Eusociality

this site is maintained by Nona Poulton under the supervision of Dr. Verna Case as part of the course requirements of Animal Behavior (Bio 323) at Davidson College, Davidson NC.

Eusociality in H. glaber

"I'd give my life for two brothers or eight first cousins!"-- J.B.S. Haldane

W.D.Hamiliton was the first to suggest that this means an individual may contribute more to future generations by helping a mother to produce more offspring than actually reproducing themselves. Charles Darwin's theory of natural selection states that an organism strives to increase its own fitness and reproductive success. Yet in eusocial societies, their exists a caste of nonreproductive workers who often sacrifice their own opportunities to survive and reproduce, for the good of other colony members (Andersson, 1984). This can be mistaken for altruism.

William Hamilton (1964) proposed a solution to this puzzle, developing a genetic model that became known as kin selection. He suggested in species that are eusocial (he did his work with the hymenoptera, but it can be extended to H. glaber as well) unusual patterns of relatedness develop between family members. Genetic probabilities show that in such a species, the relatedness of sisters (average 75%) is greater than the relatedness of a parent to his/her offspring (average 50%). Hamilton explained that by helping their parents raise siblings, some of which can breed, nonreproductive workers increase the representation of their own genetic characteristics.

Kin selection suggests that in the case of naked mole-rats, more related individuals are likely to favor each other while less related conspecific individuals are more likely to disfavor each other and treat each other in a hostile or aggressive manner. Differential treatment by the breeding female has been discovered in naked mole-rats (Sherman, 1993). Focusing on intracolonial aggression, they found that shoving behavior is strongly influenced by genetic relatedness: less-related individuals were shoved more. In all six colonies, the breeding female initiated more shoves per unit of time than did any other member of the colony. The observed relationship between the breeding female's shoving and the recipient's relatedness is evidence of nepotism in naked mole-rats. A different group found evidence that the breeding female would preferentially shove those workers who were not working as hard. The study done my Sherman, however, proved that the work level of the individual had less to do with the shove-rate than the genetic relatedness (Sherman, 1991).

Naked mole-rats are arguably the only eusocial mammal. Guidelines governing eusociality are as follows:

1. Reproductive division of labor
2. Overlap of generations
3. Cooperative care of young

The "Selfish Herd" and "Kin Selection" are both explanations of eusociality in H. glaber. The 'selfish herd' can be summarized by individuals, acting on their own behalf, trying to reduce the chances of being caught, which is not cooperation Kin selection does provide an adequate explanation of the evolution of cooperative behavior: both the gene and the individual are what natural selection acts upon and if an individual benefits from the act of a relative, carries some of same genes, then there will be an increase of frequency of that gene in gene pool (Alcock, 1993). The original individuals' life is not meaningless afterall. Kin selection provides a way around having to say that the individual acts for the good of the species, which is the obvious alternative.

Kin selection is a form of genetic natural selection in which alleles differ in rate of propagation via their influence on the fitness of kin who carry the same alleles by common descent. It helps prove the idea that an individual can benefit genetically via indirect effects on kin attributed to W.D. Hamilton (1964). Hamilton recognized two kinds of fitness, individual fitness and indirect fitness. Individual fitness is the number of offspring that an individual produces by him- or herself. Indirect fitness is fitness via propagation of genes in relatives other than offspring, who share those genes because of a common ancestor. Both of these types together is inclusive fitness (Alcock, 1993). Naked mole-rats live socially by exercising inclusive fitness maximizing: the breeders and nonbreeders are so closely related and the habitat is such that all members of the colony benefit more from indirect fitness than individual fitness.

There are ways of quantifying inclusive fitness. Hamilton derived a system based on the coefficient of relatedness, r. Quantifying the level of eusociality requires a good deal of knowledge of an individual (Keller, et. al., 1995). r = ave. proportion of identical genes in any two individuals as a result of common ancestry. r-bar is related to inbreeding, which tends to increase coefficient of relationship. This model presumes that there is no inbreeding. Hamilton's model for evolution of altruistic behavior is as follows: B x r > C; i.e., B/C > 1/r , where B is the benefit of the act (to the recipient) and C is the cost (to the altruist). In an out-bred, diploid organism, like most humans, this means that the r-bar of siblings is 0.5. In this situation, B/C > 1/(1/2), or B/C > 2. Of course, the average human has a relatedness coefficient of 0.02. The r for the naked mole-rat is 0.81, which means that 81% of the genes in an average individual are identical to 81% of the genes another individual in the colony, on average (Keller, et al, 1995). Based on the relatedness of individuals in a colony, cooperative breeding is more or less evolutionarily "worth it". Because the level of relatedness in naked mole-rats is so high, the nonworkers still benefit from helping the breeder female because they are related to her and she carries a large percentage of the same genes. This adaptive helping behavior towards kin may have served as a precondition to the evolution of eusociality (Gamlin, 1987).

