We may not always do it, but humans are hard-wired to help each other. Our instinct for altruism pushes us to reflexively care about the well-being of others, even unrelated strangers. And while we've long seen this as a uniquely human virtue, scientists are increasingly finding an altruistic streak in other species, too.

Two new studies reveal intriguing signs of selflessness in some of our closest living relatives: chimpanzees. Earlier studies have already examined altruism in chimps, including a 2007 paper that concluded they "share crucial aspects of altruism with humans." But the latest studies, both published this week in the Proceedings of the National Academy of Sciences, offer new insights on these eerily relatable apes.

This may be good news for chimps themselves, if more publicity about their wits and social skills can help inspire better protection from threats like hunting, habitat loss or mistreatment in captivity. But we also have a more selfish reason to study this: Altruistic animals, especially those closely related to us, could shed light on why human kindness evolved, how it works and maybe why it sometimes doesn't.

Before getting into that, though, let's take a look at what the new studies found:

Learning the ropes

chimp at the Leipzig Zoo A chimp lounges in an enclosure at the Leipzig Zoo in Leipzig, Germany. (Photo: Henner Damke/Shutterstock)

One study featured chimps at the Leipzig Zoo in Germany, where psychologists from the Max Planck Institute for Evolutionary Anthropology trained a small group for experiments with banana pellets as rewards. They divided the chimps into pairs, then gave one chimp in each pair a set of ropes to pull. The chimps had already learned each rope would trigger a unique result, such as rewarding only one chimp, rewarding only the other, rewarding both or deferring to the partner.

In the first experiment, one partner began by rejecting a rope that would reward only herself. But "unbeknownst to the subject," the authors write, "the partner was trained to always reject option A." She was instead taught to pull a rope letting the other chimp (the subject) decide, so "from the subject's perspective, the partner risked getting nothing for herself but instead assisted the subject in obtaining food."

Once the partner deferred, the subject could decide to reward just herself with two pellets, or pick a "prosocial option" where each chimp got two pellets. In dozens of trials, subjects chose the prosocial option 76 percent of the time, versus 50 percent in a control experiment where the partner hadn't set a tone of generosity.

That's nice, but what if a subject had to give up some of her own reward to avoid snubbing her partner? "That kind of reciprocity is often claimed to be a landmark of human cooperation," study co-author Sebastian Grüneisen tells Science Magazine, "and we wanted to see how far we could push it with the chimps."

The second experiment was nearly identical, except it made the prosocial option costly for the subject. After her partner deferred, the subject had to choose either three pellets per chimp or a "selfish option" with four pellets all for herself. That meant she'd have to forgo a pellet if she wanted to repay her partner, yet chimps still chose the prosocial rope in 44 percent of trials — a pretty high rate for an option that requires declining food. In a control version, where humans made the initial decision instead of a chimp partner, the prosocial response was just 17 percent.

"We were very surprised to get that finding," Grüneisen tells Science Magazine. "This psychological dimension to chimps' decision-making, taking into account how much a partner risked to help them, is novel."

Testing boundaries

chimpanzees grooming each other Two wild chimpanzees take time for grooming in Uganda's Kibale National Park. (Photo: snarglebarf/Flickr)

The second study looked at wild chimpanzees, using 20 years of data collected at Ngogo in Kibale National Park, Uganda. It focused on the patrol missions conducted by male chimps, who often risk injury or death by deciding to join the outings.

Patrol parties skulk the edge of their group's territory to check for intruders, a task that typically takes about two hours, covers 2.5 kilometers (1.5 miles), involves elevated cortisol and testosterone levels, and carries risk of injury. About a third of patrols meet an outside group of chimps, encounters that can turn violent.

Most Ngogo patrollers have obvious motivation to patrol, like offspring or close maternal kin in the group. (Male chimps form strong bonds with close maternal family, the authors note, but don't seem to bias their behavior toward more distant or paternal relatives.) Yet more than a quarter of Ngogo's patrolling males have no close family in the group they're guarding. And they don't appear to be coerced, the researchers say; males who skip patrols don't face any known repercussions.

These patrols are a form of collective action, achieving far more than any chimp could alone. "But how can collective action evolve," the authors ask, "when individuals receive the benefits of cooperation regardless of whether they pay the costs of participation?" They point to something called group augmentation theory: Males bear the short-term costs of patrolling despite seeing little or no direct benefit because doing so protects the group's food and may expand its territory, which can eventually boost group size and increase the male's chances of future reproduction.

These chimps presumably accept clear and present risks in hopes of uncertain payoffs sometime in the future. This may not qualify as altruism, but researchers say it could still shed light on the evolution of seemingly selfless social behaviors.

Moral history

rats and social cooperation Signs of altruistic behavior have even been recorded in rodents. (Photo: Ukki Studio/Shutterstock)

Since we don't know what animals are thinking, it's hard to prove a conscious intent to help others. But we at least can tell when an animal sacrifices its own fitness to benefit non-relatives, and anything that can compete with a self-preservation instinct must be pretty powerful. Even if these acts aren't entirely selfless — maybe driven by a sense of social obligation, or hazy hopes for an eventual reward — they still represent a level of social cooperation that should seem familiar to us.

According to Arizona State University anthropologist Kevin Langergraber, lead author of the Ngogo study, chimpanzees may offer valuable clues about how collective action and altruism evolved in our own distant ancestors.

"One of the most unusual things about human cooperation is its large scale," he tells Science. "Hundreds or thousands of unrelated individuals can work together to build a canal, or send a human to the moon. Perhaps the mechanisms that allow collective action among chimpanzees served as building blocks for the subsequent evolution of even more sophisticated cooperation later in human evolution."

In the true spirit of altruism, it's worth noting this isn't only about us. We'd certainly benefit from understanding how human altruism works, and studying other animals may help us do that by retracing its origins. But research like this also helps keep us humble, illustrating that humans don't hold a monopoly on morality. Our concepts of right and wrong may have evolved with us, but their roots run much deeper.

Hints of altruism and morality have been found not just in chimps, but across a range of primates, and research suggests their origins go back surprisingly far in the mammal family tree. A 2015 study, for example, found rats were willing to forgo chocolate to save another rat they thought was drowning.

The 'altruistic impulse'

wild baby bonobo, aka pygmy chimpanzee Bonobos, like this wild baby, are a species closely related to common chimpanzees. (Photo: Sergey Uryadnikov/Shutterstock)

Some people scoff at this view of altruism, arguing human ideas are being projected onto blind animal instincts. But as Emory University primatologist and animal-morality expert Frans de Waal wrote in his 2013 book, "The Bonobo and the Atheist," the relative simplicity of altruism in other species doesn't mean it's mindless.

"Mammals have what I call an 'altruistic impulse' in that they respond to signs of distress in others and feel an urge to improve their situation," de Waal writes. "To recognize the need of others, and react appropriately, is really not the same as a preprogrammed tendency to sacrifice oneself for the genetic good."

Other mammals don't share our whirlwind of rules, but many do have relatable, if basic, moral codes. And rather than seeing this as a threat to human superiority, de Waal argues it's a reassuring reminder that altruism and morality are bigger than us. Culture may help keep us on track, but luckily our instincts also drew a map.

"Perhaps it's just me," he writes, "but I am wary of any persons whose belief system is the only thing standing between them and repulsive behavior."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.