FLUCTUATING ASYMMETRY AS AN INDICATOR OF ENVIRONMENTAL STRESS IN SMALL MAMMALS.
INTRODUCTIONEnvironmental stress can have significant detrimental effects on animal populations (Lazic et al. 2013, 2015). Several authors have proposed that obtaining a sensitive indicator of stress is crucial for conservation biologists, since it can be used to detect signs of population disturbance before components of fitness have been affected and irreversible demographic damages have occurred (Leary & Allendorf, 1989; Teixeira et al., 2006; Delgado-Acevedo & Restrepo, 2008; Beasley et al., 2013; Lazic et al., 2013). Traditional biomarkers (molecular and cellular exams, heat shock proteins, hemoglobin adducts, etc.) may be reliable, but they are expensive and may not be applicable across species (Helle et al. 2011; Lazic et al. 2013). The developmental stability of an organism is reflected in its ability to produce an 'ideal' form under a particular set of conditions. The lower its stability, the greater the likelihood it will depart from this 'ideal' form. Ideal forms are rarely known a priori, but bilateral structures in bilaterally symmetrical organisms offer a precise expectation of symmetry against which departures may be compared. Thus, the study of bilateral traits provides a very convenient method for assessing deviations from the norm and studying the factors that may influence such deviations (Palmer & Strobeck 1986; Palmer 1994). The developmental precision that produces bilaterally symmetric structures may be negatively affected by a wide range of environmental and/or genetic stressors (Zakharov 1992).
Subtle departures from symmetry are most commonly described by frequency distributions of right-left sides of a trait (Palmer 1994). Such frequency distributions usually exhibit one of the following three patterns: directional asymmetry, antisymmetry and fluctuating asymmetry. Directional asymmetry is characterized by a normal distribution that is not centered around zero but is significantly biased towards larger traits either on the right or the left side (Fig. 1A). Examples include lateral placement of organs such as the heart and liver in humans and muscle-size asymmetries in birds (Markow 1995). Antisymmetry is distinguished by a platykurtic (broad peaked) or bimodal distribution of right-left differences around a zero mean (Palmer & Strobeck 2003) (Fig. 1B). A classic example is the claw size in male fiddler crabs. Fluctuating asymmetry is defined as small and random deviations from perfect bilaterally symmetrical traits (Ludwig 1932; Palmer & Strobeck 1986) (Fig. 1C). This asymmetry has normal distribution with zero mean. Of these three asymmetries, fluctuating asymmetry is considered as the only form of asymmetry that can serve as a useful indicator of environmental/genetic stress (Palmer & Strobeck 1986; Leary & Allendorf 1989; Leung & Forbes 1997; Polak & Taylor 2007).
The study of fluctuating asymmetry per se began with the work of a small group of researchers, including Ludwig (1932), Thoday (1953, 1956, 1958), Van Valen (1962), and Soule (1966, 1967). Early perceptions of the importance of fluctuating asymmetry were summarized by Jackson (1973), who pointed out that the level of fluctuating asymmetry can be considered as a measure of buffering capacity in development, since any non-consistent differences between paired structures could be developmental accidents. The attractiveness of fluctuating asymmetry as a potential biomarker stems from its broad application across biological systems and stressors. An additional advantage is the relative ease in taking trait measurements compared to other biomarkers that require more costly equipment or reagents (Leung et al. 2003). There are two general approaches to study fluctuating asymmetry: geometric morphometrics that provides information about traits morphology using shape and size, and linear measurements that include metrical measurements (lengths) or meristic traits, both on the left and right sides of organisms (Van Valen 1962; Palmer & Strobeck 1986). The geometric morphometrics methodology has been more recently developed and its fundamental advances over traditional approaches (linear measurements) are due to powerful statistical methods designed for the analysis of shape data rather than the use of standard multivariate methods (Rohlf & Corti 2000).
Small mammals have been widely used as model species in ecological studies to infer environmental disturbances because of their wide range of characteristics (Wolff & Sherman 2007). These organisms inhabit all continents except Antarctica and they occur in terrestrial, subterranean, arboreal, and aquatic habitats. They are crucial in their contribution to well-structured food webs (Salamolard et al. 2000; Michel et al. 2006; Baraibar et al. 2009), the consumption and dispersal of plant products (Carey et al. 1999) and mycorrhizal fungi (Maser et al. 1978) and the consumption and control of invertebrates (Elkinton et al. 1996). Most of them are very prolific, have a short life cycle and are relatively easy to capture (Steinmann & Priotto 2011; Korpimaki & Norrdahl 2013). Besides, they constitute a diverse group, with different degrees of habitat specialization, social and mating systems. All these characteristics make this group of mammals a very convenient model for ecological studies.
