Sunday, February 28, 2010

The Disperser Morph

This goes in the “just when you think you’ve seen everything” category.

One morning as Animal Caretaker Maida Ingalls prepared for her day, the early shift custodian pointed out to her that one of the naked mole-rats had escaped. This occasionally happens. A mole-rat will get out of the system of tubes and chambers, and wander aimlessly around the interior of their enclosure.

In this case, something much more dramatic had happened. The animal was spotted on the ground in front of the exhibit.

Maida quickly put him back and briefly recorded the escapee information. At first she couldn’t figure out how he got on the floor, but was able to deduce that he must have crawled between the doors and the sill, and then slid down the signs in front of the exhibit. Quite a feat for such a small animal! Surely the experience would dampen his enthusiasm for travel!

Not quite. Two days later, the same individual performed the same escape. This time there was a difference. While in his first escape, he had pushed one of the tubes out of a chamber, this time there was no visible means of escape. We did notice that one of the latrine chambers was filled especially high with bedding, almost to the very top of it. We believe he must have climbed the bedding and pushed the lid open to crawl out. So we taped down all of the chamber lids, hoping to prevent another escape. The offending mole-rat was marked with a black pen on the leg and observed for signs of trauma, then returned to the colony.

Only to escape yet again! During a cleaning later in the week, a new latrine chamber was installed without tape on the lid. Our perceptive mole-rat figured this out, filled the chamber with bedding and climbed out the top one more time. This time, when he reached the ground level, he climbed through a ventilation damper into the area below the exhibit, which could have been extremely dangerous if we hadn’t found him soon after. What was this hairless Houdini up to? Did he have a death wish?

Interestingly enough, this behavior has been described in other colonies as well, and it doesn’t appear to be entirely suicidal. This mole-rat is presumed to be a disperser morph – an individual within a eusocial group that is driven to carry its genetic material out of the colony of origin and found a new colony or integrate into an existing one. In the stable, well fed colonies kept in captivity, the disperser is usually a male, but in the wild males and females seem to become dispersers in about equal numbers.

The disperser might be thought of as a high risk, high reward behavior. The chances of survival in the wild for a solo mole-rat are slim. But any disperser hardy enough to stay alive and lucky enough to attract a partner is far more likely to produce offspring than he or she would be by remaining at home as a worker. To an animal living in an established colony, the pathways to breeding involve fighting – either with an established breeding animal, or with other contenders if the queen or her mate die. As founder of a new colony, the disperser would have ample opportunity to produce his or her own offspring rather than caring for those of others.

If our Houdini is a disperser mole-rat, his behavior is powerfully motivated and may continue. We are testing husbandry techniques to see whether high levels of enrichment will meet his dispersing need. If he is isolated from the colony, will he work to get back in? Or is getting away the whole point? Eventually, he may need to become a founder of his own, or join another organization’s colony, perhaps as a breeder. In the mean time, this peculiar behavior offers us yet another glimpse into the many adaptations of this eccentric species.


  1. Haha... What a story. Thats quite the little escape artist you have there. :D

    Have you give the idea of a "second colony" thats still connected to the first any more thought by the way?

    Yours sincerely

    Jesper K. Boesen

  2. One thing the text does not make clear enough is how the dispersing morph is heavier than the rest of the colony. Our individual was refered to by two people who found him as "fat", a term usually not used for naked mole-rats. This storing of fat would be critical to his survival in a new habitat, where food was scarce and finding it was risky.

  3. @antwatcher. Is he the molerat on the weightscale? Because allthough that one seems heavier than the scrawny ones I've seen it doesn't immediately say "heavy" molerat to me.

    Yours sincerely

    Jesper K. Boesen

  4. That is so fascinating and awesome! Tell me more!

  5. Jesper -

    That is the same guy on that scale picture above. You can see that he's weighing in at about 81 grams. He is definitely the largest mole-rat in our colony. The only exception is Galinda, one of our reproducing "queens" who has gotten up to 94 grams, but that is with about 19 babies inside her, which makes a big difference.

    I did a quick internet search on physical characteristics of mole-rats and there is rarely a discription of a non-queen mole-rat weighing more than 60 grams or so. That weight seems to be average for adults in our colony.

    The other difference between this individual and the other adults it that his fat storage seems to run the entire length of his body. I've heard him described as having a fat neck as well, which is hopefully evident in the "Wanted" picture. This might denote that those fat stores are not related to reproduction, since they're at the top of his body.

    -Brianna Todd
    Lead Animal Caretaker

  6. Hi Brianna, Thanks for the reply. Could have checked for NMR weights myself. :o ...

    I was just loosely comparing him to the mole rats I've seen on a webcam from another zoo.

    They seem to come in a few different body types:

    Skinny (young and a bit smaller?)
    Skinny (worker?)
    Heavier built (mating male? or soldier?)
    Fat (just like youre describing your disperser morph :) )

    Interesting enough the fat ones allways seem to have time for a break during their work. Where the working molerats hurry along ignoring the food the fat ones allways have time snack a little ;)

    Yours Sincerely

    Jesper K. Boesen

  7. Jesper, if you are looking at an adult mole-rat and thinking it is "skinny" it could be queen who recently gave birth, or a breeder male.

    The queen would also have a promenent spine, and probably large mammaries. She would be unusually long compared to other workers.

    The breeder male is the same size as other animals but extremely thin. He is shorter lived than other animals. It is speculated that having high turnover of breeders helps provide genetic diversity. Or that the testerterone surge the breeder male experiences is not good for his overall health. Or that he is subject to low level harrassment from other animals as they jockey for position within the colony, and that this takes a toll. We have one adult, informally called "skinny guy" who fits the breeder profile, though he has not been specifically observed breeding.

  8. @Antwatcher: I was observing through a webcam so it's a little different perspective. A few of the "skinny ones" (or the same individual multiple times) were indeed quite long compaired to the others.

    The other type of "skinny molerat" was definately shorter (allthough not massively) on average than the others.

    Thanks for the information though. It will help an aspiring NMR spotter :)

    Yours sincerely

    Jesper K. Boesen