Biology subjects

Emlen, D. J.

Publications and source records attributed to Emlen, D. J..

7 recordsLinked to original sources

bioRxiv2025

Sexual selection and flight as predictors of sexual size and shape dimorphism in stick and leaf insects

Sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) are widespread in animals, yet their macroevolutionary drivers remain poorly understood. We analyzed 196 stick and leaf insect species (Phasmatodea), a clade with exceptional diversity in size, shape, flight capacity, and mating systems, using phylogenetic comparative methods to evaluate the roles of fecundity selection, sexual selection, and ecological factors. SSD was universally female-biased and varied over an order of magnitude. Allometric scaling supported the inverse of Renschs rule, and clade-level patterns matched predictions from quantitative genetic models in which stronger directional selection on females than on males explains female-biased SSD. However, interspecific variation in SSD was not explained by female fecundity. Instead, mating system and flight dimorphism best predicted variation in SSD and SShD: males engaging in short-term mate guarding (less than a few days) were larger and stockier than searching males or those exhibiting prolonged mate guarding, reducing SSD, whereas species with flight-capable males and flightless females showed increased dimorphism, largely due to smaller males. Habitat and climate had limited effects. Sex differences in growth rate and development duration contributed equally to SSD, with females growing faster and for longer, consistent with widespread protandry. These results indicate that while fecundity selection historically drove female-biased SSD, contemporary variation is primarily shaped by male-specific selection through mating system and flight-related ecological pressures.

evolutionary biology↗

bioRxiv2025

Exaggerated male hindlegs function as pure weapons of male-male combat in thorny devil stick insects

Sexually selected weapons can function as both combat tools and agonistic signals of fighting ability, depending on whether and how males assess rivals. We investigated the function of the enlarged male hindlegs in the New Guinean thorny devil stick insects, Eurycantha calcarata in male-male and male-female interactions. Field and lab experiments showed that larger males with proportionally larger hindlegs were more likely to win fights over access to females and subsequently mate. Behavioral sequence analyses and contest cost predictors indicated that males likely use a mutual assessment strategy. Surprisingly, males did not appear to use their hindlegs as signals of fighting ability, relying instead on tactile and chemical cues to assess opponents. Hindlegs were employed only to deliver powerful squeezes in rare, escalated fights. During copulation, males also used hindlegs to stabilize their position, but as females did not appear to resist, we found no evidence for a coercive function. These findings suggest that enlarged male hindlegs in E. calcarata serve purely as force-delivering combat tools rather than signaling structures, even though males assess rivals during contests. These result highlight how understanding the specific functions and contexts of weapon use provides critical insight into the diversification of sexually selected traits. Teaser textSexually selected weapons vary widely in function, serving as signals, combat tools, or both--which shapes their evolution. In the thorny devil stick insect (Eurycantha calcarata), males use enlarged, spined hindlegs solely as force-delivering weapons in direct combat with rivals, not as displays or coercive tools. Males assess opponents using tactile or chemical cues, but their hindlegs scale proportionately with body size, reflecting selection for mechanical performance rather than signaling. These findings highlight that understanding the behavioral context and function of weapon use is essential to explaining their evolutionary diversification.

evolutionary biology↗

bioRxiv2023

Divergence time and environmental similarity predict the strength of morphological convergence in stick and leaf insects

Independent evolution of similar traits in lineages inhabiting similar environments (convergent evolution) is often taken as evidence for adaptation by natural selection, and used to illustrate the predictability of evolution. Yet convergence is rarely perfect. Environments may not be as similar as they appear (e.g., habitats scored the same may be heterogenous to the organisms). And lineages can evolve in different ways even when submitted to the same environmental challenges, because responses to selection are contingent upon available genetic variation and independent lineages may differ in the alleles, genetic backgrounds, and even the developmental mechanisms responsible for the phenotypes in question. Both impediments to convergence are predicted to increase as the length of time separating two lineages increases, making it difficult to discern their relative importance. We quantified environmental similarity and the extent of convergence to show how habitat and divergence time each contribute to observed patterns of morphological evolution in stick and leaf insects (order Phasmatodea). Dozens of phasmid lineages independently colonized similar habitats, repeatedly evolving in parallel directions on a 26-trait morphospace, though the magnitude and direction of these shifts varied. Lineages converging towards more similar environments ended up closer on the morphospace, as did closely related lineages, and closely related lineages followed more parallel trajectories to arrive there. Remarkably, after accounting for habitat similarity, we show that divergence time reduced convergence at a constant rate across more than 60 million years of separation, suggesting even the magnitude of contingency can be predictable, given sufficient spans of time. Significance statementPhasmids (stick and leaf insects) exemplify the extraordinary power of natural selection to shape organismal phenotypes. The animals themselves are charismatic champions of crypsis and masquerade; and our characterization of their adaptive radiation reveals dozens of instances of convergence, as lineages adapted to similar changes in habitat by repeatedly evolving similar body forms. Our findings show that the similarity of environmental conditions experienced by the organisms - the closeness of the invaded niches - and the extent of elapsed time since divergence, both predict the strength of morphological convergence. The phasmid radiation reveals an evolutionary process that is surprisingly predictable, even when lineages have been evolving independently for tens of millions of years.

