Wednesday, February 15, 2012

Naked Mole-Rats – Year in Review

Once again, it is time to report the previous year’s statistics for our naked mole-rat colony. The meticulously kept data helps to inform us of any long-term health trends of which we should be aware in addition to giving us insight into the general growth and well being of the colony.

Here are the 2011 highlights:

Naked Mole-Rat Colony Population:
January 2011 – 41
January 2012 – 57

Deaths by year (adults and independent juveniles):
2008 – 9
2009 – 2
2010 – 1
2011 – 2

Total recorded pregnancies: 6
Total recorded births: 6
Total pups born: 120
Average litter size: 20
Total successful litters: 3
Total surviving offspring: 18
Pup survival rate: 15%

Weight Changes Jan 2011 to Feb 2012:
Average weight difference (not including individuals born this year): 3.2 grams
Average weight difference among adults 3 years and older (not including those born in 2009, 2010 or 2011): .06 grams
Average weight difference among juveniles born in 2009 or 2010: 5.6 grams
Average weight of pups born in 2011: 22.4 grams

2011 weight changes by size – groups are divided based on their size in Jan 2011

It was another year of growth for our colony. We welcomed 18 new pups into the colony. While the pups from all three new litters have been growing rapidly, the rest of the colony has maintained a steady weight. If you look at the above graph, you will notice a steady but slight increase across all weight categories, followed by a slight decline at the end of the year. This difference may be due to a new scale that replaced our previous instrument, which broke.

Record of Pregnancies
3/4 – “Galinda”
Pups born = 25
Survived = 0

3/13 – “Elphaba”
Pups born = 17
Survived = 0

5/27 – “Galinda”
Pups born = 20
Survived = 4
Note: Pups from these two recent litters were too small at the time of the tattooing of the colony. As yet, they have no identifying markers and are known as the “Big Babies.”

6/17 – “Elphaba”
Pups born = 17
Survived = 1

8/24 = “Galinda”
Pups born = 25
Survived = 13
Note: Not tattooed but noticeably younger than the “Big Babies,” they are known as the “Little Babies.”

12/19 – “Galinda”
Pups born = 16
Survived = 0


  1. how long have you had these naked mole rats? I remember them from when my children were young and that is a long time ago.

    1. Sorry for the late reply wesleycat. You are correct that we have had naked mole-rats at PSC for a very long time. We received two adults and four pups from the Philadelphia Zoo in December of 1993. We also purchased eight more individuals from the National Zoo in March of 1994.

      Naked mole-rats can live to be up to 30 years old, so it's possible that some of those original mole-rats are still with us. Unfortunately, we haven't always kept an accurate record of births and deaths within the colony, so it is impossible to verify it one way or the other.

      Thanks for being a long-term mole-rat fan!

  2. Hello,

    thank you for the new facts about your NMR colony!

    Elphaba and Galinda are your two queens - but which mole rats in the colony are the fathers of her litters?

    Many greetings from

    1. Hi Suzy,

      You may have guessed that it is considerably more difficult to identify a father than it is a mother! For the most part, reproductive males look the same as non-reproductive males.

      The best way to determine which males are mating is to catch them in the act. Life Sciences staff members have never observed any obvious mating behavior (although clearly it is going on). It may require more steadfast observation than we are able to provide.

      We do know that there are likely 1-3 reproducing males in our colony. Who they are though, remains a mystery.

  3. What are the primary causes of such low pup survival rates since there are no natural predators, and I'm assuming there's sufficient food resources?

    And how does it compare to naked mole rat communities in the wild?

    1. Is it possible to say if the colony needs intermingling from other colonies to prevent excessive inbreeding?

  4. Re: Karen - In our colony, pup mortality is usually due to a few causes; they may be underdeveloped at birth, they may suffer a physical trauma, or they may not be getting milk. Each of these is linked to broader issues.

    For instance, our queens tend to have excessively large litters, maybe as a result of their competetion with each other. The larger the litter, the smaller and less developed her pups will be. Large litters may also mean less access to milk for each of the struggling pups. Temperature and humidity can also affect milk production.

    Trauma, on the other hand, has become less of an issue in our colony recently. For a few years, we did not have any new pups. When they did start to reproduce again, we noticed a learning curve among the adults who were caring for the babies. While at first they didn't seem to know exactly what they were doing, today they do a pretty good job of protecting and moving the pups without causing major injury.

    I am unsure how this compares to pup survival rates in the wild. I haven't been able to find much documentation on that subject. I would guess that in the wild and in captivity you would find that every colony is different and faces unique challenges to raising their young.

  5. Re: atg - Naked mole-rats have adapted over a long period of time to be highly inbred. The close genetic relationship between colony-members drives their eusocial behavior, just like in a bee colony.

    That's not to say that they never outbreed. In wild colonies, every once in a while, a strong male will leave his home colony and become a mating male in another colony. Search our archives for the "Disperser Morph" article, which will give you a lot more details on this phenomenon.

    Would we ever introduce a new male to our queens? That would be a difficult decision. Mole-rats usually reject and attack non-colony-members, often killing them, so it would be potentially dangerous.

    For now, our colony seems to have a healthy (for naked mole-rats) degree of genetic variation. If we start to notice that offspring are not surviving due to obvious mutations, we might consider taking this step.