When Minnesota dentist Walter Palmer killed Zimbabwe’s Cecil the lion last July, proponents of trophy hunting argued to an infuriated public that the pricey practice helps lions. A portion of the fees supports poor villages and conservation programs, their argument goes.

But some reports have shown that trophy hunts take a toll on Africa’s declining lions, which already face a host of severe threats, such as habitat loss, lack of prey, and greater conflict with humans. A 2012 study, for instance, found that excessive hunts have hurt lions in Tanzania.

Now new research shows why: inadequate hunting regulations. Only strict quotas and restrictions on hunting younger lions can ensure healthy populations, researchers reported on June 9 in the journal