Introduction
Bergmann’s Rule describes a negative correlation between environmental temperature and endotherms’ body size (Bergmann Reference Bergmann1847): endothermic animals are smaller in warm environments than in cold climates. The smaller surface:mass ratio of larger endotherms reduces heat loss in colder environments. The body temperature of ectotherms depends on environmental temperature (and therefore Bergmann's Rule were not defined for ectotherms; Vinarski Reference Vinarski2014), and their size might therefore depend on the length of the warm season, which is usually shorter at higher altitudes and/or latitudes (Mousseau Reference Mousseau1996). Consequently, the opposite of Bergmann’s Clines (i.e. the same pattern as predicted by Bergmann's Rule for endotherms) is often observed in ectotherms (e.g., Mousseau Reference Mousseau1996; Vinarski Reference Vinarski2014): body size decreases with decreasing environmental temperature. However, numerous studies have showed Bergmann’s Clines in ectotherms. Larger bodies absorb more heat through basking in colder environments, are a useful barrier against cold weather (e.g., Vinarski Reference Vinarski2014; Beck et al. Reference Beck, Liedtke, Widler, Altermatt, Loader, Hagmann, Lang and Fiedler2016; Brehm et al. Reference Brehm, Zeuss and Colwell2019), and function as a buffer to aid survival through seasonal unavailability of resources (Horne et al. Reference Horne, Hirst and Atkinson2017).
Studies of elevational patterns in insect body sizes have predominantly focused on interspecific patterns in communities, with both positive and negative relationships between body size and elevation detected (Shelomi Reference Shelomi2012). Body size and elevation have been positively correlated in geometrid and arctiine moths in the Costa Rican mountains (Brehm et al. Reference Brehm, Zeuss and Colwell2019), arctiine moths in the Ecuadorian Andes (Fiedler & Brehm Reference Fiedler and Brehm2021), and macromoths in the Swiss Alps (Beck et al. Reference Beck, Liedtke, Widler, Altermatt, Loader, Hagmann, Lang and Fiedler2016). However, no significant relationship between elevation and geometrid body size was found in the Ecuadorian Andes (Brehm & Fiedler Reference Brehm and Fiedler2004). Although temperature and other mechanisms are thought to be responsible for these interspecific trends (e.g., Shelomi Reference Shelomi2012, Beck et al. Reference Beck, Liedtke, Widler, Altermatt, Loader, Hagmann, Lang and Fiedler2016), the phylogenetic complexity of multispecies communities makes the results difficult to interpret (Mungee et al. Reference Mungee, Pandit and Athreya2021). Intraspecific changes in insect body size along elevation have rarely been studied (16% of 676 datasets included in Shelomi Reference Shelomi2012), but such studies are instrumental for understanding the responsible mechanisms. Most of these uncommon, intraspecific datasets mostly showed Bergmann’s Clines (Shelomi Reference Shelomi2012), including four tortricids in North America (Miller Reference Miller1974, Reference Miller1991), four geometrids and one noctuid in North Carolina (Sullivan & Miller Reference Sullivan and Miller2007), and one geometrid moth in Costa Rica (Sullivan & Miller Reference Sullivan and Miller2007). Brehm et al. (Reference Brehm, Zeuss and Colwell2019) found intraspecific Bergmann’s clines among most of 84 Costa Rican geometrid and arctiine moths they examined, and Mungee et al. (Reference Mungee, Pandit and Athreya2021) found significant but weak Bergmann’s clines in 24 sphingid species in the eastern Himalayas.
Individual generations of multivoltine ectotherms can differ in their body sizes (Chown & Gaston Reference Chown and Gaston2010; Horne et al. Reference Horne, Hirst and Atkinson2015; Horne et al. Reference Horne, Hirst and Atkinson2017). Although availability of food, water and humidity, along with predation and pathogen risk can differ seasonally (Chown & Gaston Reference Chown and Gaston2010), the main drivers of multivoltine insect body size are expected to be ambient environmental temperature and—at higher latitudes—photoperiod (Chown & Gaston Reference Chown and Gaston2010; Horne et al. Reference Horne, Hirst and Atkinson2017). Seasonal intraspecific differences in the body size of insects and other arthropods have been recently meta-analysed, and the body sizes of 86% of 102 species decreased in warmer seasons (Horne et al. Reference Horne, Hirst and Atkinson2017). This pattern was demonstrated in a North American tortricid moth Epiphyas postvittana (Danthanarayana Reference Danthanarayana1976) and in 8 of 10 butterfly species studied (Horne et al. Reference Horne, Hirst and Atkinson2017). The opposite trend of larger specimens developed during warmer period was shown in 10 European moth species (Teder et al. Reference Teder, Esperk, Remmel, Sang and Tammaru2010). Zeuss et al. (Reference Zeuss, Brunzel and Brandl2017) found a negative relationship between voltinism (i.e., number of generations per year) and body size in European lepidopterans, whilst Seifert et al. (Reference Seifert, Strutzenberger and Fiedler2022) showed the opposite trend for European moths. Nevertheless, the knowledge of intraspecific seasonal changes in lepidopteran size is limited, and no study on intraspecific seasonal differences in body sizes in tropical moths is available.
