Two weeks of single housing weakens bones in male mice, while females activate their own protective response

Two weeks of single housing weakens bones in male mice, while females activate their own protective response

Researchers at the MaineHealth Research Institute published a study on 26 September 2026: adult mice were housed either alone or in groups for two to eight weeks. Males rapidly lost bone mass and developed more fragile skeletons; females did not. The cause lies in the bone tissue of males itself: it stops responding properly to estrogen signaling, even though circulating hormone levels remain unchanged.

After two weeks without cagemates, males begin losing trabecular bone, the spongy tissue inside vertebrae and at the ends of long bones that turns over faster than the dense outer cortex and is the first to register systemic changes. The main shift occurs within the first weeks and then plateaus: by four weeks the numbers barely move further. Averaged across all time points, trabecular bone volume fraction dropped by 20% in the femur and 12% in the vertebrae, while cortical thickness fell by 8%. Three-point bending tests showed that male bones absorbed roughly a quarter less energy before fracture, breaking under lower loads. In females, neither bone structure nor strength changed at any time point.

This is the third study from Rebecca Mountain's laboratory at MaineHealth. In 2023 she identified the effect of one-month isolation in males, and in 2025 she tested and ruled out the cold-stress hypothesis: thermoneutral housing did not rescue bone. One hypothesis remained: females suffer the same damage, only more slowly. The new study compared three durations side by side and refuted it: even after two months of isolation, female bones were indistinguishable from those of group-housed mice.

Female bone does respond to isolation: within two weeks, circulating levels of both bone-turnover markers (formation and resorption) rose, and the gene-expression balance shifted toward skeletal protection.

"We hypothesize that isolation triggers a similar initial response in both sexes, but females are physiologically better equipped with buffering defenses against its consequences and are able to engage additional compensatory mechanisms to protect the skeleton"

the authors write.

In males, the enzyme aromatase converts part of their testosterone into estrogen directly within bone, helping to maintain its strength. The authors measured circulating sex hormones: testosterone in males did not differ between groups, and in females it fell below the assay's detection limit. In bone tissue of isolated males, however, expression of two estrogen receptor genes declined: ERα, which promotes osteoblasts (bone-building cells) and restrains osteoclasts (bone-resorbing cells), and GPER, which mediates a faster estrogen response. Suppression of ERα expression persisted even at the eight-week mark.

The authors flag a practical concern: single housing is routine in preclinical studies and has been treated as a neutral baseline. For males, it turns out to be a source of skeletal damage that can silently confound any experiment in which singly housed animals serve as controls. The same pattern has played out for years in the ITP, the national program that tests candidate geroprotectors in mice: seven of its drugs worked only in males.

In humans, osteoporosis in men is chronically underdiagnosed because it is perceived as a disease of women. The Canadian Longitudinal Study on Aging, which followed more than 11,000 people over 65 for three years, found that each one-point increase in the social isolation index raised fracture probability by 13%, although no measurable link to bone mineral density was detected. The mechanism identified in mice offers a possible explanation: in humans, mechanical bone quality may deteriorate in ways that standard density scans do not capture.