Currently, there is a challenge to identify vulnerable populations before irreversible demographic/genetic damage takes place. Obtaining of a reliable, general and easy-to-use biomarker of health and wellbeing of individuals that can be applied in ecological studies is therefore important. The aim of this study was to show the relevance of fluctuating asymmetry as a tool to measure environmental stress in ecological studies, with emphasis on small mammals. We summarized four decades of studies where this index was used to assess the effects of environmental disturbances on small mammals.
DATA AND METHODS
Data were compiled with Google Scholar (Mountain View, CA) using two keywords: "fluctuating asymmetry" and "small mammals". We also searched in the reference lists of selected articles additional studies that met our inclusion criteria. We applied other selection criteria to include the papers in this review. Due to the fact that the small mammals are not classified as a taxonomic group we included in this group any species of mammals with adult weights up to 1 kg. We selected only articles in English and with ecological objectives published from 1980 to 2017.
RESULTS AND DISCUSSION
We selected 27 articles (Table 1) that met our selection criteria. This small number of articles demonstrates that fluctuating asymmetry is not widely used in ecological studies of small mammal. The highest number of articles in this topic was registered between 2000-2004 (8 articles) and 2015-2017 (5 articles). We found studies carried out in several parts of the world, but most were developed in Europe (20 articles), six in the Americas and only one in Africa. In relation to those studies developed in the Americas, only two were carried out in the Neotropical region (Argentina and Brazil) and the others in United States and Canada (three and one articles respectively). The Neotropical region has suffered major transformations due to land use change and associated negative impacts on biodiversity in the last decades (Ceballos & Garcia 1995; Lowe et al. 2005; Bedano & Dominguez 2016); the lack of fluctuating asymmetry studies in small mammals in this region is remarkable.
Regardless of the approach used to analyze fluctuating asymmetry (linear measurements or geometric morphometrics) the dominant focal species were rodents and shrews. Of the total number of studies, 20 applied linear measurements, six geometric morphometrics and only one used both methodologies. Linear measurement studies used metrical o meristic measures indifferently. The oldest articles applied linear measurements and since 2002 some authors started to use geometric morphometrics. The results obtained from geometric morphometrics and linear measurements were similar with positive association between fluctuating asymmetry and environmental stress in more than 70% of the studies. Thus, both approaches would allow a proper analysis of fluctuating asymmetry in ecological studies of small mammal.
Several authors propose that multiple traits are necessary to test differences in developmental instability in linear measurement analyses (Leary & Allendorf, 1989; Palmer, 1994). Of the total of 21 linear measurement studies, 19 used more than five traits, 15 of which found a positive association between fluctuating asymmetry and environmental stress. On the other hand, Wauters et al. (1996) and Coda et al. (2016) used only one trait and found a positive association between environmental stress and developmental instability. In these studies, the authors used the length of right and left hind feet to assess fluctuating asymmetry in live red squirrels and cricetid rodents. The absence of fluctuating asymmetry could not be accurately established using only one trait.
The environmental stress factors were natural (9 studies) or anthropogenic (18 studies). The natural factors included differences in habitat suitability and only one study analyzed natural disasters (i.e., tornados) (Table 1). In relation to anthropogenic factors, most of the studies evaluated the effect of radiation emitted by nuclear power plants and waste from industries/ mining (five and eight articles respectively). We registered only two studies about the effect of agriculture on developmental instability in small mammals (Table 1), in spite of the fact that agriculture is among the predominant global changes of the last 100 years (Matson et al. 1997) and that it has led to a widespread decline in biodiversity (Benton et al. 2003).
The studies considered used measures obtained from samples of barn owl pellets, scientific collections and animals captured in field surveys that were sacrificed or not. We found only one study in which teeth obtained from barn owl pellets were used to assess fluctuating asymmetry (Amarena et al. 1993). Taking into account the small number of studies about fluctuating asymmetry in small mammals, scientific collections provide a large amount of information for studies of developmental instability (e.g. Sanchez-Char di et al. 2013; Askay et al. 2014; Maestri et al. 2015). Besides, it is possible to analyze the effects of environmental changes on individual development instability at greater spatial and time scales, which become important in populations that have undergone decreases in their abundances. Museum specimens allow the use of both geometric morphometrics and linear measurements. We found that most studies (19) used sacrificed individuals to take measurements for fluctuating asymmetry analyses, whereas only four used live animals (Table 1). Studies based on sacrificed animals used both geometric morphometrics and linear measurements, whereas only linear measurements were used with live animals. Regardless of whether measurements have been obtained from sacrificed or live animals, positive associations between developmental instability and environmental stress was observed in most studies (Table 1).
