In animals, male weapons such as antlers, horns, spurs, fangs, and tusks typically provide advantages in male contests and increase access to females, thereby enhancing reproductive success. However, such large and extravagant morphological structures are expected to come at a cost, potentially imposing trade-offs with life history traits, physiological functions, or certain behaviors (Emlen, 2001; Emlen, 2008). These costs should be manageable only by males in the best condition. The present study by Blackwell et al. (2024) examines this assumption through a comprehensive study on the European earwig, where males possess forceps-like cerci that vary widely in size within populations.

In the European earwig (Forficula auricularia), male forceps are used in male-male contests as weapons to deter competitors prior to mating (Styrsky & Rhein, 1999) or to interrupt mating pairs by non-copulating males (Forslund, 2000; Walker & Fell, 2001). Despite providing benefits in terms of mating success (Eberhard & Gutierrez, 1991; Tomkins & Brown, 2004), it remains unknown whether long or short forceps are associated with other important life-history traits.

In this laboratory study, Blackwell et al. (2024) investigated two European earwig populations, each divided into two subpopulations: one with the shortest forceps and one with the longest forceps. They examined the potential costs of long forceps on six different traits: one reproductive trait (sperm storage); three non-reproductive behavioral traits such as locomotor performance (involved in search for resources), fleeing reaction face to a risk (long forceps are supposed to be correlated with boldness), and aggregation behavior (European earwigs are facultative group-living organisms); and survival (when deprived of food and subsequently when exposed to an entomopathogenic fungus).

As males in the best condition are supposed to be those that can afford to develop large forceps, Blackwell et al. (2024) predicted that males with long forceps would perform better than those with short forceps across the investigated traits. However, their predictions were not validated, as no correlation between weapon size and male quality was detected in either population. Although the sample size is sometimes limited, the consistency of these results across different populations adds robustness to their conclusions.

By demonstrating that forceps length in the European earwig does not reliably indicate male quality, this paper challenges existing theories and highlights the complexity of evolutionary processes shaping morphological traits. Furthermore, the study raises important questions about the evolutionary mechanisms maintaining weapon size diversity, providing a fresh perspective that could stimulate further research and debate in the field, notably the search for other traits where costs might be incurred.

References

Blackwell, S.E.M., Pasquier, L., Dupont, S., Devers, S., Lécureuil, C. & Meunier, J. (2024). Relationship between weapon size and six key behavioural and physiological traits in males of the European earwig. bioRxiv, ver. 3 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.1101/2024.03.20.585871

Eberhard, W.G., & Gutierrez, E.E. (1991). Male dimorphisms in beetles and earwigs and the question of developmental constraints. Evolution, 45(1), 18–28. https://doi.org/10.2307/2409478

Emlen, D.J. (2001). Costs and the diversification of exaggerated animal structures. Science, 291(5508), 1534–1536. https://doi.org/10.1126/science.1056607

Emlen, D.J. (2008). The evolution of animal weapons. Annual Review of Ecology, Evolution, and Systematics, 39(1), 387–413. https://doi.org/10.1146/annurev.ecolsys.39.110707.173502

Forslund, P. (2000). Male-male competition and large size mating advantage in European earwigs, Forficula auricularia. Animal Behaviour, 59(4), 753–762. https://doi.org/10.1006/anbe.1999.1359

Styrsky, J.D., & Rhein, S.V. (1999). Forceps size does not determine fighting success in European earwigs. Journal of Insect Behavior, 12(4), 475–482. https://doi.org/10.1023/A:1020962606724

Tomkins, J.L., & Brown, G.S. (2004). Population density drives the local evolution of a threshold dimorphism. Nature, 431, 1099–1103. https://doi.org/10.1038/nature02936.1.

Walker, K.A., & Fell, R.D. (2001). Courtship roles of male and female European earwigs, Forficula auricularia L. (Dermaptera: Forficulidae), and sexual use of forceps. Journal of Insect Behavior, 14(1), 1–17. https://doi.org/10.1023/A:1007843227591

It is now widely accepted that anthropogenic climate change is a severe threat to biodiversity, ecosystem function and associated ecosystem services. Assessing the vulnerability of species and predicting their response to future changes has become a priority for environmental biology (Williams et al. 2020).

