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Cooperative burrowing in mice

Citation

Bedford, Nicole et al. (2022), Cooperative burrowing in mice, Dryad, Dataset, https://doi.org/10.5061/dryad.5mkkwh78c

Abstract

Animals often adjust their behavior according to social context, but the capacity for such behavioral flexibility can vary among species. Here, we test for interspecific variation in behavioral flexibility by comparing burrowing behavior across three species of deer mice (genus Peromyscus) with divergent social systems, ranging from promiscuous (P. leucopus and P. maniculatus) to monogamous (P. polionotus). First, we compared the burrows built by individual mice to those built by pairs of mice in all three species. While burrow length did not differ in P. leucopus or P. maniculatus, we found that P. polionotus pairs cooperatively constructed burrows that were nearly twice as long as those built by individuals and that opposite-sex pairs dug longer burrows than same-sex pairs. Second, to directly observe cooperative digging behavior in P. polionotus, we designed a burrowing assay in which we could video-record active digging in narrow, transparent enclosures. Using this novel assay, we found, unexpectedly, that neither males nor females spent more time digging with an opposite-sex partner. Rather, we demonstrate that opposite-sex pairs are more socially cohesive and thus more efficient digging partners than same-sex pairs. Together, our study demonstrates how social context can modulate innate behavior and offers insight into how differences in behavioral flexibility may evolve among closely related species.

Methods

Large enclosures. We measured burrow architecture as described previously (Weber & Hoekstra 2009; Weber et al. 2013; Metz et al. 2017). Briefly, at the start of the dark cycle, we introduced individuals or pairs of mice into large PVC boxes (1.2 x 1.5 x 1.1m) filled with approximately 700kg hydrated, hard-packed premium play sand (Quickrete, Atlanta, GA). In the enclosures, mice were provided food and water ad libitum along with a cotton nestlet. We removed mice from the enclosures after one (pregnancy trials) or two (standard trials) overnight periods. Then, we made casts of the resultant burrows using polyurethane filling foam (Hilti, Schaan, Liechtenstein). Next, a researcher blind to trial identity hand-measured the burrow casts. For each trial, the number of burrows and total length of each burrow was recorded. We also categorized burrow shape as either “simple” or “complex” as described previously (Weber & Hoekstra 2009): P. leucopusand P. maniculatus burrows are considered “simple” with a short entrance tunnel and terminal nest chamber, whereas P. polionotus burrows are considered “complex” with a long entrance tunnel, nest chamber, and escape tunnel. After each trial, we removed all food, feces, nesting material, and disturbed substrate from the enclosure, and then rinsed the enclosure walls with water and turned over the sand to minimize residual odors. 

Narrow, transparent enclosures. We also assayed burrowing behavior of mice directly using an acrylic chamber (5cm x 90 cm x 60 cm) with a transparent Plexiglass face (see Fig. 3A). Using a pre-cut mold, we sculpted hydrated sand into two symmetrical 45° hills and excavated an 8cm tunnel from one randomly selected hill to encourage burrowing in a consistent location. We outfitted the apparatus with an infrared illuminator frame that enabled video recording in the dark. We then introduced individuals or pairs of mice into an enclosure at the start of the dark cycle and removed animals the next morning, recording 8 hours of continuous video during the dark cycle. Mice were provided food and water ad libitum. Photographs of the resultant burrows were taken at the end of the trial. Details of photo and video analyses are provided below. Following each trial, we removed all food, feces, and disturbed substrate and rinsed the enclosure to minimize residual odors.

Funding

Howard Hughes Medical Institute

Natural Sciences and Engineering Research Council of Canada