Four nilgai calves born at Gazipur Safari Park have raised the park’s nilgai population to 13, its authorities said on Thursday.

The calves, about one and a half months old, were born in the park’s core safari and have been seen in the nilgai habitat in the deer safari area. They spend most of the day close to their mother, which remains alert while resting in the shade and leads them deep into the forest at the first sign of people or noise, the park said.

Park authorities have stepped up care and surveillance to ensure the calves’ safety, movement and growth. Their sex has not yet been identified; officials have not gone close enough to examine them due to safety concerns.

Tareq Rahman, the park’s acting officer and an assistant conservator of forests, said the four calves were first seen with nilgai in the deer safari about one and a half months ago. A staff member has since been assigned to look after them round the clock, he said.

Nilgai rescued from border areas at different times have been brought to Gazipur Safari Park, where several calves have since been born. The park last counted 11 nilgai, two of which were transferred to Dulahazara Safari Park in Cox’s Bazar. With the four new calves, the population now stands at 13, Rahman said.