Unraveling Biotypes Related to Thermotolerance During Lactation in Maternal-Line Pigs Through Principal Component and...

Heat stress imposes significant challenges to welfare, health, and productivity of lactating sows due to their limited thermoregulatory capacity and increased metabolic heat load. Genetic selection for improved heat tolerance is a promising yet complex approach, given the large number of biological processes involved in heat stress response. Therefore, the main objective of this study was to unravel biotypes related to thermotolerance in lactating sows using a multivariate approach. A total of 37 traits, including physiological, behavioural, and anatomical measures, as well as climatic resilience indicators derived from automatically recorded vaginal temperature data, were included in the study. Phenotypic data were collected between June and July 2021 in 1645 multiparous Landrace × Large White sows housed under commercial conditions in North Carolina (United States). The additive genetic correlation matrix among these traits was used for performing multivariate analyses (principal component analysis and factor analysis). Heritability estimates ranged from 0.04 for ear skin temperature (TES) to 0.39 for ear area (EA), whereas repeatability estimates ranged from 0.10 for TES to 0.58 for vaginal temperature measured at 12:00 h (TV12h). Genetic correlations were high and positive among surface temperature traits (0.76-0.79) and among vaginal temperature measures and their derived climatic resilience indicators (0.62-0.99), whereas correlations of these temperature-related traits with morphological and behavioural traits were generally low to moderate. Four principal components and common factors were extracted, explaining 71.20% of the total variance of the data. However, a factor analysis using an orthogonal Varimax rotation was superior in isolating distinct and conceptually clear biotypes. The first factor, which explained 38.80% of the total variance, was named "Core body temperature" and was defined primarily by multiple vaginal temperature measures and climatic resilience indicators. The second factor (12.61% of the total variance) was labelled "Heat dissipation measures", which grouped skin temperatures, panting scores, and respiration rate traits. The third factor (11.68% of the total variance) was interpreted as "Vaginal temperature dynamics" and showed both positive and negative loadings, contrasting vaginal temperature variability with its autocorrelation and skewness. Finally, the fourth factor (8.11% of the total variance), referred to as "Stress reactivity and morphology", was defined by a contrast between traits with positive loadings (responsiveness scores, vocalization, and hair density) and traits with negative loadings (body size and body condition). By successfully reducing 37 traits into four interpretable latent profiles, these factors could serve as breeding objectives, steering towards more holistic selection strategies to better understand how pigs adapt to increasingly challenging climatic conditions.