Our study examines intraspecific changes in sizes of 28 moth species (Lepidoptera) along an elevational gradient and in different seasons in Afrotropical rainforests on Mount Cameroon, an important biodiversity hotspot (Ustjuzhanin et al. Reference Ustjuzhanin, Kovtunovich, Sáfián, Maicher and Tropek2018, Reference Ustjuzhanin, Kovtunovich, Maicher, Sáfián, Delabye, Streltzov and Tropek2020). Forewing length is strongly correlated with body size in lepidopterans (Brehm et al. Reference Brehm, Zeuss and Colwell2019; Mertens et al. Reference Mertens, Brisson, Janeček, Klomberg, Maicher, Sáfián, Delabye, Potocký, Kobe, Pyrcz and Tropek2021), and we hypothesised that the forewing length generally increases with elevation. Additionally, we analysed inter-seasonal differences in forewing length, hypothesising that individuals will be larger in wetter seasons with less sunshine. We analysed relationships between size and elevation separately for males and females, as this is known to play a role (Baranovská & Knapp Reference Baranovská and Knapp2018; Brehm et al. Reference Brehm, Zeuss and Colwell2019).
Methods
We analysed moths sampled along the elevational gradient of Mount Cameroon, the highest mountain (4095 m a.s.l.) in West/Central Africa. Moths were attracted by light at six elevations: 350, 650, 1100, 1450, 1850 and 2200 m a.s.l. Sampling was repeated in three seasons: transition from wet to dry seasons: November/December; dry season: January/February; and transition from dry to wet seasons: April/May (Maicher et al. Reference Maicher, Sáfián, Murkwe, Przybyłowicz, Janeček, Fokam, Pyrcz and Tropek2018). See Maicher et al. (Reference Maicher, Sáfián, Murkwe, Delabye, Przybyłowicz, Potocký, Kobe, Janeček, Mertens, Fokam, Pyrcz, Doležal, Altman, Hořák, Fiedler and Tropek2020) for more details on the sampling and sites. All specimens were identified by comparison with a large reference collection of identified material and confirmed by genitalia dissection; morphospecies are therefore considered to be taxonomically valid species.
From 17,598 moths (Lepidoptera: Heterocera) of 561 morphospecies from Maicher et al. (Reference Maicher, Sáfián, Murkwe, Delabye, Przybyłowicz, Potocký, Kobe, Janeček, Mertens, Fokam, Pyrcz, Doležal, Altman, Hořák, Fiedler and Tropek2020), we selected 28 moth species (13 Erebidae: Arctiinae, 10 Erebidae: Lymantriinae, 2 Notodontidae, 2 Lasiocampidae and 1 Sphingidae; Table 1; Fig. S1) with at least 5 specimens at each of at least 2 elevations and seasons. Altogether, 3223 specimens were photographed with a scale, and their right forewing length was measured from the base to its apex using ImageJ 1.46r (Ferreira & Rasband Reference Ferreira and Rasband2012). Forewing lengths were analysed separately for each species, and wherever possible (eight species; Table 1) also for both sexes. Relationships between forewing length (continuous response variable) and elevation (continuous explanatory variable), season (factorial explanatory variable), and their interactions were tested with generalised linear models (GLMs) in glm2 package (Marschner Reference Marschner2011) in R 4.0.2 (R Core Team 2020). In several cases, the number of specimens from individual elevations or seasons was not sufficient to include one of the response variables or their interaction in the particular model. Based on the visual inspection, Poisson or quasi-Poisson distributions were applied (Table 1).
Summary of measured moths along the elevational gradient of Mount Cameroon, together with details and results of GLM models, with separated effects of elevation, season and their interaction on the forewing length of the 28 moth species. Significance – p < 0.001: ***, p < 0.01: **, p < 0.05: *, p > 0.05: n.s. Values of F-tests for models with quasi-Poisson distribution, and values of χ2 tests for models with Poisson distribution. Elevational trends – significantly positive: ↗, significantly negative: ↘, insignificant: n.s. Seasonal trends – WD: transition from wet to dry seasons, D: dry season, DW: transition from dry to wet seasons; the seasonal differences were checked visually by comparison of trends (Fig. S1), where > means that moth forewings were larger in one or two seasons than in the other one or two seasons, and ≈ means no apparent inter-seasonal difference.
Results and discussion
Of the 28 moth species, the forewing lengths of 14 species increased significantly with elevation in at least one sex and decreased significantly in 5 species. There was no significant relationship in the remaining nine species (Table 1; Fig. S2). Intersexual differences were highly variable, although no moth species showed opposite trends in males and females (Table 1; Fig. S2). Significantly positive relationships between forewing length and elevation in both sexes was observed in three species (Asythosia velutina, Galtara sp. “1” and Palaeugoa camerunensis). In two species (Afrasura cf. numida and Eilema cf. fletcheri), significantly positive trends were found only in males, and significantly negative sex-specific trends were observed in males of Ligulosia costimaculata and females of Mylantria xanthospila.
