Chill Tolerance Variability Within and Among Populations in the Dung
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Journal of Arid Environments 75 (2011) 119e124 Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Chill tolerance variability within and among populations in the dung beetle Canthon humectus hidalgoensis along an altitudinal gradient in the mexican semiarid high plateau J.R. Verdú I.U. CIBIO, Universidad de Alicante, 03080 Alicante, Spain article info abstract Article history: Thermal tolerance of individuals and populations is measured in four different populations of the dung Received 20 September 2009 beetle Canthon humectus hidalgoensis Bates, 1887 along an altitudinal gradient in the semiarid Mexican Received in revised form high Plateau. Data for thermotolerance limits may offer useful information on the ecological flexibility of 10 May 2010 individuals and populations from a biogeographical point of view. As a measure of cold thermal toler- Accepted 21 September 2010 ance, the supercooling point (SCP) is used because it is a trait which can be used to assess effects of cold Available online 16 October 2010 acclimation. Supercooling point is highly correlated with altitude and SCP frequency distributions show clear physiological variability in C. h. hidalgoensis among populations and within the population Keywords: Cold hardiness inhabiting the highest site. In the latter, SCP frequency was clearly bimodal, indicating the presence of Crystallization temperature two different groups of individuals, and thus a mechanism with two different states of cold hardiness. Environmental change Populations and individuals of C. humectus hidalgoensis were a wide physiological variability with Physiological diversity a relatively broad range of thermotolerance responses to semiarid cold conditions. The study of Scarabaeidae ecophysiological traits in different populations of this species contributes to a more realistic picture of its potential distribution by providing more complete information on the thermal niche. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction An organism’s ability to change its phenotype in response to changes in the environment, regardless of its particular genotype, is Most physiological traits in insects are conditioned by temper- called phenotypic plasticity (Chown and Terblanche, 2007; Slabber ature. The adaptations of insect species allow them to occupy and Chown, 2005; Terblanche et al., 2006) and much of the habitats according to preferred thermal ranges (altitude, latitude, geographical variation is a consequence of phenotypic plasticity etc.). Although the interspecific patterns of thermal adaptations (Ayrinhac et al., 2004; Klok and Chown, 2003). Thus, different have been widely studied, information about the variability in populations or individuals living in a given environment can thermal responses between the populations or between individ- express specific phenotypes (Scheiner, 1993). This geographical uals of the same species is scarce and biased towards a few well variation in the reactions to environmental conditions among the studied species (Ayrinhac et al., 2004; Chown, 2001; Deere et al., individuals of the same species can increase the general adaptation 2006; Hoffmann et al., 2003; Klok and Chown, 2003; Terblanche of the species (Sultan and Spencer, 2002; Van Valen, 1965). et al., 2006; Verdú and Lobo, 2008). For dung beetles, interspe- The way in which individuals adapt to diverse environments is cific thermal tolerance varies along altitudinal gradients separating one of the main questions in evolutionary biology and ecophysi- their spatial distributions according to their temperature excess ology, and is of special concern for studies of global climatic change capacity and thermal niche breadth (Gaston and Chown, 1999; using insect as indicators (Dennis, 1993; Terblanche et al., 2006). Verdú and Lobo, 2008). Thus, the range of thermal tolerance There are several physiological measures for studying tolerance to probably increases with climatic variability, as a result of variations high and low temperatures, including supercooling points (SCP), (or gradients) in latitude and altitude on both local and regional lower lethal temperatures (LLT), upper lethal temperatures (ULT), scales (for review see Chown and Nicolson, 2004; Chown and as well as critical thermal maximum (CTmax) and minimum Terblanche, 2007). (CTmin) (see Chown and Nicolson, 2004, for review). Among the measures of cold thermotolerance, SCP can be considered as a trait which can be used to assess effects of cold acclimation and provides E-mail address: [email protected]. the basic information for identifying both freezing tolerance and 0140-1963/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2010.09.010 