Biology 342 Fall 07
The rodents in the Bathygerid family are a diverse bunch: the species in this group range from solitary to social to eusocial. This page examines how these behaviors might have risen and looks at the possible preconditions necessary to evolve eusociality. By comparing the Damaraland mole-rat, a related and eusocial animal, to the naked mole-rat, we can look at the common factors that might have led them to both develop the same social system.
Evolution of Eusociality (top)
. . . From Darwin
In The Biology of the Naked Mole-Rat, Alexander et al defined eusociality as "species that live in colonies ofoverlapping generations in which one or a few individuals produce all the offspring and the rest serve as functionally sterile helpers (workers, soldiers) in rearing juveniles and protecting the colony" (1). The term eusociality was made popular by C.D. Michener (6) in his studies on the behavioral habits of bees, but it has been studied by many biologists over the years, starting, of course, with Darwin (7).
When Darwin first identified this novel behavior it puzzled him greatly. The fact that there existed sterile members of communities seemed to fly in the face of his idea of natural selection and indeed, he said in Origin of Species that these animals were "the one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory" (8). He worked through it, obviously, and came to the conclusion that sterility can evolve as a product of natural selection. He continues that the problem "disappears when it is remembered that selection may be applied to the family, as well as the individual and may thus gain the desired end."(8) In other words, the behavior can be spread by sterile individuals helping to produce functional offspring that carry the trait for altruistic behavior. The individuals do not themselves directly reproduce the young, but by working cooperatively as a colony the help ensure the maturation of the young, thus ensuring the survival of the trait. (image xi)
. . . to Haldane . . .
After Darwin, the next major leap in the study of eusociality came in mid twentieth century with J.B.S. Haldane. Haldane helped refine Darwin's hypothesis by adding that the idea of biological altruism only holds if the society in which it is present is closely related. According to biologist Maynard Smith, while in a pub together Haldane commented that "he would jump into a river and risk his life to save two brothers, but not one, and that he would jump in to save eight cousins, but not seven" (7; 18). The metaphor is exaggerated, but it makes a point about the genetics of altruistic behavior. In a brother there is a 50% chance of his same altruisticistic gene being present, while in a cousin only a 12.5% chance. Based on this, Haldane assumed that in order for an altruistic gene to dominate in a population, the individuals must be closely related. The possibility of death by jumping into the river may seem drastically different from a drone mole-rat who is sterile, but evolutionarily the two are similar: either way the host's genes will not be passed down. However, by ensuring that a close relative will survive to reproduce, the individual has a better chance of ensuring that his genes will go on to future generations. (image xii)
. . . and Hamilton
Work with eusociality really took off when W.D. Hamilton took up the field. Hamilton worked primarily as an entomologist and described the eusociality he saw in insects by saying, "The social behavior of a species evolves in such a way that in each distinct behavior-evoking situation, the individual will seem to value his neighbors' fitness against his own according to the coefficients of relationship appropriate to that situation"(9). The coefficient of relationship of which Hamilton speaks is part of his fundamental rule of altruistic behavior, known as Hamilton's rule. Hamilton's rule is states that altruism will prevail when r*b>c, with r being the coefficient of relationship (how closely related individuals are), b being the benefits of the altruistic behavior and c being the costs of the altruistic behavior. (image xiii)
Family Trees (top)
The Bathygerid rodents are all small, burrowing animals native to Africa. They live in a wide variety of habitats, from desert to savannah to coast, and have become specialized in many different ways. Both the naked mole-rat and the Damaraland mole-rat belong to this group. Originally it was thought that because both species are eusocial, they must be closely related. However, recent genetic analyses suggest otherwise. (10)
A phylogenetic tree of the bathygeridae. H. africaeaustralis is the outgroup. Note how distantly related H. glaber and C. damarensis are! (Taken from 10).
The Food-Aridity Hypothesis (top)
One suggestion for why both the naked and Damaraland mole-rats developed eusociality is called the food-aridity hypothesis. This suggests that because these animals live in a hostile desert environment where not much food grows, there is a stronger pressure for colony life. Each animal has more reasons to depend on other animals--if they can work together to get food, everybody benefits in the end. Additionally, if all animals are closely related-- a family-- every individual increases in reproductive fitness by living communally (2).
This hypothesis is supported by data on colony size, hostility of environment, food scarcity and caloric value per food item, and genetic relatedness between individuals. Both the naked mole-rat and the Damaraland mole-rat live in large groups, with a maximum size of 295 and 41 rodents, respectively. In both environments rain is very scarce, under 400 mm per year. Food items (geophytes) are scarce in both environments, with a mean digestible energy of 204.5 and 179.1 kJ per meters squared). The inter-colony relatedness index is upwards of 0.8 in the naked mole-rat (animals share 80% of their genes), with a reproductive skew of over .99 (more than 99% of animals never breed). The inter-colony relatedness is lower in the Damaraland mole-rat (statistics vary), but the reproductive skew is about .92. (10).
Alternative Explanations (top)
The biggest problem with the food-aridity hypothesis is that there are lots of animals in the world that live in hostile environments that aren't eusocial. Furthermore, even within the Bathygeridae, there are a lot of species that are solitary animals. Recently, a revision of this hypothesis has been proposed that takes these facts into account. (3)
One thing that may have helped eusociality develop in these two species is that they come from genera that have a pre-existing tendency for group living. In these species there is already a tendency to search for food and care for young communally. Often, there also seems to be a strong family structure in these groups: pair-bonded animals care for their kin, and the offspring stay close by (10).
Therefore, if we assume that in the Bathygerid rodents there is a pre-existing condition of group living, which seems to be the case, it is very sensible that eusociality would have evolved twice. Eusociality is a derived trait, but it isn't very far removed from the ancestral trait. The fact that both naked and Damaraland mole-rats live in hostile environments can be described then as not the cause of the behavior, but a factor that helps push it all the way to the extreme (3).