Reproductive Strategies

The Naked Mole Rat presents a oddity in many of its reproductive behaviors when compared to other African Mole Rats. Heterocephalus glaber presents several behaviors that are directly in contrast to many of its relatives. For one H. glaber is a eusocial mammal and splits its reproductive responsibilties up through division of labour. In a colony there is generally only one breeding female and 2-3 breeding males.1 The breeding female is morphologicaly different from the rest of the females in the colony and often displays agressive shoving behaviors towards the other females in the colony.2

The breeding female of a colony of H. glaber also produces many more young than related species. Related species such as the Damarand Mole Rat (C. damarensis) produce an average of 2-4 pups per litter, with a maximum of 6. Colonies of H. glaber produce an average of 12 pups per litter and have been observed to produce up to 27 in a single pregnancy.3 This is linked to the fact that the H. glaber colony sizes are much higher than those of the other African mole rats. The species with the next largest size of mole rat is C. damarensis which has a mean colony size of 11 individuals per colony. The largest C. damarensis colony discovered had 41 individuals The mean colony size of H. glaber colonies is 80 individuals, and the largest colony discovered contained 271 rats.4

H. glaber also inbreeds to a high degree, much higher than most other mammals. The whole colony is constructed of one single family, and breeding is almost always between brothers and sisters.5 Indeed, the average relatedness between the offspring of the Breeding female and the rest of the colony is often as high as 0.5, a value that in most other species is reserved for parent-offspring relationships. Meanwhile the relatedness between a parent and offspring is often as high as 0.81. The maximum relatedness is 1.0, a score given to monozygotic twins.6

Althought H. glaber puts the burden of creating new young for the colony upon only one female, care of the young is spread out among many of the different members of the colony. Studies have shown that from just before birth to till four weeks after huddling of all members of the colony increases, and young often beg for feeces from all members of the colony. Furthermore, nonbreeding males are often observed nudging and grooming the pups.


But perhaps the most interesting part divergence of H. glaber from its fellow African mole rats is the way in which queens sexually suppress the other mole rats in the colony. Of all the mole rats in the colony, only the breeding females and males are able to breed. Non breeding females have prepubescent ovaries and males do not create gametes in quantities anywhere near what the breeding males do.7 The Damarand mole rat (C. damarensis) also has a single breeding male and female in the colony however the non breeding males and females seem to be fully capable of breeding, and seem meerly to'choose' not to, rather than being, like in H. glaber unable to.8

Sexual Supression

The non breeding females and males of a colony of H. glaber are not merely choosing not to reproduce, they are infertile. 9 And indeed there are morphological differences between the breeding female and the non breeding females. Compared to the non-breeding females, the breeding female has prominent teats and a perforated vagina, she is also larger than many of the other females and has a visibly lengthened torso, breeding males have signifigant differences in size in comparision to nonbreeding males.10


Non-breeding females have what appear to be prepubescent ovaries and within the ovaries primordial follicles.11 Tests have also shown that non-breeding females have low urinary progestrone and estrogen levels. Ovulation appears to be inhibeted because of inadequete lutenizing hormone being secreted from the anterior pituitary. In tests, secretions of lutenizing hormone have been low or non-existant.12 The levels of the other pituitary gonadotrophin, follicle stimulating hormone, are unknown.

Scientists have theorized that these hormonal differences are caused by an insensitivity to GnRH. When tested, it was shown that the senstivity of GnRH receptors was quite low.13 However GnRH has been shown to be a self-regulating hormone whereby its receptors become more sensitive as more GnRH is released. Researchers showed that periodic injections of GnRH every four hours caused the sensitivity of GnRH receptors to up regulate.14

Furthermore, females who are suppressed are not permanently so. When removed from a colony a female H. glaber will start her ovarian cycle as early as eight days after removal.15 Also, evidence has shown that the hormone levels of some females are less suppressed than in other females.


Between breeding males and non-breeding males there are significant differences in body weight, and there are fewer cells that secrete testosterone in non-breeding males.16 Dissection of the testis have also revealed that non-breeding males have many fewer sperm. In half the testis of a Breeding male 8.6 million sperm were found, while in the non-breeding male only 1.8 million sperm were revealed. Furthermore, the sperm that were found in non-breeding males were non-motile.17

Non-breeders also evince lower urinary testosterone levels, and lower basal levels of lutenizing hormone.18 Exogenous administration of GnRH also had less of an effect upon non-breeding males. However, the testosterone levels in breeding males is variable and closely in sync with the ovarian cycle of the breeding female. The breeding male's testosterone levels peak right during the follicular stage of the ovarian cycle, right before estrus. Thus at different times in the breeding cycle non breeding males will actually have higher levels of lutenizing hormone than the breeding males.19

Finally, like the females, the suppression of sexuality is not permanent, and within five days of being removed from their home colony non-breeding males will undergo hormonal changes so that their levels are equivalent to those of breeding males and their testis will enlarge.20

Cues for reproductive suppression

It is currently unknown what cues are responsible for the morphological and hormonal changes that happen to sexually suppressed mole rats. It was once thought that primer pheromones in the urine of the reproductive might act as chemical cues causing the hormonal changes in the non-breeders. Since the mole rats share a communal bathing area, such cues might be spread to all the members of the colony.21 Furthermore, it was shown that removing the breeding female from physical but not olfactory contact was not enough to maintain the suppression on the non-breeding females in the colony.22 However, other tests with soiled bedding suggested that pheromones in the urine were also not responsible for the hormonal changes.23 Currently researchers hypothesize that direct contact with the female, including harassment may be responsible for the hormonal and behavioral changes. How this would transmit itself through the endocrine system is unknown.24

The only evidence of direct aggressive behavior in mole rats is shoving that happens between the queen and worker mole rats. It is currently unclear what this shoving is for, but there are two dominant theories. One suggests that aggression from the female indicates her reproductive dominance and willingness to fight, reducing the threat of colony members challenging her for breeding status 25. The other hypothesis is that the shoving is aimed at workers who aren't working hard enough, and work activity levels seem to vary inversely with the individuals relatedness to the breeding female. However, studies have not been able to suggest a link between work level and relatedness as predicted by the second hypothesis.26 Large animals, regardless of their relatedness to the female worked less. Furthermore, it was found that workers were more likely to be shoved when already working, rather than when resting in the nest.27 This preliminary evidence would thus suggest that the second hypothesis is incorrect.


1. Jarvis, 1981
2. Jarvis, 1981
3. Bennett and Faulkes, 2000
4. Jarvis 1991
5. Bennett and Faulkes, 2000
6. Reeve et al, 1990
7. Jarvis 1981
8. Bennett and Faulkes, 2000
9. Jarvis, 1981
10. Faulkes et. al. 1994
11. Faulkes et. al. 1991
12. Faulkes et. al. 1991, Faulkes et. al. 1990c
13. Faulkes et. al. 1990c
14. Faulkes et. al. 1990c
15. Jarvis 1991
16. Faulkes et all, 1994
17. Bennett and Faulkes, 2000, Faulkes et. al. 1994
18. Faulkes and Abbot, 1991
19. Faulkes and Abbot, 1991
20. Faulkes and Abbot, 1991
21. Jarvis, 1981
22. Jarvis, 1981
23. Faulkes and Abbot, 1993
24. Faulkes and Abbotk 1993
25. Reeve and Sherman, 1991
26. Bennett and Faulkes, 2000
27. Jacobs and Jarvis, 1996