Over the last few decades, oxygen concentrations in both the open ocean and coastal waters have been declining steadily as the result of multiple anthropogenic activities. This global trends towards hypoxia is expected to continue in the future, causing a host of negative effects on marine ecosystems. Oxygen is indeed crucial to many biological processes in the ocean, and its decrease could have strong impacts on biogeochemical cycles, and therefore on marine productivity and biodiversity (Breitburg et al. 2018).

Whenever facing such drastic environmental changes, all organisms are expected to have some intrinsic ability to adapt. At shorter than evolutionary timescales, ecological plasticity and the eco-physiological processes that sustain it could constitute important adaptive mechanisms (Williams et al. 2020)

Marine sponges seem particularly well-adapted to oxygen deficiency, as some species can survive seasonal anoxia for several months. This paper by Strehlow et al. (2023) examines the mechanisms allowing this exceptional tolerance. Focusing on two species of sponges, they used transcriptomics to assess how gene expression by sponges, by their mitochondria, or by their unique and species-specific microbiome could facilitate this trait. Their results suggest that sponge holobionts maintain metabolic activity under anoxic conditions while displaying shock response, therefore not supporting the hypothesis of sponge dormancy. Furthermore, hypoxia and anoxia seemed to influence gene expression in different ways, highlighting the complexity of sponge response to deoxygenation. As often, their exciting results raise as many questions as they provide answers and pave the way for more research regarding how anoxia tolerance in marine sponges could give them an advantage in future oceanic environmental conditions.

References

Breitburg et al. (2018): Declining oxygen in the global ocean and coastal waters. Science 359, eaam7240. https://doi.org/10.1126/science.aam7240 

Strehlow et al. (2023): Transcriptomic responses of sponge holobionts to in situ, seasonal anoxia and hypoxia. bioRxiv, 2023.02.27.530229, ver. 4 peer-reviewed and recommended by Peer Community in Zoology.  https://doi.org/10.1101/2023.02.27.530229 

Williams et al. (2008) Towards an Integrated Framework for Assessing the Vulnerability of Species to Climate Change. PLOS Biology 6(12): e325. https://doi.org/10.1371/journal.pbio.0060325 

Williams et al. (2020):  Research priorities for natural ecosystems in a changing global climate. Global Change Biology 26: 410–416. https://doi.org/10.1111/gcb.14856 

The sorting of organisms along a fast-slow continuum through correlations between life history traits is a long-standing framework (Stearns 1983) and corresponds to the pace-of-life axis. This axis represents the variation in a continuum of life-history strategies, from fast-reproducing short-lived species to slow-reproducing long-lived species. The pace-of-life axis has been the focus of much research largely in mammals, birds, reptiles and plants but less so in invertebrates (Salguero-Gómez et al. 2016; Araya-Ajoy et al. 2018; Healy et al. 2019; Bakewell et al. 2020). Outcomes from this research have highlighted variation across taxa on this axis and mixed support for, and against, patterns expected of the pace-of-life continuum. Given this, a greater understanding of the variation of the pace-of-life across-, and within, taxa are needed. Indeed, Guicharnard et al. (2023) highlight several points regarding our broader understanding of pace-of-life. In general, invertebrates are poorly represented, the variation of pace-of-life across taxonomic scales is less well understood and the relationship between pace-of-life and dispersal, a key life history, requires more attention. Here, Guicharnard et al. (2023) provide a first attempt at addressing the relationship between dispersal and pace-of-life at different scales.

The authors, under controlled conditions, investigated how life-history traits and effective dispersal covary for 28 lines from five species of endoparasitoid wasps from the genus Trichogramma. At the species level negative correlations were found between development time and fecundity, matching pace-of-life axis predictions. Although this correlation was not found to be significant among lines, within species, a similar pattern of a negative correlation was observed. This outcome matches previous findings that consistent pace-of-life axes become more difficult to find at lower taxonomic levels. Unlike the other life-history traits measured, effective dispersal showed no evidence of differences between species or between lines. The authors also found no correlation between effective dispersal and other-life history traits which suggests no dispersal/life-history syndromes in the species investigated. One aspect that was not assessed was the impact of density dependence on pace-of-life and effective dispersal, largely as this was a first step in assessing relationship of dispersal with pace-of-life at different scales. However, the authors do acknowledge the importance of future studies incorporating density dependence and that such studies could potentially lead to more generalizable understanding of pace-of-life and dispersal within Trichogramma.

