Over the course of the night on Tuesday, February 9th, Elphaba- one of our two “queen” naked mole-rats, gave birth to a surprisingly large litter of pups. When we came in on Wednesday morning, we found 22 brand new babies in the colony.
The size of the litter surprised those of us on the Animal Care team because it was more than twice the size of her largest previous litter. She also just didn’t look like she had that many pups inside her when she was pregnant. In fact, she looked relatively trim. Whereas Galinda (the other “queen”) often looks like she’s swallowed a couple of golf balls by the end of her pregnancies, Elphaba has generally remained smaller, and this pregnancy was no exception. It may go without saying that all of the babies born in this litter were very undersized.
The pups were born into a completely sterile environment, as we had just completed day 14 of our “superclean” the previous day. The older colony members seemed to do very well with the babies, taking care of them when necessary and not being too rough with them as they ran around the enclosure. Still, none of this was enough for these pups. By the end of day on Saturday, they had all passed away. None of them made it to that crucial day 5.
So why didn’t they make it? It seems there’s always something, with our naked mole-rats. This time we believe it was the sheer size of the litter. There were simply too many pups, and they were all too small for any of them to have a fighting chance.
As I explain the problems with this litter to visitors and staff, I can’t help but be reminded of one of the basic theories of animal ecology and reproductive strategy. Famously, ecologists Robert MacArthur and E.O. Wilson developed the theory of r/K selection based on their studies of island ecosystems. According to this theory, the processes of evolution push all animal species towards two main reproductive strategies, K-selection or r-selection. K-selected species have relatively few offspring, into which they invest a lot of time and energy, so that most of those offspring are likely to survive. Conversely, r-selected species have many offspring (think millions), into which they invest little time or energy, so that few of their offspring are likely to survive. There is a trade-off, essentially: Quality versus quantity. Do you want to spend more energy making babies or raising babies?
How naked mole-rats fit into this theory is a little confusing, but that’s to be expected from this eccentric species. As mammals, they and their ancestors have been pushed towards K-selection for millennia. That’s why mammals give live birth and nurse our young. For mammals, it is evolutionarily risky to have too many offspring, simply because most mammalian biological systems are not designed for many babies. At some point mammals can no longer provide the care that the offspring need to survive.
Naked mole-rats have the largest litter size of any mammal — up to 28 pups in a litter. But when litters are this large, usually few or none survive. And unlike real r-selected species, naked mole-rats don’t reproduce quickly enough to make up for this low survival rate. Their gestation period is still at about three months. In these instances they are behaving like neither r-selected nor K-selected species. They are in an evolutionary no-man’s land, which is not positive for the growth of our own mini-mole-rat population. So why have such large litters? What is the evolutionary benefit here?
For most animals, reproducing is about more than just spreading your genes around, at least in the short term. When an animal is able to produce successful and healthy offspring, they are also making a statement to their peers. They are proving their fitness, attracting better mates, and in some species, actually improving their social standing. In a naked mole-rat colony, especially one where two queens are competing for dominance of the colony, a female that gives birth to 22 babies may prove herself to be more “queen-like” than one who only produces 10.
And so the saga continues. Although it’s sad and frustrating that another litter was lost, this may just be a blip in a truly epic battle. Elphaba has proven that she can have just as many babies as Galinda (two more, even!). The gauntlet has been thrown. What will happen next? What can happen next? We will know more in just a few more weeks. Galinda is pregnant, and predicted to give birth in the mid-March. We will, of course, keep you posted.
Brianna Todd, Lead Animal Caretaker
Hi Brianna (and Terry). It's an interesting observation that NMR's are in an "evolutionary no-mans land".
ReplyDeleteI'd venture a guess and say that the conditions of their natural environment are so that the special niche of naked mole rats are not easily inhabited by insects or other species. Perhaps the arid conditions are more conduicive to larger "containers of water" (NMR's) and insects would "dry up" too fast during draughts.
The size of litters are then a balance between the large workload of gathering food from a relatively large area and yet living in a rather protected environment underground. :)
I hope Galinda's litter will fare better.
Yours sincerely
Jesper K. Boesen
If the NMR queens really are duking it out for dominance, then the smell of one queen's babies everywhere must be a good indicator to the colony who's the big mamma on the block. Might super-cleaning be removing the scents and actually encouraging bigger litters to make up for the loss of scents?
ReplyDeleteMichael, absolutely that is a possibility in a general sense. Lack of colony smell is going to send a signal saying "we need more", which may or may not coincide with other indicator such as number of interactions the queen has with workers. In this case the pups were already near due date when we began the full clean, so the litter's maximum size was set. I do not know if mole-rats can selectively reabsorb fetuses to "right-size" their litter. Some rodents will do it (one species even days before giving birth) but it is usually as a response to stress rather than the norm. What you suggest is that mole-rats start with a large litter and cut back as the standard, unless the situation requires a big batch. It seems like a biologically expensive and even risky way to control numbers but with mole-rats you never know. The plus to this method would be that large groups of future workers could be readily available on short notice.
ReplyDeleteJesper, Brianna is the expert here and she may respond later. However I think your question is a good one - why, if insects usually fill a eusocial niche, did rodents get this one? Your assumption is that insects couldn't, the question being "why"?
ReplyDeleteNormally insects are better in extreme dry conditions than rodents, so I have trouble with that idea. The diet is a possibility. Mole-rats eat high cellulose plant material, and use special gut microbes to digest it. The microbes are passed on from one worker to the next, which obligates them be at least somewhat social, but also lets them eat a food very few animals could survive on. Perhaps it is their secret. Termites similar digestive needs, so - why not them?
Hi antwatcher. I was just throwing out an idea of the dry conditions being a factor versus insects. Maybe it's the other way around and it's the sudden rains that make the mole rats more successfull there. :) It might be their digestion as you mention it. Perhaps the plants in that area are more resistant to insects and a mammal instead fills the niche.
ReplyDeleteAtleast in my oppinion the mole rats must be doing the right thing there. Otherwise they wouldnt be there. And I do like our mammalian relatives more than 6 or 8 legged crunchies. :o
Yours sincerely
Jesper K. Boesen