5.1 Evolution of eusociality in mole-rats  (Page 6/8)

Damaraland mole-rats, however, are obligate outbreeders , meaning they will only breed with foreign mole-rats (Burland et al. 2002, Burland et al. 2004). Rickard and Bennett (1997) examined C. damarensis colonies, both in the lab and the wild, from which the dominant female was removed, either due to experimental removal or a natural death. They found that the colony remained reproductively quiescent, or stopped reproduction, until an unrelated male was introduced, then a former female worker would become the new breeder (Rickard and Bennett 1997). Their work provides a strong example of the incest avoidance Damaraland mole-rats have.

For a while, it was thought that incest avoidance in Damaraland mole-rats, unlike in naked mole-rats, was sufficient to maintain the reproductive skew. Reproductive suppression was therefore unnecessary. A recent study by Burland et al. (2004) has called these ideas into question, however. They found from samples taken from wild colonies that unrelated non-breeders of the opposite sex coexisted within colonies, providing opportunities for non-breeders to mate. In addition, they found offspring of queens with different fathers than the male breeders. They suggest that either males pass through the colonies or females can briefly leave (Burland et al. 2004). Either way, non-breeders come into contact with unrelated, potential mates, yet they do not mate. Due to this, Burland et al. (2004) believe some reproductive suppression occurs in C. damarensis as well which contributes to the reproductive skew. They do not know yet at what stage suppression is achieved, ovulation, copulation, or implantation (Burland et al. 2004). In order for any outbreeding to occur, however, some mole-rats must be willing to leave their natal nest and disperse.

Dispersal and morphologically separate castes

So far this paper has examined mainly one strategy for workers, the one in which they stay in the natal colony to maximize their indirect fitness, and how that strategy impacts reproductive suppression and incest avoidance. There is however, a second strategy in which the workers leave their natal colony and disperse, either to form a new colony or invade another, in the hopes of becoming a breeder and gaining direct reproduction (O’Riain et al. 1996). Dispersal for the naked mole-rat and the Damaraland mole-rat has high ecological constraints, though, because digging a new burrow for a new colony in the hard ground is difficult. In addition, a small starting colony experiences a greater risk of not finding food when foraging, and the risk of predation is high when leaving the colony. Spinks et al. (2000) performed a within-species comparison in Cryptomys hottentotus hottentotus , or the common mole-rat, between mole-rats in an arid environment and those in a mesic environment ( [link] ). They found that the inclusive fitness for individuals is no longer maximized after reaching a certain colony size and there are greater fitness benefits in dispersing due to increased competition for resources like food. They also found, however, that the higher ecological constraints at the arid sites forced the individuals to remain in the colony longer due to the higher costs of dispersal (Spinks et al. 2000). These findings suggest that group living for H. glaber and C. damarensis was further encouraged by the large environmental constraints on dispersal.