Article

Stillwell RC, Fox CW. Environmental effects on sexual size dimorphism of a seed-feeding beetle. Oecologia 153: 273-280

Department of Entomology, University of Kentucky, S225 Agricultural Science Center North, Lexington, KY 40546-0091, USA.
Oecologia (Impact Factor: 3.09). 09/2007; 153(2):273-80. DOI: 10.1007/s00442-007-0724-0
Source: PubMed

ABSTRACT

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.

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    • "The geographic adaptations of animals, which include variations in body size, are generally genetically based (Armbruster, Bradshaw, Ruegg, & Holzapfel, 2001; Karl, Janowitz, & Fisher, 2008). Studies show that a thermocline (or other factors which vary with latitude or altitude) can result in both intra-specific and inter-specific size dimorphism (Blanckenhorn, Stillwell, Young, Fox, & Ashton, 2006; Stillwell & Fox, 2007; Teder & Tammaru, 2005). The body size of animals may also be correlated with population density (Robinson & Redford, 1986). "
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    ABSTRACT: As a general rule, spiders exhibit sexual dimorphism and their populations may differ in size according to season duration and resource availability. However, few studies have focused on dimorphism in tarantulas. Mexican redrump tarantulas, Brachypelma vagans, listed in CITES, have an exceptionally wide distribution. Surprisingly, there are no studies on the possible relationship between the abundance of tarantulas per population and the geographical areas where they are present, or on how the distribution pattern of this spider may affect individual morphological characteristics. Furthermore, there are no studies on sexual dimorphism within the genus Brachypelma. The aim of the study is to determine the existence of sexual and geographical dimorphism in populations of B. vagans. It was observed that the abundance of spiders per population may vary according to the geographical areas where they were recorded. In six localities in southern Mexico, we recorded morphological data on adult tarantulas. Sexual dimorphism was clearly observed at the site that presented numerous spiders characterized by much smaller females. Since the results of this study demonstrate differences in tarantula number of individuals per locality in southern Mexico, they make an important contribution to the conservation of this species.
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    • "Blanckenhorn et al. (2006) found for the majority of studied vertebrate and invertebrate species that the body size of male individuals changes faster with latitude than that of female individuals. Specimens of the seed-feeding beetle, Stator limbatus Horn, 1873, were smaller, but more sexually dimorphic, with males being larger than females, at lower latitudes, lower seasonality and higher moisture (Stillwell et al. 2007). However, much less is known about small-scale, or local, variability in SSD. "
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    • "Females may have displayed steeper size clines than males because their fi tness may be more sensitive to changes in season length, climatic variables, and the availability of food (Laiolo et al. 2013). Th is suggests diff erences in dispersal potential among species or populations may contribute to diff erences in geographic variation in sexual size dimorphism for a variety of taxa (but also see Stillwell and Fox 2007, 2009). As there is considerable evidence that long winged species such as M. sanguinipes are frequently dispersed to higher elevations along the Front Range of Colorado and that short winged species such as A. clavatus and M. boulderensis are not (Alexander 1951, 1964), it is important to note that some of the long winged individuals that were fi eld collected at higher sites may have originated from lower elevations. "
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