Developmental instability, of which fluctuating asymmetry is the most commonly used and recommended measure, has recently been claimed to be an objective, integrated and retrospective indicator of animal welfare. The theoretical and empirical grounds for these claims are reviewed. In theory, carefully selected composite indices of fluctuating asymmetry are valid indicators of animal welfare in the sense that they reflect the ability of the developmental processes of an animal, with a given genetic constitution, to cope with environmental stressors. Relevant scientific experiments are scant and are mainly restricted to poultry, but they are on the increase and they largely support the application of developmental instability for assessing animal welfare. A scheme for monitoring farm animal welfare based purely on measures of developmental instability would have important advantages, but cannot be recommended yet. It cannot be ruled out that certain factors are clearly relevant to the welfare status of an animal but do not notably/proportionally affect its morphogenesis. Moreover, such a monitoring scheme would not be appropriate for applications with an emphasis on problem analysis/management.

The aim of this study was to analyse the relationship between childhood socioeconomic conditions and body asymmetry in young Polish women. The study measured fluctuating asymmetry (FA), which refers to small random deviations from perfect symmetry in bilaterally paired body structures. Data were obtained from 620 female students aged from 19 to 25 years recruited from Jagiellonian University in Cracow, Poland. The research was carried out in the period from January 2016 to May 2017. A composite fluctuating asymmetry (cFA) of the women was calculated using five bilateral body traits. The lengths and widths of the women’s ears, lengths of their 2nd and 4th digits and wrist widths of the right and left sides of the body were measured twice using standard methodology. The following data were collected in a questionnaire: degree of urbanization of the woman’s place of residence during childhood, number of older siblings, parental education and woman’s dominant hand. The results showed a tendency for FA to fall with an increase in parental education, and to rise with an increase in number of older siblings. The level of FA was significantly lower in women from rural areas than in those from cities. The results of the study show that FA in early adulthood is significantly associated with socioeconomic status during childhood, and confirm that the level of FA in adulthood may be a good indicator of stress factors in the early stages of development.

Hypoxia strongly affects embryonic development during the pre-hatch period. This study was conducted to investigate the effects of oxygen supplementation (O) and a 38.5°C high temperature (HT) at high altitude (HA, 1720 m) on morphological traits during a pre-hatch period and on relative fluctuating asymmetry (relative FA) and allometric growth during an early post-hatch period in broilers. A total of 720 eggs were obtained from a 45-week-old Ross 308 broiler breeder flock raised at sea level (2 m). The eggs were divided into six incubation condition (IC) groups and were incubated at HA. O groups were exposed to 23.5% O2 for 1 h daily from either days 0 to 11 (O0–11), days 12 to 21 (O12–21) or days 18 to 21 (O18–21) of incubation. HT groups were exposed to 38.5°C daily from either days 12 to 21 (HT12–21) or days 18 to 21 (HT18–21) of incubation. A control was maintained at 37.8°C and 21% O2. The hatched chicks were raised for 6 days at HA. Embryo/chick and beak lengths and head diameter were measured during pre- and post-hatch periods. The face, middle toe and shank lengths were measured for each chick. The relative asymmetry (RA), mean RA (MRA) and allometric growth of the lengths were computed and the existence of FA was demonstrated. The IC significantly affected the embryo length, with embryos of the O0–11 group shorter than embryos of the other O groups. Chicks were longer in the O and HT groups than those in the control, except for the O0–11. We found significant interactions between the IC and each development period for beak length. During the post-hatch period, the head diameter of the O0-11 was significantly smaller than that of the other groups, but not in O12–21. The interactions among IC, age and sex were significant for the RA of the face and middle toe lengths and for MRA. All the examined bilateral traits were evaluated as allometric growth. The FA for bilateral traits was determined in both sexes. The right (R) – left (L) and IR-Ll were the lowest in females for face length and in males for shank length from the O18–21 and in males for middle toe length from the O0–11 and HT18–21 groups. Therefore, the effects of factors such as HT and O2 could mitigate the adverse effects of HA-induced hypoxia on optimal developmental stability of bilateral traits of broiler.

There is growing evidence from both experimental and non-experimental studies that fluctuating asymmetry does not consistently index stress or fitness. The widely held – yet poorly substantiated – belief that fluctuating asymmetry can act as a universal measure of developmental stability and predictor of stress-mediated changes in fitness, therefore staggers. Yet attempts to understand why the reported relationships between fluctuating asymmetry, stress and fitness are so heterogeneous – i.e. whether the associations are truly weak or non-existent or whether they become confounded during different stages of the analytical pathways – remain surprisingly scarce. Hence, we attempt to disentangle these causes, by reviewing the various statistical and conceptual factors that are suspected to confound potential relationships between fluctuating asymmetry, stress and fitness. Two main categories of factors are discerned: those associated with the estimation of developmental stability through fluctuating asymmetry, and those associated with the effects of genotype and environment on developmental stability. Next, we describe a series of statistical tools that have recently been developed to help reduce this noise. We argue that the current lack of a theoretical framework that predicts if and when relationships with developmental stability can be expected, urges for further theoretical and empirical research, such as on the genetic architecture of developmental stability in stressed populations. If the underlying developmental mechanisms are better understood, statistical patterns of asymmetry variation may become a biologically meaningful tool.