Our results are consistent with results affirming Bergmann’s Clines in moths from other biogeographic areas (Miller Reference Miller1974, Reference Miller1991; Sullivan & Miller Reference Sullivan and Miller2007; Brehm et al. Reference Brehm, Zeuss and Colwell2019; Fiedler & Brehm Reference Fiedler and Brehm2021; Mungee et al. Reference Mungee, Pandit and Athreya2021), as well as for some other insects (Shelomi Reference Shelomi2012). Although some adult moths, such as sphingids, can increase their temperature by vibrating their wing muscles (Heinrich Reference Heinrich2013), we do not expect the relationship between body surface area and thermal metabolism as predicted by Bergmann (Reference Bergmann1847). Nor can we expect that improved solar basking influences the size of these nocturnal animals (Vinarski Reference Vinarski2014; Tammaru et al. Reference Tammaru, Johansson, Õunap and Davis2018). Therefore, some other mechanisms, such as the hypothesised balancing of resource supply and demand (Horne et al. Reference Horne, Hirst and Atkinson2017), may be responsible for widespread intraspecific Bergmann’s Cline. Additionally, we hypothesise that longer wings may enable greater mobility at higher elevations with scarcer resources. A similar relationship between forewing length and mobility was already shown for intraspecific differences for some lepidopterans (e.g., Fric et al. Reference Fric, Klimova and Konvicka2006). This hypothesis is supported by the negative elevational trends for both lasiocampid species with non-feeding adults. An alternative explanation could be related to the necessity of larger wings in less dense air at higher elevations (Brehm et al. Reference Brehm, Zeuss and Colwell2019); although such evolutionary mechanism has not been studied in moths to our knowledge, it was repeatedly demonstrated in hummingbirds (e.g., Altshuler et al. Reference Altshuler, Dudley and McGuire2004).
The forewing lengths of 21 moth species were significantly different among the three seasons. In nine of these, there was an interaction between elevation and season affecting forewing lengths (Table 1). Although no consistent trends in the inter-seasonal differences of forewing length were found among the species, females from all 7 species with enough female specimens and males from 12 species analysed had longer forewings in the transition from wet to dry seasons than in one or both other studied seasons. Although their phenology and voltinism are largely unknown, most of these specimens probably developed during the wet season, i.e., the coldest months on Mount Cameroon (Maicher et al. Reference Maicher, Sáfián, Murkwe, Delabye, Przybyłowicz, Potocký, Kobe, Janeček, Mertens, Fokam, Pyrcz, Doležal, Altman, Hořák, Fiedler and Tropek2020), because all moth individuals grow during their larval stages. This suggests that the intraspecific differences in insect size results from climatic effects on physiological processes during development and may not be an adaptation for adult life (Meister et al. Reference Meister, Hämäläinen, Valdma, Martverk and Tammaru2018). However, the seasonal intraspecific differences in moth body size may also be related to differences in the accessibility and quality of food during larval development (Atkinson & Sibly Reference Atkinson and Sibly1997; Rodrigues & Moreira Reference Rodrigues and Moreira2004). Unfortunately, the larval food plants of Afrotropical moths are largely unknown.
We confirmed that environmental temperature is a key factor determining insect size along an elevational gradient and among seasons (Atkinson & Sibly Reference Atkinson and Sibly1997, Chown & Gaston Reference Chown and Gaston2010; Horne et al. Reference Horne, Hirst and Atkinson2017). Nevertheless, since there were interactions between the effects of elevation and season on size of one-third of the moth species we studied, we assume the relationship and its causality could be more complex and requires more attention.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0266467422000463
Data availability
The data that support the findings of this study are openly available in Zenodo, please visit https://doi.org/10.5281/zenodo.7254235.
Acknowledgements
We are grateful to Toomas Tammaru for fruitful discussions initiating our analyses; to Jan E.J. Mertens for his priceless help with methodology of the moths’ measurements; to Szabolcs Sáfián, Mercy Murkwe, Ishmeal N. Kobe, Jan E.J. Mertens, Pavel Potocký, Štěpán Janeček, Francis Luma Ewome, Toh Jennifer Kimbeng and several field assistants for help with the material sampling; to Łukasz Przybyłowicz for identification of arctiines; to Ewelina Sroka, Karolina Sroka, Jadwiga Lorenc-Brudecka and other staff of the Nature Education Centre of Jagellonian University for setting and dissecting the specimens; to Eric B. Fokam and staff of the Mount Cameroon NP for their assistance with permits and other logistics; and to Matthew Sweney and David Lohman for proofreading our English. The material was sampled under permits by the Cameroon Ministries of Forests and Fauna, and of Scientific Research and Innovation. Our research was funded by the Czech Science Foundation (20-16499S) and Charles University (UNCE204069).
