Lots of Little Deer Mice

If a sexually reproducing animal is to preserve its species without population loss, each pair of parents has to produce, on average, two offspring that will themselves live long enough to reproduce. This is just basic arithmetic, but there are at least three different ways that an animal can achieve this result and which one is used varies from species to species.

One approach is to maximise the chances of each of your offspring surviving. This is called the K-selection strategy, and results in the population being as close to the maximum capacity of the local habitat as possible. (The 'K' stands for 'capacity'... in German). Such animals don't need to reproduce very often, or produce very many offspring when they do, but they have to invest a lot of resources in their survival. They tend to be relatively large animals, with few predators... elephants, primates, and whales are all good examples among mammals. Humans are a particularly extreme example, given how long it takes us to raise our children, and, as with other strong K-selectors, twins are rare in our species.

The opposite extreme is the r-selection strategy, which instead maximises the rate of reproduction. What most r-selectors do is invest all their energy into producing one vast spawning of relatively vulnerable offspring just once in their lives, and then dropping dead in the hope that you've produced so many that at least some of them will survive. This is fairly common among invertebrates, but doesn't work among mammals, because the mother has to survive at least long enough to feed the young with milk. Nonetheless, there are a few species of marsupial that come close - the males drop dead from exhaustion after one frenzied breeding season, and the females live just long enough to wean their pups, and then they, too, die, sacrificing everything to give their offspring the best chance in life.

The third approach is a more moderate version of the r-selection strategy, and this is what most smaller mammal species do. Rather than breed once and die, they breed repeatedly, giving birth to large litters when they do so. Over their lifespan, even if it's a relatively short one (and it usually is) they can still manage to produce rather a lot of young.

In reality, of course, these are just three possibilities on a broad spectrum and there is considerable variation among mammals as to where they fit. One of the ways that this becomes apparent is in how large their litters are. Producing a large litter requires a lot of effort and, in the case of mammals, that effort doesn't even end once the young are born. If the litter is too large, that effort is wasted at best, and counterproductive at worst. On the other hand, if it isn't large enough, then the population will decline, and eventually die out altogether.

As in so many cases, there is a balance to be struck here, and all sorts of things can help determine what the ideal litter size is for any given species. How much food is available? How many predators are around? How many other animals are there around that want to eat the same food you do? Some of these things are difficult to estimate, but one thing that is measurable is the climate.

The closer to the poles an animal lives, the longer and harsher the winters become. This not only increases the chance that animals will die off during the winter, but means that the available time to produce and rear litters of young is shorter. It has been suggested since at least the 1940s that this means that animals living at high latitudes should tend to produce fewer, larger, litters than those living in more tropical climes. This does indeed seem to be the case, with, for example, North American animals living in the north producing larger litters than closely related species living further south. If the environment is otherwise equally fertile, and good at producing food in summer, this generally cancels out in terms of the total number of young born in any given year.

But does the same hold true within a species? Obviously, any given animal is constrained by its biology, and litter sizes will tend to be relatively constant (on average) between different members of the same species. But evolution might favour a slight reduction in litter size among individuals with the opportunity to breed and raise young more frequently, so long as multiple generations tend to stay in the same broad geographic area.

One of the most widespread and numerous mammals in the Americas is the North American deer mouse (Peromyscus maniculatus). This can be found from southern Mexico in the south to the southern Yukon Territory and Labrador in the north, taking in most of the United States, Canada, and Mexico on the way. (It is, however, absent from much of the Atlantic seaboard, as well as from Alaska and northern Canada... but that still leaves rather a lot of geography). In many ways, it's a rather typical small rodent, and it's evidently one that's highly adaptable.

Deer mice have litters that average around three to five young. With pregnancy lasting just over three weeks, and the young being weaned after a similar amount of time, it's possible for mothers to have several litters throughout the year, but exactly how many naturally depends on how long the winters are. Previous studies have shown that not only are deer mouse litters larger in populations that live at higher latitudes, but they also increase with altitude as well, but these are often comparing the various different species of deer mouse (there are at least 60) with one another rather than looking at variation within individual species.

A more recent re-examination of hundreds of museum records of pregnant North American deer mice collected from across the US and parts of Canada was able to confirm the earlier findings. (A caveat is that museum records can only tell you how many embryos a deceased mouse was carrying. This is probably larger than the eventual litter size because they won't all reach term, but the relationship between the two figures is apparently constant enough for this not to matter when analysing trends). More significantly, however, the researchers were also able to look at detailed climate records, comparing litter size with the exact local conditions, rather than simply looking at latitude.

It turned out that the best predictor of litter size was the number of frost-free days in the year where the mice happened to live. That is, the fewer such days there are, the larger the litter, as the mother produces more young per litter to compensate for the fact that she'll be able to give birth to fewer litters per year. Significantly, this is a long term climatic feature, and details such as individually bad winters or rainy years when more food is available, not affecting litter size.

This means that the mice aren't responding to a bad year by having more young, and simply happen to have more bad years the further north they live. Rather, it's likely an evolutionary trend, where mice that live in colder parts of the world and give birth to larger litters out-compete other locals, and so are more likely to pass their "large litter" genes on to the next generation. This is exactly the sort of process we would expect to cause changes in litter size when new species form and happen to colonise colder/warmer climates than their ancestors.

But there was one other finding that was also potentially interesting: litter sizes in North American deer mice seem to be have been getting slightly smaller over the last hundred years or so. It's not a huge effect, and although it does seem to be real, it's at least possible that it's just a coincidental blip in the data. But it has to be said that there are, on average, fewer frost-free days each year in modern America than there in the last century.

That doesn't prove that genetic evolution is occurring here before our eyes, because the time scale may well be too short for that. It could also be, for instance, that older mice are more likely to survive a mild winter than a harsh one, but tend to have smaller litters than their younger kin. That could be enough the push the overall average down by the small amount observed. But the end result would be the same, and might even help explain why the North American deer mouse is losing ground in some places to the otherwise less adaptable white-footed deer mouse (P. leucopus).

Which might not matter so much if the latter didn't happen to be the main one that carries Lyme disease.

[Photo from the National Park Service. In the public domain.]