TUCSON, Ariz. -- Tucson's Reid Park Zoo has four new lion cubs.
Zoo officials said the cubs were born in December.
A zookeeper could hear at least one cub on the morning of Dec. 23. The next day, it was confirmed that five cubs were born, but one did not survive.
Officials said the mortality rate for cubs up to 1 year old is close to 30 percent in zoos, and it's significantly higher in the wild.
All four cubs are gaining weight, but staff remained concerned about one male, who has slow weight gain and appears weaker than the others.
Zoo veterinarian Alexis Moreno said a litter of five cubs is unusual, but she's cautiously optimistic.
The litter was born to lions Kaya and Shombay. It's their second litter.