Morphological differences in darkling beetles (Coleoptera: Tenebrionidae) along environmental gradients
Methods:
The standard protocol was followed (Brown, 2016). Additional GLMs were run to examine possible relationships between both the mean and total size, climate zone, habitat type, altitude, precipitation, and temperature values from the data. Descriptive statistics of these various relationships were also generated.
Results:
The morphometric characteristics of the beetles considered in analysis were size (expressed as length in millimetres) and shape (analysed as two-dimensional outlines).
A corrgram was used to determine likely relationships between environmental factors and size (Figure 1). Based on this, the positive relationship between size and precipitation, and the negative relationship between size and altitude were selected for further analysis. Mean altitude, precipitation, and size values were calculated and compared for both climate zones and habitat types to discover whether or not these were significant. There was no significant relationship between size and altitude (1 degree of freedom, F=1.4, P=0.24), but a significant positive correlation was found between mean size and mean annual precipitation (1 degree of freedom, F=6.63, P=0.01).
A MANOVA test revealed a statistically significant relationship between shape and habitat type (13 degrees of freedom, Hotelling-Lawley=2.9, P=0.009) but not climate zone (8 degrees of freedom, Hotelling-Lawley=1.7, P=0.051). In order to analyse possible relationships, thin plate spline (TPS) grids were created for the average shape of beetles in each habitat and climate. This allowed for comparisons to be drawn by overlaying the average beetle shapes of different climate zones or habitat types. Those comparisons interpreted as the most biologically significant are highlighted here:
1.) Mean Annual Precipitation The mean shape of beetles found in the rainforest habitat type—the habitat with the highest precipitation (2029 mm per year)—was broader and more rounded than that of beetles found in arid scrubland—the habitat with the least precipitation (102 mm per year) (Figure 2). This suggests a link between shape and precipitation, and by extension productivity.
2.) Mean Altitude Beetles from the lowest altitude habitat, coastal scrubland (mean altitude: -25 m above sea level), were found to be broader than beetles from montane scrub, the highest altitude habitat (mean altitude: 1428 m) (Figure 3), suggesting that broad, rounded bodies are more favoured at lower altitudes than higher altitudes.
3.) Vegetation Beetles from different habitats at about the same altitude (montane forest and montane scrub) were compared (Figure 4). Montane forest beetles were found to be broader than their montane scrub counterparts.
The impact of temperature on shape was an area of interest. As the beetles in the study were collected from a wide range of habitats across many latitudes, the accuracy of mean habitat temperatures could not be confirmed. Instead, the mean shape of beetles from the climates with the highest and lowest temperatures were compared. As previously stated, climate zones as a whole were not found to affect shape in a statistically significant manner; the biological significance of the comparisons was considered instead.
Beetles from the tropical wet climate zone (mean annual temperature: 25 °C) were found to be much broader than the slender mean shape of beetles from the subarctic zone (mean annual temperature: -0.1 °C) (Figure 5).
Discussion:
Beetle size was found to correlate only with precipitation, a finding that is contrary to previous studies on the subject (Krasnov et al., 1995). Beetle shape correlated significantly with habitat type, suggesting that the selective pressures that impact shape are on a habitat-specific scale, affected by such factors as substrate or vegetation, rather than by factors on the scale of broad climatic trends.
Size
Fattorini et al. (2013) posited that beetle size is primarily dependent on temperature. Although the dataset did not show significant results as a whole when temperature and size were compared, this lack of significance may be the result of biased sample collection; collection was biased towards European and west Asian samples, with only limited samples from tropical or arctic/subarctic climates and none at all from the neotropics. Precipitation did correlate significantly with size, a phenomenon that Krasnov et al. (1996) attribute to higher precipitation being demonstrative of more vegetation, and thus more sustenance. The resource richness provided by areas of high precipitation allow beetles to reach larger sizes than would be allowed in areas of scarce precipitation and vegetation. Large size is also favoured by beetles in arid environments as a method of thermoregulation and reducing water loss (Fattorini et al., 2013).
Shape
Beetle shape is likely to be influenced by the physical characteristics of a habitat, such as vegetation and substrate, which will vary within the same climate zone much more than variables such as temperature.
Through study of the TPS grids generated for shape comparisons, precipitation and altitude were observed as having a biologically significant affect on shape. Precipitation appeared to have an effect on beetle shape, as rainforest beetles were broader than arid scrubland beetles (Figure 2). Increased precipitation allows for a build-up of vegetation, and, by extension, of decaying biotic substrate, which beetles can then exploit as a niche.
The comparison between montane forest and montane scrub (Figure 4) yields further information about the importance of vegetation in determining shape. Forest-dwelling beetles have evolved broad, rounded, flattened bodies to easily move about or burrow between layers of loose substrate—such as rotting foliage—as they forage. Conversely, narrow, cylindrical body shapes are signs of adaptation to living on the soil surface, in habitats where stony or hard substrates make burrowing useless or impossible (De Los Santos et al., 2000). It is for this reason that narrower bodies were more common in higher altitude non-forested habitats, such as montane scrub, where exposure prevents the build-up of biotic substrate. This trend is exemplified to an even greater extent in Figure 3: montane scrub beetles were narrower and less rounded than those of equivalent coastal scrub. Evidently, the relatively more extreme, exposed montane conditions facilitated the evolution of a narrower body shape. This may allow these beetles the ability to better exploit microhabitats –such as rocky crevasses or cracks that a broad beetle would be unable to enter—as shelter from hostile conditions such as wind or extreme temperatures.
A similar phenomenon was observed in the comparison between cold and warm climates (Figure 5)—beetles in tropical wet climates had broad bodies, and beetles from subarctic climates had significantly narrower bodies on average. The selective pressures on beetles of living in cold latitudes are thus likely to be similar to those associated with life in low-vegetation, high altitude environments. These same pressures also cause smaller beetle sizes in colder latitudes (Fattorini et al, 2013). Interestingly, Figure 5 shows perhaps the greatest difference in shape despite using climate zone data, which was found to be statistically insignificant in correlation to shape. This lack of statistical significance does not preclude biological significance, and it is likely that given a larger, more geographically even dataset, climate would be found to significantly correlate with shape.
Several disclaimers should be made in regards to the dataset. It is highly likely that behavioural characteristics and species-specific life histories are major determinants of both the size and shape of the beetles in this study. This functional diversity between different beetles in the study was not quantified, so major determinants of shape and size may have been ignored by subsequent analyses. For example, diurnal and nocturnal beetles are likely to have a variety of different selective pressures acting on them that could affect size and shape, as was suggested by Krasnov et al. (1996). Likewise, geographical phenomena such as island dwarfism or gigantism, which are known to occur in tenebrionid beetles, were not accounted for either (Palmer, 2002). The limited geographical spread of the data (being focused mostly in Eurasia) may also contribute to skewed data interpretation. Additionally, leg morphology, which was excluded from this study, has been shown to vary along similar ecological gradients as size and shape (De Los Santos et al., 2000). In the future, an examination of leg length and robustness would facilitate a greater understanding of the functional diversity of tenebrionid beetles.
The importance of the correlation between morphology and habitat is well expounded by Southwood (1977). The results of this study support this correlation. It is evident that understanding the distribution of tenebrionid beetles in different habitats (and also climate zones) is important in explaining the diversity that exists in the shape and size of these beetles.