Oh baby! The Milwaukee County Zoo’s newest resident has just hatched — a critically endangered northern spider tortoise.
The egg hatched on April 12, and it’s the first successful hatching for the zoo’s only mating pair of spider tortoises, a 21-year-old male and 12-year-old female.
But not for want of trying.
Proper hatching is a long, delicate, and finicky process.
The northern spider tortoise (Pyxis arachnoides brygooi) is the smallest of three subspecies, found in Madagascar. Females measure only five inches in length, and males are 4.5 inches, just slightly bigger than the palm of a typical adult hand.
The shell has a star-like webbing of black and gold that expands and becomes more intricate as the tortoises age, said Shawn Miller, curator of aquariums and reptiles.
Eggs take more than 220 days to crack
Northern spider tortoises mate during the warm, wet season when they are most active. Females lay only one “ping-pong-sized” egg at a time, and around two or three eggs each year.
The egg incubates for a whopping 220-240 days before hatching. A chicken egg, in contrast, incubates for just 21.
During the long incubation period, the egg must undergo an extensive heating and cooling process, otherwise it fails to hatch. When the temperature drops, the embryo temporarily stops its development. That break is called diapause. When the weather warms back up, the embryo resumes growing. Diapause allows the embryo to withstand harsh conditions and enter the world when the weather is more favorable.
Diapause coincides with adult tortoises going into brumation, which is similar to hibernation.
To replicate diapause at the zoo, the Aquatic and Reptile care team housed this latest egg in a wine chiller, as recommended by the Smithsonian National Zoo, which had two successful hatchings in 2015.
Steep odds of hatching
According to Melissa Spreda, the area supervisor for the Aquatic and Reptile Center, the egg was warmed to 84 degrees for six weeks, then cooled by two degrees each day until it reached 55. It was kept there for eight weeks, after which the temperature was increased daily by two degrees until it reached 86 degrees. The egg was kept at that temperature until hatching.
Even if an egg has been properly heated and cooled, there is still a chance that it won’t hatch.
The care team estimated that only 1 in 100 eggs hatch, and about 1 in 10,000 eggs result in a tortoise that lives to adulthood.
Miller said the zoo caretakers monitored the embryo’s growth and development using a technique called candling, which involves holding a small, bright light against the eggshell and illuminating an outline of the embryo inside.
The baby tortoise pipped (cracked open) its egg on April 10 and emerged on April 12. So far, the hatchling is doing well.
When can we meet it?
Visitors can expect to meet the baby hatchling about a year from now, when it is big enough to live with its mom and dad without fear of being trampled or outcompeted for food.
The trio is currently nameless, but the zoo plans to get the public involved and name each member of the family soon.
Caretakers are still monitoring the hatchling closely. To help it eat, someone must pick the baby up and directly place it into its bowl of what zoo officials jokingly call “rabbit food” — mixed greens, sweet potatoes, and carrots.
The hatchling is currently about the size of a pink school eraser, weighing in at 0.6 ounces.
At sexual maturity, the tortoise will enter a zoo ‘dating service’
Whether the hatchling is a male or female will remain a mystery for several more years.
Additionally, there are currently no plans to reintroduce the spider tortoises into the wild.
When it reaches sexual maturity, the tortoise becomes part of the Species Survival Plan (SSP) Program led by the Association of Zoos and Aquariums. Miller called it a “dating service” for endangered animals.
DNA is extracted from the spider tortoise’s nails and compared to others in the SSP database until they find its perfect (genetic) match.
The goal is to prevent close relatives from mating and ensure that future offspring are healthy and genetically diverse.
It’s not romantic, but it is necessary given their status as a critically endangered species.
Northern spider tortoise population down 80%
In 1981, northern spider tortoises occupied more than 600,000 acres of land in Madagascar. By 2010, that number decreased to 120,000, an 80% drop.
Climate change, habitat destruction, poaching, and the illegal exotic pet trade are major contributors to the sharp decline. Some people eat them; some sell them as personal pets. Miller noted that their small stature, paired with their beautifully patterned shells, are appealing to pet owners, making them more lucrative to poachers.
“They’re not fast, so it’s easy for poachers to grab them,” explained Spreda.
She went further and said, “There are no real big land predators (in Madagascar) – just people.”
