Princeton University - Council of Science & Technology

POPE PRIZE

Essay 1
The Smartest and the Fastest: Investigating the limits of nature
Charles Graham Wells

Essay 2
Oldfield Mice, Theoretical Biology and the Search for True Love
Sarah Helen Labun

Essay 3
John Horton Conway, John von Neumann Distinguished Professor of Mathematics
Lillian Beatrix Pierce

Oldfield Mice, Theoretical Biology and the Search for True Love

A special thanks to Tatjana Good for her help with researching this article.

[This essay first appeared in the Valentine's Day edition of the 'PEA News' (2002)]

An investigation of love in its natural state, to be truly thorough, would start where reproduction shed the tedium of asexuality eons ago in a eukaryote somewhere. After this first step, one could follow the process through the plankton and ferns and millennia, as DNA transfer between organisms prevailed (scientists aren't sure how) alongside the highly efficient asexual method. Then, around 500 million years ago, the Cambrian period began and things really took off (exploded, if you will). As the species of animals and plants diverged, so did the techniques for acquiring a mate. Some species developed the scatter approach- an orgy of gene passing. Others got elaborate. Caribou wandered around with racks of antlers almost too big to lift that displayed their physical vigor, both in the ability to grow such cumbersome appendages and the ability to keep their heads upright beneath them. Male fireflies flew in elaborate courtship dances. Females responded. Several thousand years of evolution passed and we ended up with a procreative world of such complexity that even an orchid, a plant, has developed reproductive systems so exact that it can deposit pollen on the correct knee cap of the correct bee to reach the stigma of the correct orchid partner somewhere in the swelter of the tropics.

Natural selection across a population and within a certain environment led each species to its respective mating systems, and conscious thought played no role in reproduction, until humans started to ask questions. Ever since Darwin, we've wanted to know how sexual selection works, what makes for the best pairing, and, most importantly, we've wanted to know who. This is not a simple case of Adam, Eve, and an indiscrete apple-eating incident; this is a matter of survival. The choice of a mate has important implications for the future of any one genetic line and the fitness of the resulting offspring that represent this future. At the same time, if any animal is going to be choosy, turning down a potential partner and taking the risk of producing no offspring, he or she needs to have good reasons for this choosiness. Many creatures, including humans, have found advantages in careful selection. At some point in history, probably in pre-history, selective humans added a new dimension to this quest for reproductive advantage. We became concerned with more than simple genetic continuance. Reproduction took on a romantic aspect and the promise of healthy offspring was less satisfying without the promise of love to go along with it. Mating got complicated. Nonetheless, our basic questions remain; we're still trying to discover how partners are chosen, on what criteria, and who these partners will be.

Oldfield mice, Peromyscus polionotus, seem an odd place to seek out the meaning of love, but if you're a biologist, and if we can define love, loosely, as the formation of long-term pair bonds in a monogamous species where both parents help to rear offspring, then these mice are perfect. Oldfield mice distinguish themselves in the rodent world through monogamy. Of the forty-six subspecies of this mouse, two subspecies (P. californicus and P. polionotus) remain with the same mate throughout their lifespan. The reasons for the selection of this particular mate remain unclear. Former Chicago graduate student Karen Koeninger Ryan and Professor Jeanne Altmann, now at Princeton University, recently conducted a series of experiments on male Oldfield mice. In 2001, they published a study revealing that males did show preference for particular partners and that males paired with a female they chose in a partner preference test sired more offspring than males paired with a female they rejected. Furthermore, the study suggests that the difference in success depends on the particular pairing, not on any universally superior quality in the female. In other words, a larger or thicker furred or bigger eyed female won't produce better litters with every male, it's the individual that matters. These results, along with a growing number of publications on male choice, added new dimensions to the literature on mate selection. Traditionally, selection experiments have focused on female selection for males with pre-identified visual cues, like a peacock's fantail or the coloration on guppies. The Oldfield mice showed that males also had partner preferences and in this case they based these on some combination of traits uniquely complementary to their own. These discoveries led to a new question: what happens when both mice (male and female) are allowed to express preference? Will they select the one mate best suited for them? Will the monogamous partnership with a pair that chose each other lead to greater reproductive success than a randomly assigned one? Tatjana Good, a fourth year graduate student in Princeton's Department of Ecology and Evolutionary Biology, is studying these mutual partner questions.

