The breeding tactics of the Taiwan Yuhina reveal a weakness in standard evolutionary theory that ignores cooperation between unrelated individuals.
By Rex Graham
An ornithologist 80 years ago noticed that the eggs in each nest had at least 2 color patterns. Obviously, a different yuhina female was responsible for each pattern. Cooperative breeding among birds has been known for decades, but one dominant female typically lays all the eggs in those nests and non-breeding “helpers” assist in feeding the young.
Non-kin Taiwan Yuhina dynamics
These cooperatively breeding species are the foundation of the “kin-selection” theory of evolution. However, the routine inclusion of non-relate helpers in cooperatively breeding birds has required more convoluted explanations of kin selection. For example, maybe nearby breeding territories are occupied by somewhat more distant relatives, so an individual wandering from one of those territories also is kin.
Kin Selection explains the origin of altruistic sociability in ants, bees and many other cooperatively breeding animals. Non-breeding, related helpers favor the genes they share with siblings even though they may forfeit their own lives and reproduction.
On the contrary, several socially monogamous pairs of unrelated Taiwan Yuhinas, residents of Taiwan’s central highlands, form nesting “coalitions.” Several unrelated pairs lay their eggs in a communal nest and everybody pitches in to feed and defend the young. About 90% of all yuhinas use these communal nests.
Alpha yuhina males are “socially monogamous” with alpha females, and beta males mate with beta females, and so on. However, the socially monogamous mating system is not really monogamous, which is true of other supposedly monogamous birds.
About 20% of the offspring in any nest are produced by matings with males outside the cooperatively breeding group, according to the Handbook of the Birds of the World. Intra-group cuckoldry occurs in 67% of nests studied.
When, a new male joins an established group, he takes his rightful place and thereby re-aligns the male hierarchy, and new monogamous pairings could result. However, if the alpha male is lost, his mate loses her status completely.
“Dominant females usually had the larger share of eggs,” a team of researchers with National Taiwan, Auburn and Cornell universities reported in The Auk. “However, if a female’s breeding position is derived from its mate’s status, how does the male control his (and her) reproductive share in the joint clutch?” They said more research is needed to understand the social dynamics of yuhina nesting.
‘Extended inclusive fitness’
The yuhina strategy exposes a shortcoming of the Kin Selection Theory, which equates an individual’s evolutionary importance to that of genetically similar relatives. Extended Inclusive Fitness Theory maintains that in addition to the importance of kin, social cooperation can also emerge and be maintained by unrelated individuals who favor one another because of similar appearance, behavior and beneficial social synergisms.
In addition to the notion that “birds of a feather flock together,” Jaffe says that friendships between unrelated chimpanzees are based primarily on personality similarities. Complex “social synergies” involving unrelated individuals can also benefit both.
Cooperation’s evolutionary origins
“Cooperation is important in a number of settings, including, behavioral interactions, biological evolution, sociobiology, cultural dynamics, and collective intelligence; yet the features allowing it to succeed are not well known,” Klaus Jaffe, an evolutionary biologist at Simon Bolívar University in Venezuela, said in a paper in the open-access journal Springer Plus.
“Focusing only on kin selection is not enough,” he said. He said the horizontal transmission of information between unrelated individuals carries selective advantages equal to those involving the vertical transmission of “good genes.” Also, mutualistic relationships between unrelated species provide evolutionary benefits to both.
“The emergence of synergies is an important phenomenon that requires deeper studies in biology, economics and in other sciences,” Jaffe said.
— Rex Graham (@TopBirdingTours) September 5, 2016