The stillbirth of twin calves came as a shock to Calgary Zoo staff who didn’t know Sparky the hippopotamus was pregnant.

The hulking herbivore gave birth Friday afternoon, but the nearly full term mammals died just prior to delivery.

Despite the efforts of staff to resuscitate the twins, a necropsy revealed the nearly full-term fetuses died due to twisted or entangled umbilical cords.

The 26-year-old hippo’s pregnancy was a surprise, given the fact Sparky is routinely given birth control. But recently, staff noticed she was gaining weight and acting differently.

“We noticed some light behavioural changes. We didn’t assume she was pregnant but we didn’t rule it out,” said zoo curator Colleen Baird.

“It was one of several things that medically could have been happening.”

But on Friday, Sparky lost her appetite and appeared increasingly uncomfortable and tired. She delivered the first of the calves around 3:30 p.m. Sadly, zoo staff recognized something was wrong when she appeared to show little interest in her offspring.

Not wanting to upset Sparky, they gingerly approached the calf and saw that it was deceased. Not long after, she delivered a second stillborn calf.

“Had they survived, it would have been something of a miracle,” said Baird. “Not only was it difficult for Sparky to become pregnant, twins are very, very rare for hippos.”

The odds of both surviving were slim. The gestation period for the massive African mammals is eight months, meaning Sparky was pregnant during the massive June flood that swamped the zoo.

Her immature partner, Lobi, nearly escaped his enclosure during the disaster because the complex was badly damaged and water levels rose high enough for him to swim over the top and roam freely about the African Savannah building.

This was Sparky’s fourth pregnancy. Her first calf in 1991 — sired by her longtime mate Foggy — was also stillborn.

Twin births are exceedingly unusual for hippos, zoo staff say. Sparky, who weighs more than 2,000 kilograms, has been at the Calgary Zoo since 1988.