The accumulation of multiple stressors over a short period, with the result that an animal who could have coped with each stressor individually cannot cope with all of them together.

Common in dogs (a stressful walk, then a vet visit, then unfamiliar visitors), in horses (a new arrival, then a worming, then transport, then a new environment), in cats (a household renovation, then a new pet, then a vet visit), and across other companion species. Trigger stacking is one of the most useful frameworks for understanding why an animal who is normally fine suddenly seems to fall apart over what looks like a minor incident.

The mechanism is partly physiological and partly behavioural. Physiologically, each stressor produces some level of stress response, and the responses do not fully resolve before the next stressor arrives. The animal’s stress system is therefore operating from an elevated baseline by the time the third or fourth stressor arrives, and the same stressor that would normally be manageable produces a disproportionate response. Behaviourally, the animal’s capacity to cope is depleted by the earlier challenges, and behaviours that depend on cognitive control (working through fear, complying with cues, tolerating handling) become more difficult.

Trigger stacking explains many of the apparently random behavioural events that occur in domestic and captive animals. The horse who has been fine all week and suddenly explodes at the float on Saturday morning may have stacked stressors across the week. The dog who has been fine with visitors before and suddenly snaps at one may have stacked stressors that day. The cat who has been managing a multi-cat household and suddenly attacks a housemate may have stacked stressors over a longer period.

The framework is a routine consideration in clinical animal behaviour work. Behavioural problems that look like they emerged out of nowhere often, on careful history-taking, turn out to involve trigger stacking that the handlers had not noticed. The clinical intervention is partly about identifying and reducing the stressors involved, and partly about ensuring sufficient recovery time between stressors so that the stress response can fully resolve.

The concept also has implications for prevention. Spacing out potentially stressful events (vet visits, transport, social changes, training challenges) reduces the cumulative load and tends to produce better welfare and behavioural outcomes than clustering them together.