Smile big, hippos. Your pearly whites are helping scientists monitor the health of Africa’s elephants. A recent study revealed that the teeth of decades-old hippo skeletons hold information about Africa’s changing plant life and could model the further decline of elephant populations and savanna ecosystems.

In the past, getting long-term information about Africa’s changing savannas was difficult because there were no records, so scientists started looking in new places for data. Plants native to the savanna’s grasslands are slowly being pushed out, and scientists are becoming concerned that local herbivores are losing their main food source. Since hippopotamus teeth continuously grow throughout the hippos’ lives, they contain diet records that can span over decades.

Scientists from the University of Utah and Braunschweig University in Germany recovered three hippo tusks from the Mweya Peninsula, in Uganda’s Queen Elizabeth National Park, and used stable-isotope analysis to examine diet patterns in the enamel. Through their research of the hippos’ eating habits, the scientists discovered that a shift in the park’s fauna over the past 50 years has left elephants with a dwindling supply of food, making it harder for populations to recover.

The problem first emerged in the mid 1970s. Queen Elizabeth National Park once boasted the highest density of herbivores on the planet, but the former Ugandan president Idi Amin stopped management of the country’s national parks and elephants were massacred for ivory. From 1972 to the mid 1980s, poachers intensely hunted the area. Elephant numbers dropped from 4,139 to 150, and the number of hippos fell from about 12,000 to 4,000.

Hippos are the landscapers of their semi-aquatic environments, building water channels and feeding on ground vegetation to transform tall, grassy areas into grazing fields. They prevent soil from hardening and flooding from rainfall, which in turn promotes healthy plant growth for the savanna. When the population declined, the soil compacted and rainfall pooled into the savanna’s wooded area, encouraging the growth of invasive plants and trees. This change in plant life made it difficult for some herbivores, like elephants, to find food in the park.

“In African savannas, a lot of the biodiversity of grazers rely on grasses predominantly,” said Kendra Chritz, one of the study’s researchers and a postdoctoral fellow at the University of Utah. “As you decrease the diversity of grasses available in a savanna, you also in turn decrease the diversity of these grazers.”

Woody plants also stifled natural fires that grasses need for germination. This further decreased the food supply for elephants, as young grass is high in protein. While hippos were able to survive on their new diet, elephants are pickier and often had to move around to find their dinner.

“The problem is that [elephants] will have to change their range or they’ll have to go elsewhere to find the diets that they prefer,” Chritz said. “So it might then push them to the fringe of the national park or outside the park, where they’re more vulnerable to poaching or hunting.”

Hippo tusks can be helpful for predicting future ecological conditions, but African governments will need to take substantial steps to protect parks for later generations of megaherbivores. Park managers are trying to maintain elephant populations by closely guarding herds from poachers and creating controlled burns to promote new grass growth and get rid of invasive plants. Unfortunately, Chritz doesn’t think this problem is unique to Queen Elizabeth National Park.

“In East Africa as a whole, if we allow poaching to continue at the rate that it’s going and lose elephants as quickly as we are, a lot of savanna ecosystems that we know and love could potentially see the same kind of state in Queen Elizabeth,” Chritz said. “These elephants are really important for these savanna systems, and they really help maintain health and diversity of these areas.”