120 J.R. Verdú / Journal of Arid Environments 75 (2011) 119e124 freezing avoidance strategies (Bale, 2002; Bennett et al., 2005; xerophilous landscape dominated by leguminosae (Prosopis, Mimosa, Carrillo et al., 2006; Sinclair et al., 2003). The frequency distribu- etc.) and cactaceae (Opuntia, Echinocactus, Ferocactus, etc.); 2) Los tion of SCP (or crystallization temperature) has been widely used Mármoles National Park which is dominated by a mosaic of grass- for analyzing the factors that influence freezing in arthropods lands, xerophilous shrublands and patches of pineeoak forest; 3) (Chown and Nicolson, 2004). Venta del Río which is dominated by crassicaule scrubland (Opuntia, In this paper the thermal tolerance of four populations of the Fouquieria, Yucca, Ferocactus, Myrtillocactus); and 4) Barranca de dung beetle Canthon humectus hidalgoensis Bates, is examined along Metztitlán, dominated by crassicaule and submountainous scrub- an altitudinal gradient. This subspecies is endemic to central Mexico, land (Prosopis, Yucca, Acacia, Opuntia, Myrtillocactus, Isolatocereus). In a region with a dominant pattern of frost resistance in plants that is the field, prior to transport to laboratory, all beetles were immedi- characterized by depression of the freezing point (FP) (Larcher, ately acclimated at 10e15 C (close to ambient temperature) in 2003). C. humectus hidalgoensis belongs to a lineage of North Amer- portable refrigerated chambers in order to avoid the high tempera- ican taxa with a Mexican High Plateau distribution pattern (Halffter, tures during the transport within the vehicle. Prior to SCP 1976). There is little knowledge of the natural history of many measurement, body mass (fresh weight) was measured with an Scarabaeinae species, but it is known that some species belonging to ANDs HM-202 high-precision balance (accuracy of 0.1 mg). All the Canthon genus (such as Canthon cyanellus and Canthon humectus) specimens were deposited in the entomological collection of the can live more than two years (M. Favila, personal communication University of Alicante (CEUA). and personal observations). For C. humectus hidalgoensis, this longevity implies an ability to endure two climatic extremes: the 2.2. Chill tolerance measurement frequent frosts of winter and the extreme dryness of summer. Previous observations allow the classification of this subspecies as Supercooling is analyzed because it is the major physiological a freeze-avoiding (sensu Bale, 1993, 1996, 2002) and endothermic determinant of overwintering mortality (Bale, 2002). For the SCP taxon (Verdú et al., 2006, 2007), and therefore, as a taxon that likely and body mass measurements in the laboratory, 25, 31, 21 and 24 has the ability to supercool as overwintering strategy. Considering individuals from each of the sites mentioned above, respectively, the environmental heterogeneity of an altitudinal gradient, it is were used. All specimens were captured in September (8e10) 2007. hypothesized that different populations may have different ther- According to results of several works (Bennett et al., 2005; Carrillo motolerance strategies. According with Bale (2002) the ability to and Cannon, 2005; Ohlsson and Verhoef, 1988; Verdú and Lobo, supercool is undoubtedly the most important component of the 2008; Worland, 1996), several methodological requirements are overwintering strategy, thus, to determine the thermotolerance of C. considered in order to minimize factors that can influence the humectus hidalgoensis, SCP is measured. physiological measurements: only mature specimens were selected The purpose of this paper is to explore variability in the thermal according to external age-grading methods proposed by Tyndale- tolerance of individuals and populations for four different pop- Biscoe (1984) that permits identification of individuals with ulations of this xerophilous dung beetle. Data for thermotolerance approximately the same age; a sex ratio of 1:1 was tested in each limits may offer useful information on the physiological flexibility of population using a binomial test (Table 1); in order to obtain individuals and populations from a biogeographical point of view. homogeneous environmental conditions among populations, all This would allow us to predict distributions in different environ- beetles used in the experiments were maintained in plastic mental scenarios (such as climate warming) and to explore future containers (20 Â 10 Â 6 cm) with vermiculite as substrate for 5 days genetic differentiation among populations of this species. at a constant temperature of 10 C and 75% RH prior to the labora- tory experiments; in order to avoid the effect of diet and gut content 2. Materials and methods in SCP measures, all beetles were starved for 3 days