(CNN)An elephant's tusks are among its defining features -- they help the animal lift heavy branches, topple trees, strip bark, fight, and dig holes for water and minerals.
But an increasing proportion of female elephants in Mozambique's Gorongosa National Park have been born without these crucial tools, and scientists say it's an evolutionary response to the brutal killing of elephants for their ivory tusks during the country's 15-year civil war.
Elephant experts working in the park had begun to notice the phenomenon after the war ended in 1992. Field data and analysis of old video footage from the park found that the proportion of tuskless female elephants increased more than threefold between 1972 and the year 2000. It was a period during which the elephant population plummeted from roughly 2,000 to about 250 individuals, said Ryan Long, an associate professor of wildlife sciences at the University of Idaho.
"During the war, Gorongosa was essentially the geographic center of the conflict," Long said via email. "As a result there were large numbers of soldiers in the area and a lot of associated motivation... to kill elephants and sell the ivory to purchase arms and ammunition. The resulting level of poaching was very intense."
Scientists now have a better understanding of the genetic basis for this tusklessness and why it only appears to affect female elephants, according to a study that published in the journal Science on Thursday.
The analysis showed that tuskless females were over five times more likely to survive during the 28-year period than their tusked female counterparts, hence the adaptation was very unlikely to be a chance occurrence.
Tusklessness does occur naturally -- and only in females -- even in the absence of poaching, but usually only in a small minority of elephants. In Gorongosa in the 1970s, 18.5% of female elephants didn't have tusks, while three decades later 51% did.
"Evolution is simply a change in heritable characteristics within a population over successive generations, and based on the results of our study, the shift toward tusklessness among female elephants at Gorongosa fits this definition perfectly," said Long, an author of the study.
"The fact that it occurred so rapidly is rare indeed, and is a direct function of the strength of selection," he said via email. "In other words, it happened so quickly because tuskless females had a MUCH higher probability of surviving the war, and thus a MUCH greater potential for passing their genes on to the next generation."