Catch‐up growth in the rhinoceros beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae): Smaller neonates gain relatively more body mass during larval development

Abstract

Body size often varies among conspecific neonates. As larger adults generally have higher fitness than smaller conspecifics, it is adaptive for smaller neonates to subsequently gain relatively more size increments during larval development (catch‐up growth). Although catch‐up growth has been suggested in insects, inappropriate methods have been used to examine the size dependence of growth increments. Therefore, it remains unclear to what extent catch‐up growth is common among insects. The present study examined the size dependence of growth increments among larvae of Trypoxylus dichotomus using reduced major axis regression of final to initial body masses. Catch‐up growth was found consistently for larval instars. Furthermore, simulations of the size increments revealed that not only sexual divergence of the mean size, but also catch‐up growth within sexes plays a role in the development of sexual divergence in the body size distribution of T. dichotomus. The significance of catch‐up growth in body size evolution was discussed.

Per‐stage growth rates of head and body sizes in scarab larvae (Coleoptera: Scarabaeidae) decrease across ontogeny following a species‐specific pattern

“…2016; Vendl et al . 2018; Hoshizaki 2019). Such negligence might be valid if body growth characteristics were similar between instars, especially if body size growth rate followed Dyar's Rule irrespective of the insect lineage; however, this condition has not yet been met.…”

“…The insects observed here were a subset of those used in a previous study by the author (for details, see Hoshizaki 2019). The insects were reared at 25 ± 1°C and a light cycle of 15 h light: 9 h dark (LD 15:9).…”

“…The initial and final body masses were the same as previously used (Hoshizaki 2019). The method of measuring the initial and final masses was as follows.…”

“…An overwhelmingly large proportion of the adult body mass in insects is acquired in the last instar, which is why many previous studies focused on this developmental stage. In contrast, body growth studies during early instars remain relatively sparse (Nijhout et al 2006;Tammaru et al 2010;Christiansen 2013;Grunert et al 2015;Pélissié et al 2016;Vendl et al 2018;Hoshizaki 2019). Such negligence might be valid if body growth characteristics were similar between instars, especially if body size growth rate followed Dyar's Rule irrespective of the insect lineage; however, this condition has not yet been met.…”

“…The Japanese rhinoceros beetle (Trypoxylus dichotomus) inhabits lowland forests and farmlands of the main Japanese islands. Examinations of various aspects of larval growth in T. dichotomus reported the condition and size dependencies, development of sexual size dimorphism and timing of metamorphosis (Karino et al 2004;Plaistow et al 2005;Iguchi 2006;Johns et al 2014;Kojima 2015aKojima , 2015bKojima , 2019Hoshizaki 2019).…”