Biology 342 Fall 07
A eusocial lifestyle may not seem like it makes very much sense evolutionarily. Why should a system arise in which only one out of a large group of animals passes on her genes? However, there may be certain advantages to having adapted this way of living, especially considering the harsh environmental circumstances.
According to Hamilton's Rule (7) eusociality will only arise if the costs are outweighed by the benefits times the coefficient of relatedness. The costs to eusociality are clear: a single individual is very unlikely to breed and as such cannot directly pass on its genes. However, eusociality is clearly one mode of life, so it must have advantages in terms of the species, if not the single animal.
The first obvious benefit is to the queen and the young. While in other species there are only one or two parents to look after them, in a eusocial community the care of the young is shared amongst the workers. The division of labor is also highly effective in ensuring positive returns on work for all individuals (17). This can be readily seen in food harvesting. All animals in the colony share the food, which means that so long as there is an adequate supply of food available, all animals will be well fed. There is no competition for food, so each less stress is put on each individual to maintain it survival. Another direct benefit of communal living is that there is no struggle for living space. Territory is inherited from one generation to its successor: there are no battles for preferred areas or the best nesting sites and the initial work of creating an elaborate colony system is already done. (21).
Mole-rat enjoying a delicious lunch (ix).
Naked mole-rats live in an inhospitable climate. Conditions are extremely dry and rainfall is scarce, leading there to be very little food available. The lifestly of communal living that the mole-rat has adapted best utilizes this food and maximizes the number of individuals that can thrive off of such limited resources. (For more on this please see the Food-Aridity Hypothesis on the Phylogeny page.) In fact, the naked mole-rat is so well aware of the scarcity of its food source that they have developed a system to make the most of it. The mole-rat will hollow out a tuber, harvesting the inner tissue, but leaving much of the shell and root system intact. This way the tuber can regrow and the mole-rat can come back next season for another harvest (for more information see Attenborough video on Home page).
Taro, a common edible root. A very tasty tuber (x).
In such extreme conditions the likelihood that a mother mole-rat would be able to survive and raise young on her own is not very good. With food sources being so widely spread, it would be nearly impossible for the mother to have the energy to dig tunnels to the food while at the same time nursing her babies. Likewise, it would be difficult for a single pair to perform the same task. The elaborate maze of tunnels created by the naked mole-rat community is the best way to ensure that there will be food enough to support the adults and raise the children. So, the mole-rats all rely on each other and in doing so each increases their own reproductive fitness (2).