Naked mole rats survival depends on their queen

It is very curious that many of the species that are eusocial (which means that there is differentiation of breeding and non-breeding members and highly organized division of labor within the colony) are creatures that form extensive burrows and tunnels, either underground or in trees. Think ants and termites (virtually all ant and termite species are eusocial) as well as some beetles (1). Even for bees, some of their species live eusocial ground-nesting lives like bumblebees.

Creating underground nests means protection from predators, but less availability to food, oxygen, water, and warmth (2). Thus, it is possible to imagine how eusociality developed over time, particularly for underground-living creatures.

Development of eusociality could have started with simple systems, like that of a bumblebee. A female bee starts with her first brood of eggs. When these grow to be adults, a dominant female's pheromones could evolve to suppress the fertility of the new bees. The net result is the formation of two castes of bees: a queen and her newly minted female worker bees. Given that such social cooperation makes survival of the whole colony more likely, it is easy to see how eusociality could be selected for (3). 

Having a nesting site (hives, burrows, sponges, tree tunnels, house walls, etc) is a prerequisite condition for a eusocial species (1). They need a place to call home, where the brood can be watched over, protected, and nourished over several generations. 

However, that is not sufficient condition, as there are many animals that create nests (above and underground) that are not eusocial.  

Of all the mammals that form nests, or burrows, or tunnels (think  rabbits, moles, voles, badgers, gophers), only 2 have developed eusociality. The  are the Naked Mole Rat and Damaraland mole-rat (2, 4, 5, 6) (though some also include a few related mole-rat species (5).

Clearly something must have pushed those two mammals to having 1 queen!

Why did the naked mole rat develop eusociality? there are a number of hypotheses on how eusociality develops. For our naked mole rats, several lines of evidence suggests the following (5):

- They evolved from an ancestor that had established social tendencies

- Cooperative brood care was prominent in the ancestral species of  mole rats, as females typically had long pregnancies with slow postnatal development for the progeny.

- Females needed help during their pregnancy as well as with brood care. A female mole rat is unable to store enough fat reserves to hold her through her long pregnancy and lactation period. In fact, cooperative monogamy has been found to be an ancestral trait for mole rats

- Finally, living underground was a disincentive for members of the colony to disperse, allowing continuous breeding (reinforcing the need for cooperative monogamy)

The monogamous mating system with cooperative brood care set the stage for a dominant female system to evolve that suppresses reproductive maturation in the rest of the colony (except for 2-3 males, called Pashas - 6).

The queen is crucial for several key elements in the colony: 

- Establishing the dialect that the colony uses: each colony has its own set  of chirps with different patterns and pitch. This way they can recognize outsiders, whom they would be hostile to. When the queen dies, the colony becomes a free-for-all, and the colony loses its distinctive dialect. 

- Providing the grandmother effect (6): Given the longevity of the queen (sometimes into the decades) and the long lives of the colony members (naked mole rats live into their 30s), they are available to train the young to serve the needs of the colony. Compare that to other rodents (like rats and mice). 

- Maintaining the social hierarchy: The queen maintains the social (and reproductive) order within the colony by shoving breeder males and non-breeding members (8). 

- Increasing the number of pups born: Early in her reign the queen has much few pups than later. The primary reason is that the queen's spine must elongate to accommodate larger litter sizes and the queen must grow in size. As both of those physiological parameters take place, larger litters become possible (9)

- Providing a large colony size: Having a stable reigning monarch focused on breeding means that the colony size is large, allowing for increased foraging and protection for all its members. Interestingly, the size of a naked mole rat colony (almost up to 300 mole rats) is similar to human groups: Studies from Neolithic villages suggest that optimal numbers are when the group is not larger than 100–150 individuals (6).

When the naked mole rat queen dies, several females fight to the death to claim the throne. 

The thing is that without a dominant female, the whole social order will be disrupted. For example, it has been shown that when worker females are removed from the colony, they start ovulation and become fertile (7). 

There would be a problem when there is more than one dominant female. Offspring from stable colonies have been shown to be better parents when given the opportunity to breed compared to offspring from unstable colonies (for example, where coup d'états have taken place) (6).