Of all the causes philanthropy takes up, donors and fundraisers are often most effective when it comes to filling in a super-specific gap—tackling a finite problem that falls outside of normal funding channels. That’s especially the case when there’s an important, and even heartwarming outcome donors can witness.

An example of such a success story currently unfolding is the expansion of Chimp Haven, a 200-acre sanctuary in Louisiana that’s been providing a natural home for hundreds of chimpanzees retired from biomedical research by the NIH.

Funded early on with a federal contract, the nonprofit facility is growing to meet the needs of the final remaining chimps still living in NIH research facilities, having raised around $15 million of a $20 million fundraising campaign. The effort is ongoing, but half of its goal was met with a Chicago couple’s $10 million gift, which allowed construction to begin.

The funding and expansion will go a long way toward seeing the effort through to its finish, a process that’s been underway since research on chimpanzees was phased out and ultimately ended in 2015.

While the federal agency supports lifetime care for the animals, Chimp Haven and sanctuaries like it offer a different experience. The facility includes a huge wooded environment where chimps can live out their days alongside large groups of other chimps. Experts and those who visit describe a moving experience of seeing them behave and socialize more like chimpanzees in the wild.

As for how the NIH came to have as many as 1,500 chimpanzees in captivity, it’s a story of well-intentioned researchers, a long campaign by animal welfare advocates, and a dramatic, albeit gradual reversal by the agency.

In 1960, the U.S. government first established a network of primate research centers, and in the 1980s, began breeding them intensively for AIDS research. But as scientists learned more about HIV, it became clear that chimps weren’t practical test subjects. In the 1990s, research using chimpanzees declined, and by 2000, a series of laws and policies began phasing out the practice. Finally, in 2015, the agency announced it would retire them all, the same year they were declared endangered, effectively banning chimpanzee research.

There’s been an ongoing, sometimes strained, process of moving the animals to sanctuaries. That process is now approaching its endgame, as decisions are being made regarding where the remaining 270 chimps will live out their days—many have health conditions that make it difficult to relocate them.

The biggest donors in the latest campaign by Chimp Haven are Kimbra and Mark Walter, who give through their Walter Family Foundation. The Chicago couple give to social justice causes, helping low-income youth, but another major interest is animal welfare. The Walters own a 17,000-acre animal refuge in Florida, preserves in Africa, and are backers of the Lincoln Park Zoo. The Walters’ wealth comes from Mark Walter’s career as head of investment firm Guggenheim Partners; he’s now worth an estimated $3.2 billion. The couple stays mostly out of the public eye, but Mark Walter did become an owner of the Dodgers in 2012.

Primates are a priority for a handful of high-profile donors, including the family behind the Park Foundation, and Jon Stryker’s Arcus Foundation, another big backer of sanctuaries.

It’s a sad story, but also one that is moving toward a happy resolution, and an interesting funding story, as well. Here, we’ve got a public sector problem that nobody wanted to end via euthanasia, and a combination of federal funds, animal welfare advocacy, and private donations, all seeking the best solution possible.

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