Research Conversation

# How Do Mice Behave in Space? A Comprehensive Review, Key Insights, Open Questions, and Tailored Recommendations *(last literature search May 2023)* --- ## 1. Why study mouse behavior in orbit? Mice are the most widely used mammalian model in biomedicine. Their short life-span, well-understood genetics, and small mass make them ideal passengers on the International Space Station (ISS). Behavioural read-outs are not merely “cute trivia”: they are early, sensitive indicators of changes in vestibular, muscular, skeletal, immune, circadian, and cognitive systems that also matter to astronauts. --- ## 2. What have we actually observed? ### 2.1 Locomotion and “race-tracking” • Within the first 48–72 h after launch, most mice show disorganized, floating movements and frequent “pushing off” from habitat walls. • By Day 4–5 they adopt a highly stereotyped continuous running pattern along the habitat walls—nick-named “race-tracking” (Ronca et al., 2019 Sci Rep 9:4717). • Younger animals (16 weeks) developed coordinated group “wheel-like” circling, whereas 32-week-old mice did not, underscoring an age effect (NASA-CR-20220018919, ref 527). ### 2.2 Circadian rhythm and activity cycles • Overall activity is lower for the first 2–3 days post-launch, then rebounds to pre-flight levels (Shiba et al., 2017 MHU-1 mission). • Core clock gene expression in liver and mammary tissue is shifted under altered gravity (Casey, reviewed in Wade 2005). • 2 g hyper-gravity (centrifuge-kept ISS mice) re-entrain their rhythm faster (6–13 days) than rats at the same g-level (Murakami 2000; Fuller 2002). ### 2.3 Feeding, drinking and grooming • Food and water intake normalize after ~3 days; mice anchor themselves with tails or forelimbs while licking the water lixit. • Grooming increases during the initial adaptation phase—interpreted as a stress surrogate—but returns to baseline once race-tracking emerges. ### 2.4 Social behaviour • Group-housed females (Rodent Research-1) maintained stable social hierarchies; no excessive aggression recorded. • Coordinated group circling (only in younger cohorts) suggests that microgravity can induce novel collective behaviors not seen on Earth (Ronca 2019). • Isolation, when required for some experiments, increases stress markers (TP-20205007191) and should be minimized. ### 2.5 Sensorimotor / vestibular function • Space-reared pups display delayed righting reflexes and poorer balance upon return (Neurolab data, Krasnov 1991; Abe 2008). • Otoconia (ear stone) size is increased in µg and decreased in hyper-g, pointing to gravity-dependent inner-ear plasticity. ### 2.6 Cognitive and affective domains Direct in-flight cognitive testing is scarce, but hind-limb unloading and radiation analogues on Earth demonstrate: • Minor deficits in novel-object recognition and spatial memory after 30 days unloading (Neurobiol Learn Mem 78:199). • Combined ionizing radiation + unloading synergistically disrupts metabolic pathways related to the brain (Raber et al., 2021 Front Physiol 12:746509). --- ## 3. Physiological links to behaviour Behaviour is the surface read-out of deeper system changes: System | Behavioural correlate | Key flight finding ---|---|--- Bone & muscle loss | Reduced spontaneous activity early on | µg mice lose ≤2 %/wk tibial BMD, which partly recovers with voluntary exercise in habitat (Fu et al., 2021 NPJ Microgravity 7:19) Cardio-vascular | Shorter exploratory bouts | Fluid shift → elevated intracranial pressure may affect motivation Immune | Sickness behaviour (lethargy) | Shift toward Th2 cytokine bias in µg mice (RR-1) Microbiome | Anxiety-like behaviour | Spaceflight alters Firmicutes/Bacteroidetes ratio; causality to behaviour under investigation --- ## 4. Factors that modulate mouse behaviour in orbit 1. Age (younger adapt faster, show group circling). 2. Sex (most data from females; limited male datasets suggest more aggression if food restricted). 3. Strain (C57BL/6 standard; B6D2F1 show higher baseline activity; direct comparisons rare). 4. Housing density (3–5 animals per 23 × 14 × 10 cm cage works well). 5. Mission duration (acute disorientation < 72 h, then stable). 6. Gravity level (µg, 0.3 g, 1 g onboard centrifuge). 