2003 and 2004 as well). It’s more complicated than you might imagine, although the use of artificial incubators can help increase the number of hatches. Even getting the young monitors to eat can be difficult.

“Beccarii hatchlings can be notoriously difficult to get started eating,” explains Animal Keeper Bryan Summerford, noting that it may take several weeks for the young to meet this milestone after hatch, as they can live off absorbed yolk from the egg for quite some time. “With the first clutch of eggs, keepers attempted various feeding strategies, some quite novel—like offering pureed chicken and a variety of insects. We found that the hatchlings preferred squished dubia roaches, which is not a commonly offered food item for this species. Once they began eating, they soon transitioned to whole insects, but crickets became their favorite item.”

Eventually, the first set of young started eating a delicious and nutritious diet of dubia roaches, crickets, mealworms, and later, superworms. Armed with that experience, keepers were able to get the 2025 offspring eating just three days after they hatched! As all the young black tree monitors grew older, they were able to transition to eating rodents, which they would also learn to do in the wild. As arboreal animals, they live high in the canopy and would not necessarily encounter rodents so quickly.

Until they are a year old, young black tree monitors will be offered crickets, dubia roaches, mealworms, and baby mice.

The Animal Welfare Science Program—Taking Action!

Reptile wellbeing is sometimes overlooked by researchers, who may prefer to focus on larger, rarer, more elusive animals—even though reptiles need help too, and they make up a larger part of zoo and aquarium populations. Lincoln Park Zoo’s Animal Welfare Science Program is hoping to rectify this issue by exploring the quality of life of non-mammalian species in zoo environments. Learn more about that work in this blog post and check out some of the exciting science that zoo staff are doing every day.