A flat coat is the spiny mouse's calm baseline
A spiny mouse relaxed around other animals wears her coat flat. The stiff, spiny guard hairs along her back lie down against her body when she feels secure, because puffing that coat up is her way of looking bigger to something she fears. Watch for the opposite set: a puffed, bristled silhouette, a hunched back and wide, staring eyes mean the nearby animal is worrying her even if she has not run. A relaxed spiny mouse holds a level back, her large upright ears in their natural position, and her whiskers moving gently as she sniffs at her own unhurried pace.
She keeps doing ordinary things in company
Continuing normal behaviour is the strongest sign a spiny mouse is relaxed around another animal. She comes out of her hide while the other pet is in the room, takes seeds from her dish, grooms her face and flanks, and explores her enclosure edges without constantly checking over her shoulder. A spiny mouse who chooses to rest in an open corner, or who sleeps in a nest spot near the side closest to the other animal's space, is showing she does not expect trouble from that direction. Brief pauses to look up are ordinary vigilance; resuming her meal within seconds is what separates a pause from alarm. Group behaviour adds another layer, because spiny mice are social with their own kind: a female who grooms a cagemate, shares a food pile without chasing, and piles into a communal sleeping heap while another species is visible in the room is showing deep security. Hoarding every seed into a defended corner, by contrast, or dragging bedding to block her hide entrance during visits, tells you she is managing a threat, not enjoying company.
Tail, movement and voice give the finer detail
In a relaxed spiny mouse, the tail is carried in a loose, natural curve behind her as she walks, not tucked tight under her body or whipped side to side. Her movement stays fluid: she trots, climbs and turns smoothly instead of darting in short bursts from cover to cover. Sound matters too. A comfortable spiny mouse is mostly quiet around other animals, with only soft contact noises inside her group. Repeated sharp squeaks, chattering aimed at the other animal, or lunging at the bars when it passes are stress signals, not friendliness, however bold they may look.
Build the picture over several meetings
Read a spiny mouse's relaxation across several sessions, not one quiet minute. An animal can sit still because she is calm or because she is frozen with fear, and the difference shows in what happens next: the relaxed spiny mouse blinks, shifts her weight, yawns or starts grooming, while the frozen one stays locked and then explodes into flight. Keep other animals at a barrier at first, watch whether the puffing, hiding and squeaking fade session by session, and separate them permanently if those signs intensify or she stops eating. Recording each session's signals in one shared place, KinsLe Family Sharing lets every family member check the same spiny mouse notes before the next introduction step so nobody advances things faster than she is ready for.