7. Radiation environment (ISS ~80 mSv/6 mo; little behavioural effect alone, but interacts with unloading). --- ## 5. Remaining uncertainties • Cognition in-flight: Lack of miniaturized operant chambers leaves a data gap in learning/memory during exposure. • Long-term (> 180 d) behaviour across a full murine life-span is unknown. • Sex differences and reproductive behaviours in µg remain understudied. • Interaction of space radiation spectra with behaviour not yet mapped (most studies use simplified ground beams). • Translation to human sensorimotor adaptation needs better quantitative scaling laws. --- ## 6. Synthesis & conclusions 1. Mice undergo a predictable two-phase behavioural adaptation: initial disorientation/stress, followed by efficient, often novel locomotor strategies tailored to weightlessness. 2. Microgravity perturbs circadian regulation but the system can re-entrain; hyper-g recovery is faster in mice than rats. 3. Social cohesion is largely preserved; in fact, some cooperative behaviours emerge uniquely in µg. 4. Behavioural read-outs align with—and sometimes precede—physiological changes, validating mouse behaviour as an early biomarker for astronaut health risks. 5. Age, housing, and gravity level must be rigorously controlled to compare across missions. --- ## 7. Recommendations (audience-specific) ### 7.1 Scientists • Develop shoebox-sized automated operant boxes (touch-screens, nose-pokes) that satisfy ISS power/crew constraints. • Adopt standardized behavioural “core measures” (locomotor counts, social interaction index, sucrose preference) to enable meta-analysis. • Pair behavioural endpoints with omics in the same individuals to link genes → circuits → behaviour. ### 7.2 NASA program managers • Maintain dual-gravity habitats (µg and 1 g centrifuge) on every Rodent Research mission to create internal controls. • Fund an open data portal for raw video and telemetry to crowd-source behavioural annotation via AI. • Incentivize cross-consortia age- and sex-balanced study designs to close key gaps before Artemis-era missions. ### 7.3 Politicians & policy-makers • Investing <$10 M/yr in rodent behavioural payloads de-risks multibillion-dollar human exploration by revealing neuro-vestibular and mental-health hazards early. • International partnerships (JAXA MHU, ESA ICE-Cub) can share cost and enlarge sample sizes—support policies that ease biological sample return across borders. ### 7.4 Venture capital & future payers • Space-validated drug leads (e.g., sclerostin antibody for bone loss) originated from mouse data; similar pipelines for neuro-protectants and circadian modulators are ripe. • Opportunities exist in miniaturized, radiation-hardened behavioural hardware and AI video-analytics—areas with dual-use terrestrial markets (e.g., smart-farm animal monitoring). ### 7.5 General public • Mice are pioneers: by watching how they run in circles or lose bone, we safeguard the health of future astronauts—and eventually tourists—without putting humans at first risk. ### 7.6 Kids • Imagine floating in your bedroom: at first you’d bump into walls, but soon you’d invent a new game of “orbit tag.” That’s exactly what mice do in space—science in action! Keep asking “why” and you might design the next mouse hotel in orbit. ### 7.7 Potential healthcare payers / insurers • Early-warning biomarkers discovered in mice (e.g., shifts in activity patterns predicting immune suppression) can translate to wearable analytics for astronauts and tele-medicine patients, reducing downstream treatment costs. Supporting this research is a preventive-health investment. --- ## 8. Key citations 1. Ronca AE et al. “Behavior of mice aboard the International Space Station.” Sci Rep 2019; 9:4717. 2. Shiba D et al. “Mice in microgravity: Mouse Habitat Unit-1 mission.” NPJ Microgravity 2017. 3. Wade CE. “Rodent responses to gravity conditions: A review.” NASA TP-20180002559, 2018. 4. Fu J et al. “Bone health in space-faring rodents and primates: meta-analysis.” NPJ Microgravity 2021; 7:19. 5. NASA Contractor Report 20220018919. “Evaluation of models used to assess effects and countermeasures.” 2023. 6. TP-20205007191. “Summary report for the technical interchange on behavioural core measures.” 2020. --- *Prepared for Space Biology stakeholders, June 2024*