Review written by Natalie Wong (2025, EEB)
The question of what drives animals to cooperate with one other is a compelling one. After all, such behavior contradicts the notion lodged in popular imagination that nature is dominated by ruthless Darwinian battles among feral creatures. Yet, these instances of cooperation serve as a reminder that looking out for others is not limited to the Homo sapiens realm. Several explanations for cooperation in social animals exist, but perhaps the most well-established is kin selection: the idea that organisms can indirectly boost their own fitness by performing actions that help ensure the survival of those with whom they share genetic information.
But what happens when animals are not “kin”? Do they still exhibit cooperative tendencies, and if so, why? Dr. Christina Riehl and her lab in the EEB department sought to address these inquiries in a recent set of studies they conducted with greater anis (Crotophaga major). These birds are unique in that adults form communal nests with two to three unrelated breeding pairs and raise the resulting offspring indiscriminately. Parental duties include defending the clutch against myriad predators while simultaneously trying not to be eaten oneself. Indeed, eggs and nestlings are vulnerable to snakes, mammals, and the avian species yellow-headed caracaras, and adults often fall victim to birds of prey. With so many dangers to contend with, it is perhaps unsurprising that anis have a way of sounding the alarm. In a display of referential signaling, or the act of conveying information to receivers in response to certain aspects of the surrounding environment, anis give special calls to their social groups when a predator is nearby. Riehl’s advisee, PhD candidate Amanda Savagian, discovered something intriguing about these vocalizations in her 2017 research. They seemed to be dichotomous, presenting as either a high-frequency call for aerial threats (the “high cackle”) or a low-frequency one for everything else (the “scold”). What remained to be determined, however, were the nuances of the behavior, its evolutionary origin story, and the factors causing it to persist today.
To build off of Savagian’s work and further characterize such a multilayered system, Riehl’s team, led by postdoctoral research associate Joshua LaPergola, first set out to conduct a long-term observational field investigation. After four years, they found that 97% of high cackles were in response to aerial predators and 93% of scolds were in response to nonaerial predators. They also worked out which predator species were associated with each type of call: high cackles warned of flying raptors like hawks and caracaras in the majority of cases (26 of 35), while scolds signaled primarily humans, monkeys, snakes, or crocodiles (242 of 260 cases). This data not only corroborates Savagian’s documentation of dichotomous alarm calling, but also provides robust evidence that this complex communication is more widespread than previously believed. Distinction between aerial and nonaerial predators was thought to have evolved exclusively in primates, but the fact that anis and other birds do this as well implies that it actually evolved multiple times in different groups of organisms. For anis particularly, separation in signaling might have occurred because it was advantageous to have a second call specific to flying foe, whose positions are easier to predict because they can only take an overhead approach.
From there, the lab carried out a follow-up experiment in which they played audio recordings of the two alarm calls, plus a pigeon song as a control, to wild anis and analyzed their responses on videotape. It appeared that the birds would usually look around by moving just their heads immediately after hearing a scold, but tended to move their whole bodies to fly to cover within seconds of hearing a high cackle. Furthermore, any reaction to the pigeon song was significantly delayed. The automatic nature of either head- or whole-body-movement for the alarm calls but not the control indicated that these behaviors were specialized to each type of vocalization.
At this point, Riehl’s team concluded that anis engage in complex referential signaling that elicits appropriate responses from conspecifics (members of the same species). The piece that tied everything together came from DNA sampling of 27 of the breeding groups in the wild. If the birds didn’t share a significant portion of genetic material between them, they were considered unrelated, belonging to different “families.” The results revealed that, within any given communal nest, adult anis had almost no relatedness, and offspring of different parents were as unrelated to each other as they were to random offspring from other nests. This scenario is akin to comparing one’s ancestry to that of a perfect stranger and finding only incidental overlap in genetic similarity. Why, though, would birds be giving alarm calls to warn non-relatives? If kin selection wasn’t the answer, there had to be another explanation.
LaPergola et al. theorized that anis’ communal breeding system encourages cooperation. Since adults care for all progeny in the shared nest, not just their own, they have a stake in each other’s survival. Essentially, whether an offspring lives to reproductive age and passes their—and therefore their parents’—genes on to the next generation is largely influenced by the quality of parental care they receive. If unrelated breeding pairs did not help each other escape predators, all adults in the nest would be at heightened risk, rendered unable to provide effective protection, and their young would suffer as a result.
Ultimately, the Riehl lab’s research represents a major advance in the field of behavioral ecology and fills a crucial gap in scholarship by demonstrating that individuals within a species can use referential signaling systems—and intricate ones, at that—even if they are not genetically related. This challenges the idea that cooperative alarm calling is favored only by kin selection, which has been the predominant view since the behavior was initially observed in primates that lived among family members. The team’s future studies on greater anis will continue to explore the subtleties of their interactions with one another while potentially uncovering additional clues as to the evolution and maintenance of referential signaling not only in birds, but across the animal kingdom.
Riehl shared that she and her colleagues are working on designing follow-up experiments that they hope will elucidate the ways in which greater anis learn to distinguish between birds of prey species. As she describes, “greater anis recognize caracaras (predatory hawks) as a threat, but they know to ignore snail kites (which are also hawks, but specialize on snails and don’t prey on anis). These two hawks appear very similar to human eyes, but anis know which one is the real threat.”
She goes on to explain how this recognition has to be learned, given that greater anis and snail kites previously occupied different geographical ranges at the Panama study site and thus would not have had much chance, if any, to interact over time. In addition to this investigation, Riehl revealed that another study is in the works to aid “in understanding the functions of a different call (the “yelp” - it sounds like a yelp!),” which they suspect “acts as a recruitment signal to call group members to the nest for cooperative defense.” The decoding of the anis language is no doubt only just beginning.
The original article discussed here was published in PNAS on May 1, 2023. Please follow this link to view the full version.