There are other advantages of kin selection to an individual, other than passing on the same genes. Parental care is good because the breeder female can devote all her time with the offspring. This is explained in more detail in the mating section.

Evolution of this social behavior, however, poses a problem. There is much debate over how eusociality could possibly be selected for, using the traditional view of natural selection. There are 'requirements' that must be met by a species in order for eusociality to result:

1. Colony size: Colonies must be big enough so that there are enough individuals to do the work of those who breed.

2. Gradual metamorphosis: Eusociality most likely evolved from polyandronous groups that were selected for by the absence of predators. These groups grew larger and larger because of the unique habitat in which they live. It is hard to tell what selection pressures drove the naked mole-rat to live underground, but it was most likely predatorial pressure. The harsh environment cannot reach them underground, which is yet another reason they are so safe. Their habitat is in a unique situation because they rarely get contact with other colonies. This low dispersion rate is conducive to a gradual evolutionary rate.

3. Habitat: The habitat must be expandable, long-lasting, have a renewable and inexhaustible food source, and be very uniquely safe from predators. See social system and nmr home page for more information about their habitat.

4. Genetic predispositions: By helping rear siblings the workers pass on their genes to the next generation so there is an obvious genetic predisposition towards helping. It is difficult to tell what behaviors arose from being in a eusocial environment and which actually helped caused it. There are many distinct instances of behavior that are unique to the naked mole-rat. For example, if one individual is bitten by a poisonous rufous-beaked snake, it will seal off the tunnel that leads to the main burrow, if possible, before it dies. This is a very distinct example of altruism: acting for the good of the species.

5. Subsoclialty (extensive parental care): it is difficult to predict whether naked mole-rat pups needed extensive parental care or providing extensive parental care helped the pups survive, but either way, naked mole-rats provide extensive parental care, where the breeding female is constantly nursing and caring for the young. Please read more about the parental care on the mating system page.

6. Clumping on clumped resources: clumped resources would make it hard for a solitary lifestyle to exist, or even for small groups to be able to defend the resource. See social system and nmr home page for more information about necessary resources.

7. Kin recognition: The development and maintenance of eusocial behavior, may hence be associated with sibling recognition and selection. Kin discrimination is defined as the differential treatment of members of the same species in a way that depends on their genetic relatedness (Sherman and Holmes, 1985). Being able to recognize relatives is very important to the idea of sociality for obvious reasons. Naked mole-rats have an amazing memory for mainly olfactory cues, which allows them to differentiate relatives and remember them. Also, their vast array of vocalizations help give them a basic language to aid them in kin recognition and predator defense. Holmes and Sherman (1982) further suggest that there exist at least four proximal mechanisms of kin recognition: (1) spatial distribution; (2) association; (3) phenotype matching; and (4) recognition alleles. The data they gathered implicates both association and phenotype matching as primary mechanisms for kin recognition, even though their work was done in ground squirrels.

Eusociality may have evolved via inbreeding in naked mole rats, too, which have a high degree of relatedness within a colony and great genetic differentiation between colonies. Thus true altruism is not an evolutionarily stable strategy (ESS) because it can be invaded by a selfish morph. Because altruism can be invaded by cheaters, altruism is not a stable strategy over long periods of time. The idea of an ESS was originated by John Maynard Smith, and it is very accurate tool for predicting the evolutionary outcome of some species (Alcock, 1993).

Balanced against the benefits of sociality is the reproductive sacrifice made by most of the colony members. However, genetic-relatedness studies of the naked mole-rat suggest that the costs of altruism of non-breeders are offset by inclusive fitness benefits, resulting from a high degree of intra-colony genetic relatedness. Thus, the naked mole-rat is truly a social animal and has evolved, solving the problems listed above to form a social environment that violates no law of natural selection, yet is quite unique (and seemingly maladaptive) among mammals.