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Similarly, Damaraland mole-rats have a dispersive morph made up of bigger mole-rats who do less work (Hazell et al. 2000, Scantlebury et al. 2006). Scantlebury et al. (2006) found that these mole-rats work little until after rains, when they substantially increase their work in comparison to normal workers. The disperser mole-rats are also significantly bigger than the queen, who is significantly larger than the workers (Scantlebury et al. 2006). The disperser morph in both species has an advantage that allows it to attempt the second strategy, dispersing and gaining direct reproduction. Its larger fat stores allow it to travel longer distances without starving and provide a good reserve of energy when beginning reproduction (Scantlebury et al. 2006). Overall, the disperser morph, while costly to its natal colony in terms of energy consumed and not used (O’Riain et al. 1996, Scantlebury et al. 2006), benefits itself through the potential for direct reproduction and provides gene flow for these highly unusual and fascinating mammalian species (Faulkes et al. 1997a).


The evolution of eusociality in mole-rats can be explained primarily by the Aridity Food Distribution Hypothesis which uses the environmental factors of aridity and unequal food distribution to explain why certain species of mole rats evolved to be eusocial. This also accounts for the separate evolution of eusociality in two different species. As a eusocial species, the mole-rat has tiers of breeders, workers, and dispersers. The situations in which mole-rats will and will not breed is not entirely understood, but appears to be more socially or hormonally driven, as even mole-rats who are not mole-rats do preserve their ability to reproduce, even if they do not use it. The dispersal tier of the mole-rat eusocial hierarchy is essential to allowing for enough outbreeding to sustain healthy offspring, and allows large, successful colonies to spread their genes even further. The entire hierarchy of eusociality in mole-rats is certainly intricately evolved, from intrinsic differences to social cues that allow the mole-rat to survive in harsh conditions.

Discussion questions

  1. In mole-rats, non-breeders do not lose reproductive capabilities. If non-breeder tiers of the mole rat did engage in reproductive behavior, how might this interfere with the eusocial structure of the colony? Would this behavior increase that individual’s fitness, why or why not?
  2. Eusociality appears to have evolved twice separately in the mole-rat. What conditions would likely allow for eusocial evolution in other mole-rat species, or even in other organisms that are typically solitary?


  • Arid climate - characterized by severe lack of available water
  • Basal clade - the first to diverge from the lineage in a cladogram
  • Dimorphism - two distinct body forms of an individual within a species
  • Eusocial - applies to a society with reproductive division of labor, overlapping generations, and cooperative care of young
  • Fractal dimension - the extent that the burrow fills the area it inhabits
  • Fitness - ability of an individual to stay alive and pass on genes
  • Inbreeding - breeding with relatives, leads to less genetic variation
  • Inbreeding depression - a decrease in offspring size, fertility, and fitness due to inbreeding as recessive deleterious traits are more likely to manifest themselves in homozygous individuals
  • Inclusive fitness - the combination of direct fitness and indirect fitness gained
  • Indirect fitness - the fitness gained by helping a relative and impacted by the degree of relatedness
  • Kin Selection - altruistic acts directed towards kin that lead to greater overall inclusive fitness because of an increase in indirect fitness
  • Mesic - having a moderate supply of moisture.
  • Shoving - nose-to-nose pushing for protracted periods of time
  • Outbreeding - reproduction between two different populations , leading to more genetic variation
  • Reproductive skew - distribution of breeding between members of a population where some members hold more breeding rights than others


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  • Burland TM, Bennett NC, Jarvis JUM, Faulkes CG. 2004. Colony structure and parentage in wild colonies of cooperatively breeding Damaraland mole-rats suggest incest avoidance alone may not maintain reproductive skew. Mol Ecol. 13:2371-2379.
  • Ciszek D. 2000. New colony formation in the “highly inbred” eusocial mole-rat: outbreeding is preferred. Behavioral Ecology. 11:1-6.
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About the author

A portrait of the author.

Sheena Shah-Simpson is an undergraduate from Arlington, Texas, majoring in Biochemistry and Cell Biology. In her free time she likes to play, or attempt to play, musical instruments, read, hang out with her friends, listen to good music, and dance. While studying animal behavior she learned fascinating things about conflicts of interest, between species, within species, within individual families (parent-offspring conflict is particularly interesting), etcetera. Writing this chapter, Sheena also learned many extraordinary things about such varied topics as incest and different morphologies within just one family of animals, the ever-fascinating mole-rats.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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