Stress is a rather abstract and recently introduced concept. It has been discussed a lot over recent decades, and during this period, our understanding of it has been affected by research results.

Stress was initially defined, in a way that covered any reaction to demands on the animal system, as an effective response to environmental challenges that is essential for survival .

Nowadays, however, it is normally assumed to be inherently negative: often the term “stress” describes the animal’s state when it is challenged beyond its behavioural and physiological capacity to adapt to its environment. Stress demands a redistribution of resources and a rise in energy expenditure.
When an animal is stressed the catecholamines transmit the signal via ACTH (which is released in the hypothalamus) to the adrenal cortex resulting in the secretion of cortisol and corticosterone (together named glucocorticoids). Illustration: Lena Holm