Ontogeny of sexual size dimorphism revisited: Females grow for a longer time and also faster

Abstract

Sex-specific mechanisms of the determination of insect body sizes are insufficiently understood. Here we use the common heath moth, Ematurga atomaria (Lepidoptera: Geometridae) to examine how larval growth trajectories differ between males and females. We monitored the development of 1379 larvae in controlled laboratory conditions. Sexually dimorphic development times during the first four instars were associated with sexual size dimorphism (SSD) in the beginning of the fifth (last) instar, when females were on average 15% heavier than males. Similarly, the duration of the last instar was about 13% longer in females. Further, we specifically focussed on the estimates of differential (instantaneous) growth rates of the larvae based on 24h mass increments of the 2 nd , 3 rd , 4 th and 5 th day in the beginning of the last instar. We calculated ‘allometric’ differential growth rates as the per-day increase in cube-root-transformed mass of the larvae. We found that allometric growth rates were slightly but significantly larger in females than in males. As this measure of growth rate (in contrast to the relative growth rate, based on the ratio of masses recorded at consecutive measurements) did not depend on body size, it allows an unambiguous separation of the effects of sex and size. We conclude that in accordance with an emerging general pattern, larger female body size in E . atomaria is achieved primarily by means of a longer growth period. Furthermore, our study shows that the differential growth rate can also be sexually dimorphic and contribute to SSD. This contribution, however, is lower than that of the development time by an order of magnitude. In addition to development periods and growth rates, other parameters of the non-linear growth curves of insect larvae also need to be considered in the context of SSD determination. In particular, weight loss prior to pupation was shown to be considerably larger in females than in males.

Ontogenetic Basis of Among-Generation Differences in Size-Related Traits in a Polyphenic Butterfly

“…This pattern differs from the emerging general picture according to which, when "needed, " larger sizes of insects are attained through prolonging growth periods and not through increasing instantaneous growth rates. This applies to comparisons among sexes (Tammaru et al, 2010;Teder et al, 2014;Sõber et al, 2019), populations within species (Vellau et al, 2013;Meister et al, 2018), and also seasonal generations (Esperk et al, 2013). Our comparison of seasonal generations in A. levana broadens the view indicating that exceptions to this rule can exist.…”

“…As the perhaps most intuitive explanation, such long developmental periods may be selected for to facilitate physiological preparation for overwintering, whereas warning appearance and group living habits in A. levana-likely reducing larval mortality rates-may be seen as permissive for such a strategy to evolve. As an aside, the conclusion about flexibility of larval growth rates in A. levana is supported also by sex-related difference in this variable, detected in this study (c.f., Tammaru et al, 2010;Stillwell et al, 2014;Sõber et al, 2019).…”

“…A particular question to be answered in studies on such reaction norms is that about the proximate basis of the differences in body sizes. The individuals that ultimately attain larger adult sizes may (1) be larger from the beginning, (2) grow for a longer time, or (3) grow faster (Blanckenhorn, 2000;Sõber et al, 2019). As one particular implication, the answer to this question has direct relevance to the problem of evolutionary ecology of growth rate as a separate life-history trait (Arendt, 1997).…”

Searching for mates may shape the immune response and parental investments

“…Both species in this study developed under the same environmental conditions and their nymphal stage numbers (5) were the same. It has been revealed that there is a positive relationship between longer development period and body size in some insect species (Sõber et al 2019).…”

Nutritional Ecology of the Southern Green Stink Bug <i>Nezara viridula</i> (Hemiptera: Pentatomidae) on Selected Varieties of Cowpea and Tomato

“…Fifth instars of southern green stink bug have previously been reported to cause a greater damage to seeds than any other stage [44]. Females required a longer developmental time probably because they need greater amount and higher quality of nutrients for reproduction than do males [45,46]. Third and fourth instars that fed on pods of MS required a longer developmental time, a phenomenon usually associated with sub-optimal diets.…”