Latitudinal insect body size clines revisited: a critical evaluation of the saw-tooth model

Abstract

Summary1. Insect body size is predicted to increase with decreasing latitude because time available for growth increases. In insects with changing voltinism (i.e. number of generations per season), sharp decreases in development time and body size are expected at season lengths where new generations are added to the phenology of a species, giving rise to saw-tooth clines in these traits across latitudes. Growth rate variation may affect the magnitude of variation in body size or even reverse the saw-tooth cline. 2. In this study, we analyse latitudinal body size clines in four geometrid moths with changing voltinism in a common laboratory environment. In addition to body size, we measured larval development time and growth rate and genetic correlations among the three traits. 3. The patterns of clinal variation in body size were diverse, and the theory was not supported even when saw-tooth body size clines were found. Larval development time increased and growth rate decreased consistently with increasing season length, the clines in these traits being uniform. 4. The consistencies of development time and growth rate clines suggest a common mechanism underlying the observations. Such a mechanism is discussed in relation to the complex interdependencies among the traits.

Population dependent effects of photoperiod on diapause related physiological traits in an invasive beetle (Leptinotarsa decemlineata)

“…Even though there is some empirical evidence for Bergmann's rule also in ectotherms, the mechanisms are not fully understood and debated (e.g. van Voorhies 1996, Mousseau 1997, Blanckenhorn & Demont 2004, Kivelä et al 2011). The trend is partly accredited to the so-called temperature-size rule, which states that ectotherms grow larger at lower temperatures (von Bertalanffy 1960, Atkinson & Sibly 1997, Chown and Gaston 1999, Angiletta et al 2004.…”

“…In these species, individuals from populations at high latitude are expected to show an increased rate of weight gain compared to lower latitude populations, to compensate for the shorter time available for preparation. As noted by several authors, there are inherent physiological interdependencies between growth rate, development time and adult weight gain, and therefore care must be taken when analyzing them in isolation (Blanckenhorn & Demont 2004, Kivelä et al 2009, Nijhout et al 2010, Kivelä et al 2011.…”

“…Terblanche & Anderson 2010), energy conversion (e.g. Chown & Nicholson 2004) and increased growth rate (Chown & Gaston 1999, Blanckenhorn & Demont 2004, Dmitriew 2011, Kivelä et al 2011. Thus development and preparation for reproduction, migration or overwintering is possible in a short time window (Blanckenhorn & Demont 2004, Chown & Nicholson 2004, Piiroinen et al 2011.…”

Spatial Selection and Local Adaptation Jointly Shape Life-History Evolution during Range Expansion

“…Furthermore, high-latitude populations tend to compensate for the low temperatures that plastically reduce growth rate through the evolution of genetically faster growth rates (i.e., countergradient variation; see Conover and Schultz 1995). In some instances, development time can moreover share an underlying mechanism with growth rate (Kivela et al 2011), and together these two traits can impact adult size, leading to either bigger, smaller, or equal-sized individuals in more northern regions (Blanckenhorn and Demont 2004). Apart from these climatic changes, many range-shifting species may also suffer from changes in habitat quality and quantity.…”

“…At this point, voltinism abruptly decreases (i.e., loss of a generation), in turn allowing a sudden relaxation of development speed. This mechanism of a step-by-step reduction in voltinism with increasing latitude creates a typical sawtooth pattern in development time (i.e., alterations of gradual increase and sudden decrease in development speed; see Roff 1980;Kivela et al 2011;Levy et al 2015). Furthermore, high-latitude populations tend to compensate for the low temperatures that plastically reduce growth rate through the evolution of genetically faster growth rates (i.e., countergradient variation; see Conover and Schultz 1995).…”