Socially transferred materials: why and how to study them

Reviewing the female accessory glands of insects: Where they come from, what they do, and where they are going

Female accessory glands (AGs) are structures associated with the reproductive system of many arthropods. In insects, the glands are mostly related to oviposition by secreting egg coatings and adhesives. Nevertheless, the AGs are versatile concerning their function, and show a high diversity of chemical compounds. Here we aim to review the AGs in insects, highlighting their distribution among the orders, origins, location, chemical nature of the secretions, development, endocrine control, and morphological features. We also propose dividing the glands into epidermal and mesodermal, according to their origin, avoiding the general term "accessory gland" to refer to non-homologous structures. The main functions of the AGs are: I) secretion of the egg coverings, adhesive, and lubricant, II) oviposition pheromones, III) egg fertilization, IV) antibiotic activity, V) nutrient production, VI) anti-desiccation/thermal tolerance. We also discuss the Dufour's and venom glands of Hymenoptera and, based on recent studies, argue that they are not homologous to the AGs of other insects. Given the vast diversity of insect species and reproductive mechanisms, it is quite plausible that unknown functions of the AGs remain to be explored. Finally, the potential use of the glands as targets to control disease vectors, crop pests, and food supply is discussed. We expect our study to guide researchers interested in the function of AGs and how they evolved in the different insect groups.

Ecological change and conflict reduction led to a social circulatory system in ants

Behavioral innovations can be ecologically transformative for lineages that perform them and for their associated communities. Many ecologically dominant, superorganismal, and speciose ant lineages use mouth-to-mouth social regurgitation behavior - stomodeal trophallaxis - to share exogenous and endogenous materials within colonies. This behavior is less common in other species-poor, less cooperative ant lineages. How and why trophallaxis evolved and fixed in only some ant clades remains unclear, and whether this trait could be indicative of superorganismality has yet to be established. Here we show that trophallaxis evolved in two main events, in non-doryline formicoids around 130 Ma and in some ponerines around 90 Ma, lineages that today encompass 86% of all ant species. We found that trophallaxis evolved in lineages that began drinking sugary liquids and that had reduced intra-colonial conflict by constraining worker reproductive potential. Evolution of trophallaxis increased net diversification. Causal models indicate that trophallaxis required low reproductive conflict and contributed to the large colony sizes of the ants that use it. This suggests that the evolution of social regurgitation was enabled by both social conflict reduction and opportunistic inclusion of nectar and honeydew in the ant diet during the shifts in terrestrial ecosystems toward flowering plants.

Integrating computer vision and molecular neurobiology to bridge the gap between behavior and the brain

The past decade of social insect research has seen rapid development in automated behavioral tracking and molecular profiling of the nervous system, two distinct but complementary lines of inquiry into phenotypic variation across individuals, colonies, populations, and species. These experimental strategies have developed largely in parallel, as automated tracking generates a continuous stream of behavioral data, while, in contrast, 'omics-based profiling provides a single 'snapshot' of the brain. Better integration of these approaches applied to studying variation in social behavior will reveal the underlying genetic and neurobiological mechanisms that shape the evolution and diversification of social life. In this review, we discuss relevant advances in both fields and propose new strategies to better elucidate the molecular and behavioral innovations that generate social life.

Revisiting the ecology and evolution of burying beetle behavior (Staphylinidae: Silphinae)

Investigating fundamental processes in biology requires the ability to ground broad questions in species-specific natural history. This is particularly true in the study of behavior because an organism's experience of the environment will influence the expression of behavior and the opportunity for selection. Here, we provide a review of the natural history and behavior of burying beetles of the genus Nicrophorus to provide the groundwork for comparative work that showcases their remarkable behavioral and ecological diversity. Burying beetles have long fascinated scientists because of their well-developed parenting behavior, exhibiting extended post-hatching care of offspring that varies extensively within and across taxa. Despite the burgeoning success of burying beetles as a model system for the study of behavioral evolution, there has not been a review of their behavior, ecology, and evolution in over 25 years. To address this gap, we leverage a developing community of researchers who have contributed to a detailed knowledge of burying beetles to highlight the utility of Nicrophorus for investigating the causes and consequences of social and behavioral evolution.

