Rodent Societies: An Ecological & Evolutionary Perspective

and use wood shavings and cotton batting to make them-
selves comfortable in an assortment of nest boxes. Within
these simulated mouse cities, the nightly activities of dozens
of mice are observed and recorded under the dim red light
of an artificial evening. Here, mice maneuver throughout
their surroundings, strategically placing and responding to
odors and displays rich in context and meaning. Territorial
competition is exhaustively played out among males, usu-
ally requiring days of escalated contests before territory
holders prevail. But it doesn’t end there. Subordinate males
continue their attempts at a coup d’état by a persistent pro-
gram of insurgency. Territorial males are continually probed
and challenged, and we have demonstrated that the greatest
loss of fitness for some genotypes emerges during attempted
overthrows rather than the initial territorial establishment
(Meagher et al. 2000). Females are free to exercise choice
of mates, often seeking matings from dominant males in
neighboring territories. As all of these complicated social
interactions play out, we obtain hundreds of litters over the
course of several months. But the critical point is that these
are not a random set of pups, but a highly selected set that
are anticipated to be genetically superior to a set whose
parents were not exposed to sexual selection. In turn, the

fitness differentials of the parental generation can be deter-

mined from the analysis of these offspring.

Case 1: Mating Preferences,

Odors, and MHC Diversity

In 1975, during the midst of excitement involving the

recently discovered immunological role of the major histo-

compatibility complex, Yamazaki and his colleagues (Ya-

mazaki et al. 1976; Yamazaki et al. 1978) made the unex-

pected observation that male laboratory mice preferred to

mate with females carrying dissimilar MHC genes (also

known as the H-2 complex in mice). Since the mice studied

were genetically identical except at the MHC region, only

genes residing within this large genetic cluster could be re-

sponsible for the observed mating behavior. As tiny sentries

positioned on the surface of cells, MHC molecules help de-

cipher the antigenic universe, binding bits of proteins (anti-

gens) from self and foreign sources for presentation to the

T-cells of the immune system (fig. 5.2; Zinkernagel and Do-

herty 1974). T-cells recognize this antigen-bound complex,

targeting for destruction any cells bearing foreign anti-

gens bound to the MHC. Without MHC molecules, T-cells

would be incapable of distinguishing self from non-self, ef-

fectively rendering the immune system defenseless against

pathogens, parasites, and autoimmune disease. At a cellu-

lar level, self /non-self recognition helped explain the role

of MHC in the acceptance or rejection of tissue grafts,

Sexual Selection: Using Social Ecology to Determine Fitness Differences 59

Figure 5.1 Interior view of a seminatural population enclosure for mice. The
5m by 10m enclosure is subdivided into eight subsections by 0.6 m-high hard-
ware cloth. Each subsection contains food, water, and nest boxes.

Figure 5.2 Depiction of a killer T cell receptor recognizing an MHC-bound

peptide.