Mountain gorilla populations rise when they do this

By Mark Waghorn via SWNS

Mountain gorilla populations rise when they stick to their own groups, according to new research.

Encounters with outsiders give females more opportunities to move - delaying the period between successful births.

Infants are usually born about four years apart. Mothers switching extended this interval by nearly eight months - or a year-and-a-half if done multiple times.

Lead author Dr. Robin Morrison, of the Dian Fossey Gorilla Fund, said: "Female choice of which group to live in is an important part of gorilla life history but is limited to when groups interact with each other.

"Historically, the low rates of group interactions meant that females had relatively few opportunities to move, as they generally only transfer when they are not pregnant or don't have a dependent offspring.

"The significant increase in group interactions provided us with the first opportunity to study the implications of females changing groups multiple times."

The study analyzed more than five decades of demographic data - shedding fresh light on a rare conservation success story.

Mountain gorillas are currently the only great ape whose numbers are increasing thanks to long-term investment by the governments of Rwanda, Uganda and the Democratic Republic of the Congo.

But with about 1,000 individuals left, they remain critically endangered. The findings highlight the challenges the iconic species faces within a limited habitat.

Dr. Morrison said: "Our study shows female movement patterns and their reproductive consequences follow broader trends driven by social dynamics across the population.

"It adds to our growing understanding of how social dynamics can impact conservation - particularly when groups are living at a high density."

Fortunately, over the last several years the groups have been able to spread out - suggesting group instability, high mortality and slowed reproduction may be coming to an end.

Dr. Morrison said: "We are already beginning to see infant mortality, as well as female transfer rates, return to what was more typical in the 1980s and 1990s."

The population in Rwanda began a slow recovery in the mid-1980s, demonstrating the benefits of strengthened conservation practices.

They included anti-poaching patrols, daily monitoring, the economic benefits of ecotourism, community engagement and veterinary care.

This allowed the subpopulation monitored by the Fossey Fund to grow at relatively high rates from the 1980s through the mid-2000s.

Co-author Dr. Tara Stoinski, the Fossey Fund's president, said: "The mountain gorillas of Rwanda are one of the longest-studied animals on the planet.

"This research really emphasises the value of such an investment to understanding big-picture questions.

"We often think of extrinsic conservation threats, such as climate change, poaching or habitat loss, but these data highlight some of the intrinsic challenges species may face as habitat shrinks.

"This is critical information that can help inform longer-term conservation strategies to ensure the continued growth of the population."

In 2007, stability started to change as these now very large groups began to split from each other.

Co-author Felix Ndagijimana, Rwanda program director for the Fund, said: "Historically, as a result of the poaching and habitat disturbance in the 1970s and early 1980s, the mountain gorilla group density was low in the area of the Volcanoes National Park where the Fossey Fund works.

"Groups interacted infrequently, maybe every other month, and there was stability in group leadership.

"One of the groups we monitor grew to 65 individuals, which is six times larger than the average mountain gorilla group."

An earlier study by the same team revealed that increased interaction rates resulted in higher male and infant mortality, contributing to a significant slowing of the population growth rate. But mortality alone did not explain this pattern.

Ndagijimana said: "The group density tripled in the space of only two years, as young males left to form their own groups."

This increase led to more spatial overlap and group interactions. Such interactions range from tolerant to highly aggressive, and this aggression can even be lethal.

The study was published in the journal Animal Conservation.