The Biology and Social Life of Earwigs (Dermaptera)

Earwigs are often known for the forceps-like appendage at the end of their abdomen, urban legends about them crawling into human ears, and their roles as pest and biological control agents. However, they are much less known for their social life. This is surprising, as many of the 1,900 species of earwigs show social behaviors toward eggs, juveniles, and adults. These behaviors typically occur during family and group living, which may be obligatory or facultative, last up to several months, and involve only a few to several hundred related or unrelated individuals. Moreover, many individuals can alternate between solitary and group living during their life cycle, an ability that probably prevailed during the emergence of social life. In this review, I detail the diversity of group living and social behavior in earwigs and show how further developing this knowledge in Dermaptera can improve our general understanding of the early evolution of social life in insects.

Social administration of juvenile hormone to larvae increases body size and nutritional needs for pupation

Social insects often display extreme variation in body size and morphology within the same colony. In many species, adult morphology is socially regulated by workers during larval development. While larval nutrition may play a role in this regulation, it is often difficult to identify precisely what larvae receive from rearing workers, especially when larvae are fed through social regurgitation. Across insects, juvenile hormone is a major regulator of development. In the ant Camponotus floridanus, this hormone is present in the socially regurgitated fluid of workers. We investigated the role the social transfer of juvenile hormone in the social regulation of development. To do this, we administered an artificial regurgitate to larvae through a newly developed handfeeding method that was or was not supplemented with juvenile hormone. Orally administered juvenile hormone increased the nutritional needs of larvae, allowing them to reach a larger size at pupation. Instead of causing them to grow faster, the juvenile hormone treatment extended larval developmental time, allowing them to accumulate resources over a longer period. Handfeeding ant larvae with juvenile hormone resulted in larger adult workers after metamorphosis, suggesting a role for socially transferred juvenile hormone in the colony-level regulation of worker size over colony maturation.

Deciphering the ecoevolutionary recipe of milk microbiomes

In a recent article, Keady et al. analyzed mammalian milk microbiomes across 47 species and found their assembly to be largely determined by stochastic (i.e., random) processes. In many ways, host-associated microbiomes are not random, but random events may have an underappreciated role in microbiome assembly, persistence, and ecology.

What Amphibians Can Teach Us About the Evolution of Parental Care

Parenting is considered a key evolutionary innovation that contributed to the diversification and expansion of vertebrates. However, we know little about how such diversity evolved. Amphibians are an ideal group in which to identify the ecological factors that have facilitated or constrained the evolution of different forms of parental care. Among, but also within, the three amphibian orders-Anura, Caudata, and Gymnophiona-there is a high level of variation in habitat use, fertilization mode, mating systems, and parental sex roles. Recent work using broad phylogenetic, experimental, and physiological approaches has helped to uncover the factors that have selected for the evolution of care and transitions between different forms of parenting. Here, we highlight the exceptional diversity of amphibian parental care, emphasize the unique opportunities this group offers for addressing key questions about the evolution of parenting, and give insights into promising novel directions of research.

The royal food of termites shows king and queen specificity

Society in eusocial insects is based on the reproductive division of labor, with a small number of reproductive individuals supported by a large number of nonreproductive individuals. Because inclusive fitness of all colony members depends on the survival and fertility of reproductive members, sterile members provide royals with special treatment. Here, we show that termite kings and queens each receive special food of a different composition from workers. Sequential analysis of feeding processes demonstrated that workers exhibit discriminative trophallaxis, indicating their decision-making capacity in allocating food to the kings and queens. Liquid chromatography tandem-mass spectrometry analyses of the stomodeal food and midgut contents revealed king- and queen-specific compounds, including diacylglycerols and short-chain peptides. Desorption electrospray ionization mass spectrometry imaging analyses of ¹³C-labeled termites identified phosphatidylinositol and acetyl-l-carnitine in the royal food. Comparison of the digestive tract structure showed remarkable differences in the volume ratio of the midgut-to-hindgut among castes, indicating that digestive division of labor underlies reproductive division of labor. Our demonstration of king- and queen-specific foods in termites provides insight into the nutritional system that underpins the extraordinary reproduction and longevity of royals in eusocial insects.