Colonies
Merriam-Webster defines eusocial as "living in a cooperative group in which usually one female and several males are reproductively active and the nonbreeding individuals care for the young or protect and provide for the group" (Merriam-Webster). Naked mole-rat colonies are the perfect example of a eusocial community. The colonies average seventy to eighty members that spend their entire lives digging tunnels underground to find food.However, the burrows do not only provide food for the mole-rats, but also shelter, defense, and thermoconformation. Because the mole-rats spend their lives underground, they are almost completely blind. Hair becomes obsolete because the mole-rats regulate their body temperatures by being in certain areas of their burrow. If mole-rats are cold, they huddle together or bask in the shallower parts of the tunnels. However, if the mole-rats become to hot, they will migrate deeper into cooler parts of the burrow.
The burrowing system contains many tunnels that connect to larger communal areas. There is a single chamber in which breeding and sleeping occurs, a separate chamber where food found whilst digging is stored and consumed, and a chamber for defecation and urination (in this chamber, some naturalists, like Sir David Attenborough in the video below, believe the breeding female gives off a pheromone in her urine that sterilized the other individuals).
The burrowing system contains many tunnels that connect to larger communal areas. There is a single chamber in which breeding and sleeping occurs, a separate chamber where food found whilst digging is stored and consumed, and a chamber for defecation and urination (in this chamber, some naturalists, like Sir David Attenborough in the video below, believe the breeding female gives off a pheromone in her urine that sterilized the other individuals).
https://www.youtube.com/watch?v=VBRig7K70Z4&feature=youtu.be
For the Good of the Community
There is a distinct hierarchy in the colonies of naked mole rats which divides between the breeding and non-breeding and splits even more within the non-breeding individuals. This hierarchy takes an entire overthrow of those that are reproductive to change (van der Westhuizen, 2013) where the boundaries lie. van der Westhuizen et al. (2013) surmise that the mole-rat exhibits "the most extreme form of socially induced infertility or reproductive suppression...in any vertebrate." In each colony, there is only one female that is reproductive, the queen. The queen is usually the largest as well as oldest female in the colony. Only one to three of the largest/oldest males are used for breeding. The rest of the colony is non-breeding and make up the worker caste. Jarvis (1981) observed three subsections within the worker caste, (1) frequent workers, (2) infrequent workers, and (3) non-workers.
The photograph above shows a gravid queen huddling with other individuals of the colony. The queen's only worry is reproduction and with litters that average around twenty pups, she can have her hands full. However, the queen might have to worry about possible attacks, especially when she is heavily pregnant (van der Westhuizen, 2013). The workers take care of food foraging, defense, maintenance of the tunnels, and care for the young. Jarvis (1981) deduced that the frequent workers were smaller and moved food, soil, and carried the children while infrequent workers were larger and spent most of their time resting. The infrequent workers protect the burrow and save up as much energy until they are called into duty while the non-working caste of mole-rats take care of the young in the communal areas. According to Jarvis (1981), when adolescents are weaned from the queen, they automatically become frequent workers. To move up to infrequent workers or non-workers, the mole-rats have to grow and those that do not grow quickly stay frequent workers longer than those that attain larger sizes. The largest mole-rats will likely become breeders, the top caste level. However, if the breeding female become aggressive, the larger non-breeding females and males may be killed to "maintain suppression of reproduction of non-breeding animals" (van der Westhuizen, 2013). Hierarchy can be observed as the animals move throughout the tunnels--older/larger mole-rats will walk over those of the lower castes while the younger/smaller ones shove their way underneath.
Naked mole rat society is one of "diplodiploid eusocial animals...[in which] both males and females within a colony work but do not participate in reproduction" (Herrera, 2012). While the hierarchy does not allow every mole-rat to pass their direct genes onto the next generation, the entire colony is so closely related that lower caste individuals still have a large amount of their DNA passed on without procreating.
The heading photography is from http://www.the-scientist.com/?articles.view/articleNo/32136/title/Underground-Supermodels/.