![]() Larger size in colder areas could thus follow. For such species the length of the favourable season does not set the limits for development time, but instead e.g. They emphasised that species with short life spans, inhabiting ephemeral habitats, could be expected to increase their mass at higher latitudes. There is clearly a need to explore the causes of this variation ( Chown and Gaston 1999 Johansson 2003 Blanckenhorn and Demont 2004).Ī possible explanation for the occurrences of variable patterns of body size with latitude in insects was suggested by Chown and Gaston ( 1999). Still, some reports described an increase in size with increasing latitude also occurring in arthropods ( Chown and Gaston 1999 Blanckenhorn and Demont 2004). decreasing size with increasing latitude (or altitude), has often been reported this is the so-called “converse Bergmann's rule” ( Mousseau 1997), that has been well documented ( Mousseau and Roff 1989 Nylin and Svärd 1991 Blanckenhorn and Fairbairn 1995 Telfer and Hassall 1999 Johansson 2003). In insects and other arthropods the opposite size trend, i.e. For instance, Boyce ( 1979) argued that an increased degree of seasonality with latitude can select for increased body size. It has been suggested that larger size at higher latitudes in ectotherms may be due to non-adaptive responses to low temperatures ( Van Voorhies 1996), but adaptive explanations can also be found to explain increased size with latitude, other than thermoregulation. The underlying selection is in this case generally assumed to be related to thermoregulation the allometric relationship between body mass and surface area selects for bigger animals in cold areas. ![]() ![]() One of the best known intra-specific geographical patterns in animals is “Bergmann's Rule”, depicting a size trend with larger size at higher latitudes as is found especially in the endothermic mammals and birds ( Ashton et al. That said, the study of latitudinal and altitudinal variation has proven to be a much more complex and challenging subject than perhaps was originally thought when various “rules of thumb” regarding geographical variation was first proposed. As long as we cannot predict trends in the characteristics of organisms along such relatively simple gradients, we have little hope of predicting or even understanding biological variation in cases where the underlying environmental causes are less evident. To the extent that this is not the case would be evidence of our imperfect understanding of the processes behind local adaptation. It might be expected that geographical gradients in climate, and in length of the favourable season, should translate to biological gradients (for instance clines in animal life history traits) in a fairly predictable manner. The study of latitudinal life history trends within species has a strong potential to illuminate processes of local adaptation, which in turn can provide information on how populations may respond to factors such as climate change. Hence, we found support for both adaptive phenotypic plasticity and local genetic adaptation, with gene-environment interactions explaining the observed field patterns. This is indirect evidence for field patterns being shaped by end-of-season cues cutting development short, and also suggests counter-gradient variation, as butterflies from the time-stressed populations over-compensated for decreasing larval development time by increasing their growth rates, thus obtaining higher mass. ![]() In contrast, under laboratory conditions with a constant long day-length there was a different pattern, with the butterflies pupating at a higher mass when individuals originated from southern populations under time stress to achieve a second generation. This is in accordance with the “converse Bergmann” pattern and with the “saw-tooth model” suggesting that insect size is shaped by season length and number of generations along latitudinal transects. Further north, the size of the field-collected butterflies again decreased with latitude (with the exception of the northernmost collection sites). Sizes of the field-collected butterflies tended to smoothly decrease northwards in a latitudinal cline, but suddenly increase at the latitude where the life cycle changes from two to one generations per year, hence allowing more time for this single generation. The study of latitudinal body size clines can illuminate processes of local adaptation, but there is a need for an increased understanding of the relative roles of genetic variation, environmental effectstions or this reason, we combined an investigation of a museum collection of the common blue butterfly Polyommatus icarus (Rottemburg) (Lycaenidae: Polyommatini) from Sweden with a common-garden experiment in the laboratory, using strains reared from individuals collected from three different latitudes.
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