Canada's largest zoo was closed to the public and its animals were being cared for by management on Thursday after more than 400 employees walked off the job in a contract dispute.
The Toronto Zoo said it had "a robust contingency plan" for the job action, noting that the health and welfare of its animals were top priorities.
A spokeswoman said the animals will be taken care of by "highly qualified management staff, many who were former keepers themselves."
The walkout at the Toronto Zoo began at midnight Wednesday after bargaining teams failed to agree on the key issue of job security, said the Canadian Union of Public Employees Local 1600.
"We are incredibly disappointed to have to take strike action, but the Toronto Zoo's refusal to move on job security left us with no alternative," local president Christine McKenzie said in a statement.
The 400 workers who walked off the job include zookeepers, maintenance staff, administration staff, ride operators, public relations staff and concession workers.
The zoo said it has been "more than fair and reasonable" during contract talks.
Spokeswoman Jennifer Tracey said in a statement that the union "has not provided any flexibility" and the zoo's proposals include a wage increase and "satisfactorily address the issues of job security."
Tracey added that the facility in east-end Toronto is a not-for-profit charitable organization and the union demands are "simply unaffordable and do not reflect the financial realities facing the zoo."
She said the employees are "well paid, receive an employer-paid pension plan, multiple weeks of vacation, and are covered by excellent health and dental benefits which includes more than 100 sick days per year."
McKenzie said the union's bargaining committee remains ready to quickly resume negotiations and urged zoo management to provide their negotiating team with the "flexibility they need to conclude bargaining."
"We have numerous animals on the verge of giving birth, a new health centre supposed to open, and thousands of animals that won't be getting the level of care they should be getting," she said. "Ultimately, that ball is in the employer's court."