The big three U.S. airlines have all this week banned the shipment of hunting trophies, although it is unclear how many -- if any -- they have been carrying in recent years.

Delta Air Lines was the first to announce the change Monday, saying that it would no longer accept lion, leopard, elephant, rhinoceros and buffalo trophies. American Airlines and United Airlines soon followed.

Effective immediately, we will no longer transport buffalo, elephant, leopard, lion or rhino trophies. — American Airlines (@AmericanAir) August 4, 2015

American spokesman Ross Feinstein said it's largely symbolic because his does not serve Africa. United, which only has one flight to Africa, also announced Monday afternoon its own restriction. United said its records indicate no shipments of these types of trophies in the past.

The moves come after an American dentist killed a well-known lion named Cecil in Zimbabwe last month in an allegedly illegal hunt, setting off a worldwide uproar. The dentist, Walter James Palmer, lives in Minnesota, which is a major hub for Delta.

As recently as May, Atlanta-based Delta had said that it would continue to allow such shipments -- as long as they were legal. At the time, some international carriers prohibited such cargo.

Delta has the most flights of any U.S. airline to Africa. Several foreign airlines announced similar bans last week.

Delta would not answer questions from The Associated Press about why the decision was made now and how many hunting trophies it has shipped in recent years. The company only issued a 58-word statement noting that prior to Monday's ban, "Delta's strict acceptance policy called for absolute compliance with all government regulations regarding protected species."

Henry Harteveldt, a travel industry consultant, noted that the airline was probably responding to pressure following the news of Cecil's killing. The airline was the subject of a petition on change.org to ban such shipments.

"I don't think there was much of this shipment taking place, so there is minimal revenue loss and big PR gain for them," he said.