The use of exomorphological traits seem to be a useful tool to assess the relationship between developmental instability and environmental stress, since it can be used with live animals, is easy to obtain and shows similar results to those obtained from measurements of internal traits of sacrificed animals. However, to increase accuracy several measurement of each trait should be taken (repeatability of the measure) and as much traits as possible should be considered.
PERSPECTIVES
Few studies have evaluated the relationship between developmental instability (using fluctuating asymmetry as a proxy) and environmental stress in small mammals. These studies clearly show that fluctuating asymmetry is a tool to assess this relationship, allowing us to suggest that this approach will prove to be increasingly important in future ecological studies.
Fluctuating asymmetry of exomorphological traits may provide a valuable indicator of environmental stress in small mammals. Particularly, the use of non-invasive technique in live wild animals using these traits would be a valuable and inexpensive tool for studies in conservation biology as an alternative to sacrificing animals. To our knowledge, there are no studies that use geometric morphometrics with exomorphological measurements to evaluate fluctuating asymmetry. However, it would be possible to implement this approach with these traits, since they have been already used in studies of leg shape and locomotion in rodents (Rivas & Linares 2006).
This review shows the importance of including fluctuating asymmetry in ecological studies as a reliable, cheap and fast biological indicator of the effect of environmental stress on mammals, especially in the Neotropical region, where there is a noticeable absence of these kinds of studies.
Recibido 10 agosto 2016. Aceptado 20 de octubre 2017. Editor invitado: J. Priotto. Editor asociado: E. Lessa
ACKNOWLEDGMENTS
We are grateful to Enrique Lessa and the reviewers for their helpful comments and suggestions on improving this manuscript.
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Jose A. Coda (1), Juan Jose Martinez (2), Andrea R. Steinmann (1), Jose Priotto (1), and M. Daniela Gomez (1)
(1) Universidad Nacional de Rio Cuarto, Rio Cuarto, Argentina. CONICET (Consejo Nacional de Investigaciones Cientificas y Tecnicas), Argentina. [Correspondencia: <[email protected]>]
(2) Instituto de Ecorregiones Andinas (INECOA), Universidad Nacional de Jujuy, CONICET, San Salvador de Jujuy, Argentina.
Caption: Fig. 1. Three common distributions of right-left in bilateral traits: A) directional asymmetry, B) antisymmetry, C) fluctuating asymmetry. f: frequency of the measured trait.
Table 1 List of the articles selected to evaluate the use of fluctuating asymmetry as a tool to measure environmental stress in ecological studies on small mammals in chronological order. Articles authors (Year) Animals source Pankakoski (1985) Capture and sacrifice Owen & McBee (1990) Capture and sacrifice Zakharov et al. (1991) Capture and sacrifice Amarena et al. (1993) Barn owls pellets Vasil'ev & Vasil'eva (1995) Capture and sacrifice Vasil'ev et al. (1996) Capture and sacrifice Wauters et al. (1996) Capture without sacrifice Zakharov et al. (1997a) Capture and sacrifice Zakharov et al. (1997b) Capture and sacrifice Badyaevet et al. (2000) Capture and sacrifice Nunes et al. (2001) Capture and sacrifice Gileva & Nokhrin (2001) Capture and sacrifice Oleksyk et al. (2002) Capture and sacrifice Marchand et al. (2003) Capture and sacrifice Knopper & Mineau (2004) Capture without sacrifice Oleksyk et al. (2004) Capture and sacrifice VeliCkovic (2004) Capture and sacrifice VeliCkovic (2007) Capture and sacrifice Wojcik et al. (2007) Capture and sacrifice Hopton et al. (2009) Capture without sacrifice Sanchez-Chardi et al. (2013) Specimens from collections Askay et al. (2014) Specimens from collections Shadrina & Volpert (2014) Capture and sacrifice Maestri et al. (2015) Specimens from collections Coda et al. (2016) Capture without sacrifice Shadrina & Volpert (2016) Capture and sacrifice Yalkovskaya et al. (2016) Capture and sacrifice Articles authors (Year) Approach (trait) (a) Pankakoski (1985) LM (meristic) Owen & McBee (1990) LM (metrical) Zakharov et al. (1991) LM (metrical and meristic) Amarena et al. (1993) LM (metrical) Vasil'ev & Vasil'eva (1995) LM (meristic) Vasil'ev et al. (1996) LM (meristic) Wauters et al. (1996) LM (metrical) Zakharov et al. (1997a) LM (meristic) Zakharov et al. (1997b) LM (meristic) Badyaevet et al. (2000) LM (metrical) Nunes et al. (2001) LM (metrical) Gileva & Nokhrin (2001) LM (metrical) Oleksyk et al. (2002) GM Marchand et al. (2003) GM and LM (metrical) Knopper & Mineau (2004) LM (metrical) Oleksyk et al. (2004) GM VeliCkovic (2004) LM (metrical) VeliCkovic (2007) LM (meristic) Wojcik et al. (2007) LM (meristic) Hopton et al. (2009) LM (metrical) Sanchez-Chardi et al. (2013) GM Askay et al. (2014) GM Shadrina & Volpert (2014) LM (meristic) Maestri et al. (2015) GM Coda et al. (2016) LM (metrical) Shadrina & Volpert (2016) LM (meristic) Yalkovskaya et al. (2016) GM Articles authors (Year) Stressor Results (b) Pankakoski (1985) Habitat suitability + (natural causes) Owen & McBee (1990) Industries/Mining - Zakharov et al. (1991) Habitat suitability + (natural causes) Amarena et al. (1993) Industries/Mining + Vasil'ev & Vasil'eva (1995) Radiation - Vasil'ev et al. (1996) Radiation + Wauters et al. (1996) Habitat suitability + (human activities) Zakharov et al. (1997a) Habitat suitability + (natural causes) Zakharov et al. (1997b) Habitat suitability - (natural causes) Badyaevet et al. (2000) Habitat suitability + (human activities) Nunes et al. (2001) Industries/Mining + Gileva & Nokhrin (2001) Radiation + Oleksyk et al. (2002) Radiation + Marchand et al. (2003) Habitat suitability partial (human activities) Knopper & Mineau (2004) Pesticides - Oleksyk et al. (2004) Radiation + VeliCkovic (2004) Industries/Mining partial VeliCkovic (2007) Industries/Mining + Wojcik et al. (2007) Habitat suitability + (natural causes) Hopton et al. (2009) Natural disasters + Sanchez-Chardi et al. (2013) Industries/Mining + Askay et al. (2014) Habitat suitability - Shadrina & Volpert (2014) Industries/Mining + Maestri et al. (2015) Habitat suitability + (natural causes) Coda et al. (2016) Habitat suitability + (human activities) Shadrina & Volpert (2016) Habitat suitability + (natural causes) Yalkovskaya et al. (2016) Industries/Mining + Articles authors (Year) Focal specie group Pankakoski (1985) Rodent Owen & McBee (1990) Rodent Zakharov et al. (1991) Shrew Amarena et al. (1993) Rodent and shrew Vasil'ev & Vasil'eva (1995) Rodent Vasil'ev et al. (1996) Rodent Wauters et al. (1996) Rodent Zakharov et al. (1997a) Shrew Zakharov et al. (1997b) Shrew Badyaevet et al. (2000) Shrew Nunes et al. (2001) Rodent Gileva & Nokhrin (2001) Rodent Oleksyk et al. (2002) Rodent Marchand et al. (2003) Rodent Knopper & Mineau (2004) Rodent Oleksyk et al. (2004) Rodent VeliCkovic (2004) Rodent VeliCkovic (2007) Rodent Wojcik et al. (2007) Shrew Hopton et al. (2009) Rodent Sanchez-Chardi et al. (2013) Shrew Askay et al. (2014) Rodent Shadrina & Volpert (2014) Rodent and shrew Maestri et al. (2015) Rodent Coda et al. (2016) Rodent Shadrina & Volpert (2016) Rodent and shrew Yalkovskaya et al. (2016) Rodent Articles authors (Year) Study area (country) Pankakoski (1985) Finland Owen & McBee (1990) EEUU Zakharov et al. (1991) Russia, Finland Amarena et al. (1993) Italy Vasil'ev & Vasil'eva (1995) Russia Vasil'ev et al. (1996) Russia Wauters et al. (1996) Belgium Zakharov et al. (1997a) Russia Zakharov et al. (1997b) Russia Badyaevet et al. (2000) EEUU Nunes et al. (2001) Portugal Gileva & Nokhrin (2001) Russia Oleksyk et al. (2002) Ukraine Marchand et al. (2003) France Knopper & Mineau (2004) Canada Oleksyk et al. (2004) Ukraine VeliCkovic (2004) Serbia VeliCkovic (2007) Serbia Wojcik et al. (2007) Poland Hopton et al. (2009) EEUU Sanchez-Chardi et al. (2013) Spain Askay et al. (2014) Africa (5 countries) Shadrina & Volpert (2014) Russia Maestri et al. (2015) Brazil Coda et al. (2016) Argentina Shadrina & Volpert (2016) Russia Yalkovskaya et al. (2016) Russia (a) Approaches to study FA: geometric morphometrics (GM) and linear measurements (LM) that include metrical or meristic measurements. (b) Positive and negative relationships between FA levels and stressors are indicated with + (plus) and -(minus), respectively.