A disparity between locomotor economy and territory-holding ability in male house mice

Female behavior drives the formation of distinct social structures in C57BL/6J versus wild-derived outbred mice in field enclosures

BACKGROUND

Social behavior and social organization have major influences on individual health and fitness. Yet, biomedical research focuses on studying a few genotypes under impoverished social conditions. Understanding how lab conditions have modified social organizations of model organisms, such as lab mice, relative to natural populations is a missing link between socioecology and biomedical science.

RESULTS

Using a common garden design, we describe the formation of social structure in the well-studied laboratory mouse strain, C57BL/6J, in replicated mixed-sex populations over 10-day trials compared to control trials with wild-derived outbred house mice in outdoor field enclosures. We focus on three key features of mouse social systems: (i) territory establishment in males, (ii) female social relationships, and (iii) the social networks formed by the populations. Male territorial behaviors were similar but muted in C57 compared to wild-derived mice. Female C57 sharply differed from wild-derived females, showing little social bias toward cage mates and exploring substantially more of the enclosures compared to all other groups. Female behavior consistently generated denser social networks in C57 than in wild-derived mice.

CONCLUSIONS

C57 and wild-derived mice individually vary in their social and spatial behaviors which scale to shape overall social organization. The repeatable societies formed under field conditions highlights opportunities to experimentally study the interplay between society and individual biology using model organisms.

Musculoskeletal mass and shape are correlated with competitive ability in male house mice (Mus musculus)

Intense physical competition between males for mating opportunities is widespread among mammals. In such agonistic encounters, males with combinations of morphological, physiological and behavioral characters that allow them to dominate an opponent have greater fitness. However, the specific physical traits associated with competitive ability are poorly understood. Larger body size is often correlated with fitness in mammals. Interestingly, fitness is maximized at intermediate body masses in male house mice (Mus musculus), a species with a polygynous mating system in which males compete physically for access to reproductive resources. Here, we used competition trials in semi-natural, mixed-sex population enclosures to directly measure competitive ability in male house mice based on control of a preferred nesting site. We tested the hypothesis that the musculoskeletal systems of male mice demonstrating high competitive ability are more specialized for competition by comparing the masses of 10 major muscle groups and eight bones as well as a set of 12 skeletal shape indices associated with anatomical specialization for fighting performance in a set of nine winners and 20 losers. Winning males possessed several traits hypothesized to enhance performance in male-male contests: relatively greater mass in several muscle groups and bones of the forelimb and hindlimb and larger scapular surface area. Unexpectedly, no measurements of the head and neck differed significantly between winners and losers. These results identify musculoskeletal traits associated with competitive ability in male house mice and suggest that our current understanding of mammalian fighting performance is incomplete and more nuanced than previously considered.

Transitions in paternal social status predict patterns of offspring growth and metabolic transcription

One type of parental effect occurs when changes in parental phenotype or environment trigger changes to offspring phenotype. Such nongenetic parental effects can be precisely triggered in response to an environmental cue in time-locked fashion, or in other cases, persist for multiple generations after the cue has been removed, suggesting multiple timescales of action. For parental effects to serve as reliable signals of current environmental conditions, they should be reversible, such that when cues change, offspring phenotypes change in accordance. Social hierarchy is a prevalent feature of the environment, and current parental social status could signal the environment in which offspring will be born. Here, we sought to address parental effects of social status and their timescale of action in mice. We show that territorial competition in seminatural environments affects offspring growth. Although dominant males are not heavier than nondominant or control males, they produce faster growing offspring, particularly sons. The timing, effect-size, and sex-specificity of this association are modulated by maternal social experience. We show that a change in paternal social status is sufficient to modulate offspring weight: from one breeding cycle to the next, status-ascending males produce heavier sons than before, and status-descending males produce lighter sons than before. Current paternal status is also highly predictive of liver transcription in sons, including molecular pathways controlling oxidative phosphorylation and iron metabolism. These results are consistent with a parental effect of social experience, although alternative explanations are considered. In summary, changes in paternal social status are associated with changes in offspring growth and metabolism.

Sexual dimorphism in postcranial skeletal shape suggests male-biased specialization for physical competition in anthropoid primates

Sexual dimorphism often arises as a response to selection on traits that improve a male's ability to physically compete for access to mates. In primates, sexual dimorphism in body mass and canine size is more common in species with intense male-male competition. However, in addition to these traits, other musculoskeletal adaptations may improve male fighting performance. Postcranial traits that increase strength, agility, and maneuverability may also be under selection. To test the hypothesis that males, as compared to females, are more specialized for physical competition in their postcranial anatomy, we compared sex-specific skeletal shape using a set of functional indices predicted to improve fighting performance. Across species, we found significant sexual dimorphism in a subset of these indices, indicating the presence of skeletal shape sexual dimorphism in our sample of anthropoid primates. Mean skeletal shape sexual dimorphism was positively correlated with sexual dimorphism in body size, an indicator of the intensity of male-male competition, even when controlling for both body mass and phylogenetic relatedness. These results suggest that selection on male fighting ability has played a role in the evolution of postcranial sexual dimorphism in primates.

A disparity between locomotor economy and territory-holding ability in male house mice

Both economical locomotion and physical fighting are important performance traits to many species because of their direct influence on components of Darwinian fitness. Locomotion represents a substantial portion of the total daily energy budget of many animals. Fighting performance often determines individual reproductive fitness through the means of resource control, social dominance and access to mates. However, phenotypic traits that improve either locomotor economy or fighting ability may diminish performance in the other. Here, we tested for a predicted disparity between locomotor economy and competitive ability in wild-derived house mice (Mus musculus). We used 8 week social competition trials in semi-natural enclosures to directly measure male competitive ability through territorial control and female occupancy within territories. We also measured oxygen consumption during locomotion for each mouse using running trials in an enclosed treadmill and open-flow respirometry. Our results show that territory-holding males have higher absolute and mass-specific oxygen consumption when running (i.e. reduced locomotor economy) compared with males that do not control territories. This relationship was present both before and after 8 week competition trials in semi-natural enclosures. This disparity between physical competitive ability and economical locomotion may impose viability costs on males in species for which competition over mates is common and may constrain the evolution of behavioral and phenotypic diversity, particularly in natural settings with environmental and resource variability.