A pleasant addition was the link to potential implications for biocontrol. This addition showed an awareness by the authors of how insights into pace-of-life can have an applied component. The results of the study highlighted that selecting for specific lines of a species, to maximise a trait of interest at the cost of another, may not be as effective as selecting different species when implementing biocontrol. This is especially important as often single, established species used in biocontrol are favoured without consideration of the potential of other species which can lead to more efficient biocontrol.    

REFERENCES

Araya-Ajoy, Y.G., Bolstad, G.H., Brommer, J., Careau, V., Dingemanse, N.J. & Wright, J. (2018). Demographic measures of an individual's "pace of life": fecundity rate, lifespan, generation time, or a composite variable? Behavioral Ecology and Sociobiology, 72, 75.
https://doi.org/10.1007/s00265-018-2477-7
 
Bakewell, A.T., Davis, K.E., Freckleton, R.P., Isaac, N.J.B. & Mayhew, P.J. (2020). Comparing Life Histories across Taxonomic Groups in Multiple Dimensions: How Mammal-Like Are Insects? The American Naturalist, 195, 70-81.
https://doi.org/10.1086/706195
 
Guicharnaud, C., Groussier, G., Beranger, E., Lamy, L., Vercken, E. & Dahirel, M. (2023). Life-history traits, pace of life and dispersal among and within five species of Trichogramma wasps: a comparative analysis. bioRxiv, 2023.01.24.525360, ver. 3 peer-reviewed and recommended by Peer Community in Zoology.
https://doi.org/10.1101/2023.01.24.525360
 
Healy, K., Ezard, T.H.G., Jones, O.R., Salguero-Gómez, R. & Buckley, Y.M. (2019). Animal life history is shaped by the pace of life and the distribution of age-specific mortality and reproduction. Nature Ecology & Evolution, 3, 1217-1224.
https://doi.org/10.1038/s41559-019-0938-7
 
Salguero-Gómez, R., Jones, O.R., Jongejans, E., Blomberg, S.P., Hodgson, D.J., Mbeau-Ache, C., et al. (2016). Fast-slow continuum and reproductive strategies structure plant life-history variation worldwide. Proceedings of the National Academy of Sciences, 113, 230-235.
https://doi.org/10.1073/pnas.1506215112
 
Stearns, S.C. (1983). The Influence of Size and Phylogeny on Patterns of Covariation among Life-History Traits in the Mammals. Oikos, 41, 173-187.
https://doi.org/10.2307/3544261

The Western honeybee, Apis mellifera L., is one of the best-studied social insects. It shows a reproductive division of labour, cooperative brood care, and age-related polyethism. Furthermore, honeybees regulate the temperature in the hive. Although bees are invertebrates that are usually ectothermic, this is still true for individual worker bees, but the colony maintains a very narrow range of temperature, especially within the brood nest. This is quite important as the development of individuals is dependent on ambient temperature, with higher temperatures resulting in accelerated development and vice versa. In honeybees, a feedback mechanism couples developmental temperature and the foraging behaviour of the colony and the future population development (Tautz et al., 2003). Bees raised under lower temperatures are more likely to perform in-hive tasks, while bees raised under higher temperatures are better foragers. To maintain optimal levels of worker population growth and foraging rates, it is adaptive to regulate temperature to ensure optimal levels of developing brood. Moreover, this allows honeybees to decouple the internal developmental processes from ambient temperatures enhancing the ecological success of the species. 

In every system of thermoregulation, whether it is endothermic under the utilization of energetic resources as in mammals or the honeybee or ectothermic as in lower vertebrates and invertebrates through differential exposure to varying environmental temperature gradients, there is a need for precision (low variability) and accuracy (hitting the target temperature). However, in honeybees, the temperature is regulated by workers through muscle contraction and fanning of the wings and thus, a higher number of workers could be better at achieving precise and accurate temperature within the brood nest. Alternatively, the amount of brood could trigger responses with more brood available, a need for more precise and accurate temperature control. The authors aimed at testing these two important factors on the precision and accuracy of within-colony temperature regulation by monitoring 28 colonies equipped with temperature sensors for two years (Godeau et al., 2023).

They found that the number of brood cells predicted the mean temperature (accuracy of thermoregulation). Other environmental factors had a small effect. However, the model incorporating these factors was weak in predicting the temperature as it overestimated temperatures in lower ranges and underestimated temperatures in higher ranges. In contrast, the variability of the target temperature (precision of thermoregulation) was positively affected by the external temperature, while all other factors did not show a significant effect. Again, the model was weak in predicting the data. Overall colony size measured in categories of the number of workers and the number of brood cells did not show major differences in variability of the mean temperature, but a slight positive effect for the number of bees on the mean temperature. 

