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Eusociality and Reproduction in the Naked Mole-Rat
Gray Lyons
The Naked Mole-Rat is most commonly known as the only truly eusocial mammal. Naked mole-rat colonies are similar to some eusocial insect colonies in the nature of the dominant breeding queen, but are unique by the existence of a limited number of breeding male individuals (Sherman et al., 1992). Generally, only one to three males in the colony are breeding, while the others are reproductively suppressed like the non-breeding females. For the lay reader, eusocial behavior can best be related to ants or termites - where many individuals work selflessly to the benefit of a dominant queen. Scientists usually define eusociality more specifically as the existence of reproductive altruism, overlap of adult generations, and permanent philopatry (Burda et al., 2000).
Naked Mole Rats are characterized by female dominance - namely a colony that is ruled by one breeding female and sustained by an army of monomorphic workers. This breeding system is most closely characterized as polyandry (one female with many male mates) since the queen will copulate with one to three breeding males. However, the breeding system may also be characterized as male monogymy since the males will mate only with a single queen. Males generally follow an age-related hierarchy whereby the oldest males enjoy breeding rights with the female. Due to the high levels of genetic relatedness, the males are largely non-competitive for reproduction.
In a recent study, researchers removed breeding males from a colony of naked mole-rats and monitored both behavioral and physiological signs to understand the fruition of select breeding males. Although urinary levels of testosterone and cortisol were monitored, no correlation could be inferred from cortisol levels, but testosterone levels did correlate with dominance rank. Additionally, age and size correlated with dominance rank. The researchers noted, however, that virtually no aggressive interactions or other competitive manifestations occurred in the ascension of breeding males. This lack of competition is surprising, considering the relatively hostile competition that exists among females to assume the queen position. The authors attribute this discrepancy to either the “lower reproductive skew for males compared to females or queen control over male reproduction” (Clarke and Faulkes, 1998).
Female Naked Mole-Rats are more aggressive and will exert dominance to in the form of shoving behavior. This behavior includes prolonged pushes involving nose to nose contact. This aggression occurs within colonies and is mainly instigated by breeding individuals (generally one female and between one and three male individuals). The prevailing theories explaining this behavior are the threat reduction hypothesis (that is, suppressing the reproduction in non-breeding individuals) and the work conflict hypothesis (that is, inciting work activity in non-breeding individuals). In a recent article, the authors found no evidence to support the work conflict hypothesis, but they did note that the onset of shoving behavior coincided with reproductive activity and shoving strongly correlated with reproductive status. Queen shoving may have several functions, but this report ads evidence to only one of those functions: augmenting the reproductive success of the queen and breeding male individuals (Clarke and Faulkes, 2001).
In accordance with the eusocial nature of naked mole-rats, the it would be expected that naked mole-rats recruit colony mate to food sources. The successful forager attracts other colony members by auditory calls and displaying food (for scent, rather than sight). The colony mates will follow the scout’s original route through the labyrinth of tunnels, even if shorter alternate routes exist. However, if the route is replaced with clean tunnels, the colony mates showed no preference in routes to seek out the food, implicating the role of olfaction in food recruitment (Judd and Sherman, 1996).
It is easy to speculate on the evolutionary origins of the eusocial behavior of the Naked Mole-Rat. Presumably, the evolution of kin selection is inextricably tied to the subterranean habitiat: an ancestral rodent species found that they could escape some of the predation inherent in burrowing by closing the top of the burrow to surface predators. This habitiat led to rampant in breeding which produced individuals with high genetic similarity such that kin selection was more benefitial than individuality in the colony (Sherman et al., 1991). The intracolony genetic relatedness has been confirmed in a recent study, which explains the evolution of reproductive altruism. The same study also has implications predicting the intrercolony aggression described in Basic Biology and Habitat of the Naked Mole-Rat.
Faulkes et al. (1997) used a variety of genetic tests available in 1997 to quantify the level of genetic divergence in the naked mole-rats. Multilocus DNA fingerprinting was performed using restriction endonucleases and sequence-specific minisatellite probes. In addition, mtDNA regions, including the conserved cytochrome-b sequence, were sequenced and manually read by polyacrylamide gel electrophoresis. Thereafter, the data were analyzed by computer algorithms to construct unrooted phylogenetic trees. The authors found the individuals within a single colony to be nearly monomorphic to the point where they observed “a single individual can be representative of the whole colony.” Conversely, the authors observed a more divergent macrogenetic structure among the various colonies they tested in Ethiopia, Somalia, and Kenya. These results may be expected considering the secluded nature of naked mole-rat colonies and their documented propensity for eusocial inbreeding.
While the many behavioral attributes of Naked Mole-Rat reproduction are interesting, the species is unique in other areas. They are known to defy many common assumptions regarding mammalian biology. Some of these characteristics are addressed in the next section, Assorted interesting Facts About the Naked Mole-Rat.
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This web site was completed in partial fulfillment of the requirements for Biology 323, Animal Behavior, at Davidson College in the Spring Semester 2004