The vertebrate stress response comprises a suite of behavioural and physiological traits that must be functionally integrated to ensure organisms cope adaptively with acute stressors. Natural selection should favour functional integration, leading to a prediction of genetic integration of these traits. Despite the implications of such genetic integration for our understanding of human and animal health, as well as evolutionary responses to natural and anthropogenic stressors, formal quantitative genetic tests of this prediction are lacking. Here, we demonstrate that acute stress response components in Trinidadian guppies are both heritable and integrated on the major axis of genetic covariation. This integration could either facilitate or constrain evolutionary responses to selection, depending upon the alignment of selection with this axis. Such integration also suggests artificial selection on the genetically correlated behavioural responses to stress could offer a viable non-invasive route to the improvement of health and welfare in captive animal populations.

Kryptolebias marmoratus exposed to 4 ng l(-1) of ethinyl oestradiol (EE2) for 30 days experienced significant changes in endogenous 17β-oestradiol (E2) and 11-ketotestosterone (KT) and qualitative changes in gonad morphology. Both hermaphrodites and males showed a significant decrease in E2, whereas only males exhibited a significant decrease in KT. Exposure to EE2 resulted in a decrease in spermatid and spermatocyte density in males and an increase in the number of early stage oocytes in hermaphrodites.

22The stress response is a product of selection for an integrated suite of behavioural and 23 physiological traits that facilitate coping with acute stressors. As such, genetic variation 24 in the stress response is expected to reflect genetic variation in, and genetic covariation 25 among, its behavioural and physiological components. Such genetic integration among 26 stress response components has yet to be formally demonstrated using multivariate 27 quantitative genetics, despite its profound implications for optimising human and 28 animal health and understanding the responses of wild populations to natural and 29 anthropogenic stressors. Here we use a laboratory population of wild-derived 30 Trinidadian guppies (Poecilia reticulata) to determine levels of genetic variation in 31 behavioural and physiological components of the acute stress response, and to establish 32 whether such variation is integrated into a single major axis of genetic (co)variation. 33 First, using a novel method to characterise behavioural components of the stress 34 response from a widely used Open Field Trial paradigm, we find genetic variation in, 35 and genetic covariation among, behavioural parameters that characterise movement 36 patterns under stress. Second, we find a strong genetic component to variation in both 37 the endocrine response to a confinement stressor and the rate at which this response 38 attenuates following repeated exposures to the stressor. Finally, we show that these 39 behavioural and physiological components of the stress response align on a major axis 40 of genetic (co)variation as predicted, suggesting correlational selection in the past has 41 led to genetic integration. This genetic integration could either facilitate or constrain 42 future responses to selection, depending upon the extent to which the direction of 43 selection aligns with this major axis of genetic covariation among stress response traits.44 This genetic integration also suggests that, while stress-related disease typically arises 45 from physiological stress responses, selection on the genetically correlated behavioural 46 responses could offer a viable non-invasive route to the genetic improvement of health 47 and welfare in captive animal populations.

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