evolutionary biology↗

bioRxiv2023

Evolution of horn length and lifting strength in the Japanese rhinoceros beetle Trypoxylus dichotomus

Rhinoceros beetle (Trypoxylus dichotomus) males have pitchfork-shaped head horns, which they use to pry rival males from the trunks of trees. In the largest males these horns can be three times the length of horns in the two closest sister species. Because this weapon functions as a lever, longer horns should lift with less force than shorter horns (the paradox of the weakening combatant) unless other elements of the weapon system (e.g., input lever length, muscle mass) evolve to compensate. We used next-generation sequencing approaches to consolidate 23 sample locations into 8 genetically distinguishable populations, reconstructing their historical relationships and providing a comprehensive picture of the evolution of this horn lever system. We show that head horns likely increased in length independently in the Northern and Southern lineages. In both instances this resulted in weaker lifting forces, but this mechanical disadvantage was later ameliorated, to some extent and in some locations, by subsequent reductions to horn length, changes in muscle size, or by an increase in input lever length (head height). Our results reveal an exciting geographic mosaic of differences in weapon size, weapon force, and in the extent and nature of mechanical compensation. Reconstructing the evolution of this weapon system offers critical insights towards meaningfully linking mating system dynamics, selection patterns, and diversity in sexually selected traits.

evolutionary biology↗

bioRxiv2023

DISCO: A deep learning ensemble for uncertainty-aware segmentation of acoustic signals

Recordings of animal sounds enable a wide range of observational inquiries into animal communication, behavior, and diversity. Automated labeling of sound events in such recordings can improve both throughput and reproducibility of analysis. Here, we describe our software package for labeling sound elements in recordings of animal sounds and demonstrate its utility on recordings of beetle courtships and whale songs. The software, DISCO, computes sensible confidence estimates and produces labels with high precision and accuracy. In addition to the core labeling software, it provides a simple tool for labeling training data, and a visual system for analysis of resulting labels. DISCO is open-source and easy to install, it works with standard file formats, and it presents a low barrier of entry to use.

bioinformatics↗

bioRxiv2022

The draft genome sequence of Japanese rhinoceros beetle Trypoxylus dichotomus

Beetles are the largest insect order and one of the most successful animal groups in terms of number of species. The Japanese rhinoceros beetle Trypoxylus dichotomus (Coleoptera, Scarabaeidae, Dynastini) is a giant beetle with distinctive exaggerated horns present on the head and prothoracic regions of the male. T. dichotomus has been used as research model in various fields such as evolutionary developmental biology, ecology, ethology, biomimetics, and drug discovery. In this study, de novo assembly of 615 Mb, representing 80% of the genome estimated by flow cytometry, was obtained using the 10x Chromium platform. The scaffold N50 length of the genome assembly was 8.02 Mb, with repetitive elements predicted to comprise 49.5% of the assembly. In total, 23,987 protein-coding genes were predicted in the genome. In addition, de novo assembly of the mitochondrial genome yielded a contig of 20,217 bp. We also analyzed the transcriptome by generating 16 RNA-seq libraries from a variety of tissues of both sexes and developmental stages, which allowed us to identify 13 co-expressed gene modules. The detailed genomic and transcriptomic information of T. dichotomus is the most comprehensive among those reported for any species of Dynastinae. This genomic information will be an excellent resource for further functional and evolutionary analyses, including the evolutionary origin and genetic regulation of beetle horns and the molecular mechanisms underlying sexual dimorphism.

genomics↗

bioRxiv2021

Multi-modal locomotor costs explain sexual size but not shape dimorphism in a leaf-mimicking insect

In most arthropods, adult females are larger than males, and male competition is a race to quickly locate and mate with scattered females (scramble competition polygyny). In this context, smaller males may be favored due to more efficient locomotion leading to higher mobility during mate searching while larger males may benefit from increased speed and higher survivorship. Understanding how body size affects different aspects of the locomotor performance of males is therefore essential to shed light on the evolution of this widespread mating system. Using a combination of empirical measures of flight performance and substrate adhesion, and modelling of body aerodynamics, we show that large body size impairs both flight and landing (attachment) performance in male leaf insects (Phyllium philippinicum), a species where relatively small and skinny males fly through the canopy in search of large sedentary females. Smaller males were more agile in the air, ascended more rapidly during flight, and had a lower risk of detaching from the substrates on which they walk and land. Our models revealed variation in body shape affected body lift and drag, but tradeoffs with weight meant that effects were negligible, suggesting that flight costs do not explain the evolution of strong sexual dimorphism in body shape in this species. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=177 SRC="FIGDIR/small/451325v1_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@193d0adorg.highwire.dtl.DTLVardef@1693bccorg.highwire.dtl.DTLVardef@1d50b53org.highwire.dtl.DTLVardef@1da8572_HPS_FORMAT_FIGEXP M_FIG C_FIG

ecology↗