The Oldfield mouse study is a form of biological alchemy; an experimenter puts together the right combination of feed, monitoring and specially designed cages, and in the end, hopefully, mutual selection (let's call it 'love' for stylistic purposes) will emerge. Laboratory work transforms the fields to sawdust-lined terrariums. A redlight replaces the moon that illuminates these mice's nocturnal activities. Data processing requires a more precise term for "living happily ever after", and so it becomes "reproductive success", measured by days before a first litter, pup survival until weaning, pup weight at weaning, and number of litters produced in a lifetime. The experiment's greatest innovation is to relocate the search for true love into a 'mutual mate choice apparatus'. This invention allows four mice, two males and two females, to observe the presence of the opposite gender nearby and to express preference for one mate or the other. Plexiglass dividers separate the mice. Flirtation occurs when either mouse spends time in the hypothetical preference zones, a small space on either side of the plexiglass. After observing interactions in the mutual mate choice apparatus, Tatjana calculates whether she has seen either a mutual preference, a one sided preference, or disinterest. She then arranges the pairings and videotapes a six hour-long honeymoon of one of the new mouse couples placed in a terrarium together before they begin a lifetime of cohabitation in the mouseroom next door.

The Oldfield mouse experiment's initial results can satisfy the romantic in all of us. Not only do mutually selected pairs appear to have greater reproductive success, but they also show more affectionate behavior, spending over twice as much time as other pairs in displays of huddling together, grooming each other, and playful wrestling. In the mouseroom, miniscule gray babies lay with both their mother and father rolled into one big familial ball of mouse fur in the nested bedding of each house. It is tempting to stop here. Mice are cute, and mice in love are even cuter. Science continues, though, and there are other, less lovable experiments to perform. In the next stage of the mouse mating experiment, Tatjana will use a retro-orbital bleeding technique to obtain blood samples from her subjects at different times to freeze along with fecal samples at negative 80°C until they can be analyzed using 125I-radioimmunoassay kits for concentrations of testosterone, estrogen, and corticosterone. If her hypothesis holds, correlations will appear between hormone levels and mouse behavior. Perhaps being with a particular female changes a male's hormone concentrations so that he wants to groom her more often. Perhaps this change in him influences her behaviors and hormone concentrations. Watching mice cuddle only took us so far. Now it's time for a hormonal analysis of reciprocal interaction and stimulation. Once the 125I-radioimmunoassay work begins, it becomes much more difficult for a lay observer to understand the appeal of the results.

In a room full of cages and equipment and tables of data that deconstruct to the deci-second the "onset and duration of sexual activity, ie lordosis and mounting", the mouse equivalent of true love loses some luster. Cinderella has gone into reverse. The carriage is back to a pumpkin, not only are the footmen mice, but so is Prince Charming, and the fairy godmother's magic was only our own belief in a world with too much dignity to suffer this rude manhandling in the basement of Guyot Hall.

Watching as the pursuit for a scientific understanding of love, if only in the Oldfield mouse sense, leads the intrepid investigator to the analysis of plasma and fecal samples and endless hours of frame-by-frame replays of acts of copulation that last, on average, one second could be unsettling. But it isn't unsettling. Because the pursuit has not ended, nor does it show any signs of slowing. If we can correlate patterns of hormonal change with differences in mating behavior (or lack thereof) we may be getting one step closer to understanding the evolution of monogamy, but this is only one step. This tells us very little about why some mice are monogamous and others, living in almost identical environments, are not. Or about whether pairing disinterested mice will discourage monogamy. Or about the repeatability of mate choice in the face of multiple options. And this is only mice.

Experiments build off of each other, they offer a basis for more questions as much as they offer any answer. Scientific inquiry leads to some insights, but it also leaves the romance of exploring the unknown thankfully undiminished. Sandwiched somewhere in the remarkable evolution of the mutually selected long-term mates may be the biological basis for love, but while we wonder and question and test theories of this basis, in the end, we don't want to find that final answer. Humans don't court each other in anticipation of a moment when they have charted each detail of their mutual attraction and can move on to the next experiment or relationship. We want to learn more. We seek the excitement of continual discovery. On closer examination, the mice do show us something about love; they show us the importance of closer examination. Our capacity for amazement never ceases, and it is in this capacity that science and love and P. polionotus reach a mutual understanding.

Happy Valentine's Day