A veil of ignorance can promote fairness in a mammal society

H. H. Marshall, R. A. Johnstone, F. J. Thompson, H. J. Nichols, D. Wells, J. I. Hoffman, G. Kalema-Zikusoka, J. L. Sanderson, E. I. K. Vitikainen, J. D. Blount, M. A. Cant. A veil of ignorance can promote fairness in a mammal society. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-23910-6

Abstract: Rawls argued that fairness in human societies can be achieved if decisions about the distribution of societal rewards are made from behind a veil of ignorance, which obscures the personal gains that result. Whether ignorance promotes fairness in animal societies, that is, the distribution of resources to reduce inequality, is unknown. Here we show experimentally that cooperatively breeding banded mongooses, acting from behind a veil of ignorance over kinship, allocate postnatal care in a way that reduces inequality among offspring, in the manner predicted by a Rawlsian model of cooperation. In this society synchronized reproduction leaves adults in a group ignorant of the individual parentage of their communal young. We provisioned half of the mothers in each mongoose group during pregnancy, leaving the other half as matched controls, thus increasing inequality among mothers and increasing the amount of variation in offspring birth weight in communal litters. After birth, fed mothers provided extra care to the offspring of unfed mothers, not their own young, which levelled up initial size inequalities among the offspring and equalized their survival to adulthood. Our findings suggest that a classic idea of moral philosophy also applies to the evolution of cooperation in biological systems.

Popular version: Mongooses solve inequality problem -- ScienceDaily

Introduction

The idea that impartiality or ignorance on the part of decision-makers promotes cooperation and fairness in human societies has a long pedigree in philosophy and economics1,2,3. Individuals that are blind to their own gains are predicted to allocate resources for the good of the group rather than themselves1, typically reducing inequality2. In biology, an analogous argument has been proposed as a mechanism for the evolution of cooperation among self-interested agents4,5. Meiosis, for example, ensures that each allele has an equal chance of ending up in any given offspring, creating a “Mendelian veil of ignorance”4, which aligns the fitness interests of each gene with that of the organism4,6,7,8. In insect societies, uncertainty over relatedness promotes cooperative behaviour: workers cooperate to raise the offspring of other workers when relatedness to offspring is uncertain, but kill such offspring when they can discriminate worker-laid vs. queen-laid eggs5. Our aim was to understand if a veil of ignorance over kinship can also promote fairness in an animal society, in the sense of a redistribution of resources to reduce initial inequalities, analogous to Rawls’2 redistributive concept of fairness in human societies.

We studied fairness in the allocation of postnatal care in wild groups of cooperatively breeding banded mongooses Mungos mungo9. The veil of ignorance arises in this system because multiple females (mean ± s.d. = 5.0 ± 2.6, n = 84 litters) synchronize birth to the same morning in a shared underground den. There is good evidence that this extreme birth synchrony removes cues to the parentage of offspring. First, mothers are observed to suckle pups in synchronously produced litters without any apparent discrimination, and pups move from female to female to suckle in a single suckling session10. Second, after pups emerge from the den parents and other helpers do not preferentially care for more closely related offspring in communal litters11. Third, on rare occasions when cues to parentage are available (e.g., in the minority of breeding attempts that are asynchronous12, or when older females are reproductively suppressed using contraceptives13), females kill the pups of other females rather than care for them, suggesting that such cues are absent in natural, synchronous litters.

The communal litter (hereafter “litter”) is suckled underground and guarded at the den by babysitters for the first month, after which the pups emerge from the den and form one-to-one caring relationships with particular adults, called escorts11,14,15. Escorts, who can be any adult male or female in the group, feed and protect the pup in their care until it reaches nutritional independence at around 90 days old (see “Methods”). Both escorts and pups contribute to maintaining the association14,16,17,18. Escorts individually recognize and preferentially respond to the calls of “their” particular pup16,18, and actively seek out their pup if it becomes separated or lost14. Escorts almost exclusively provision the pup with which they are associated14,19, and young pups receive almost all their food from their escort19. Pups, for their part, aggressively defend access to their escort14,15,17 and beg continuously for food16. Pups who receive more escorting are heavier at independence20 and heavier pups are more likely to survive21.

Below we develop a simple game theoretical model to investigate how the veil of ignorance over parentage in banded mongoose communal litters might influence fairness in the distribution of postnatal care among the offspring. We test our model by creating inequalities among helpers and among offspring through the targeted provisioning of pregnant females, and then measuring whether mothers and helpers act to reduce or amplify these inequalities.