Unfortunately, the temperature was a poor predictor of colony size. The latter is important as the remote control of beehives using Internet of Things (IoT) technologies get more and more incorporated into beekeeping management. These IoT technologies and their success are dependent on good proxies for the control of the status of the colony. Amongst the factors to monitor, the colony size (number of bees and/or amount of brood) is extremely important, but temperature measurements alone will not allow us to predict colony sizes. Nevertheless, this study showed clearly that the number of brood cells is a crucial factor for the accuracy of thermoregulation in the beehive, while ambient temperature affects the precision of thermoregulation. In the view of climate change, the latter factor seems to be important, as more extreme environmental conditions in the future call for measures of mitigation to ensure the proper functioning of the bee colony, including the maintenance of homeostatic conditions inside of the nest to ensure the delivery of the ecosystem service of pollination.

REFERENCES

Godeau U, Pioz M, Martin O, Rüger C, Crauser D, Le Conte Y, Henry M, Alaux C (2023) Brood thermoregulation effectiveness is positively linked to the amount of brood but not to the number of bees in honeybee colonies. EcoEvoRxiv, ver. 5 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.32942/osf.io/9mwye 

Tautz J, Maier S, Claudia Groh C, Wolfgang Rössler W, Brockmann A (2003) Behavioral performance in adult honey bees is influenced by the temperature experienced during their pupal development. PNAS 100: 7343–7347. https://doi.org/10.1073/pnas.1232346100

Human African Trypanosomiasis (HAT), or sleeping sickness, is caused by trypanosome parasites. In sub-Saharan Africa, two forms are present, Trypanosoma brucei gambiense and T. b. rhodesiense, the former responsible for 95% of reported cases. The parasites are transmitted through a vector, Genus Glossina, also known as tsetse, which means fly in Tswana, a language from southern Africa. Through a blood meal, tsetse picks up the parasite from infected humans or animals (in animals, the parasite causes Animal African Trypanosomiasis or nagana disease). Through medical interventions and vector control programs, the burden of the disease has drastically reduced over the past two decades, so the WHO neglected tropical diseases road map targets the interruption of transmission (zero cases) for 2030 (WHO 2022).

Meaningful vector control programs utilize traps for the removal of animals and for surveillance, along with different methods of spraying insecticides. However, in existing HAT risk areas, it will be essential to understand the ecology of the vector species to implement control programs in a way that areas cleared from the vector will not be reinvaded from other populations. Thus, it will be crucial to understand basic population genetics parameters related to population structure and subdivision, migration frequency and distances, population sizes, and the potential for sex-biased dispersal. The authors utilize genotyping using nine highly polymorphic microsatellite markers of samples from Chad collected in differently affected regions and at different time points (Ravel et al., 2023). Two major HAT zones exist that are targeted by vector control programs, namely Madoul and Maro, while two other areas, Timbéri and Dokoutou, are free of trypanosomes. Samples were taken before vector control programs started.

The sex ratio was female-biased, most strongly in Mandoul and Maro, the zones with the lowest population density. This could be explained by resource limitation, which could be the hosts for a blood meal or the sites for larviposition. Limited resources mean that females must fly further, increasing the chance that more females are caught in traps. 

The effective population densities of Mandoul and Maro were low. However, there was a convergence of population density and trapping density, which might be explained by the higher preservation of flies in the high-density areas of Timbéri and Dokoutou after the first round of sampling, which can only be tested using a second sampling. 

The dispersal distances are the highest recorded so far, especially in Mandoul and Maro, with 20-30 km per generation. However, in Timbéri and Dokoutou, which are 50 km apart, very little exchange occurs (approx. 1-2 individuals every six months). A major contributor to this is the massive destruction of habitat that started in the early 1990s and left patchily distributed and fragmented habitats. The Mandoul zone might be safe from reinvasion after eradication, as for a successful re-establishment, either a pair of a female and male or a pregnant female are required. As the trypanosome prevalence amongst humans was 0.02 and of tsetse 0.06 (Ibrahim et al., 2021) before interventions began, medical interventions and vector control might have further reduced these levels, making a reinvasion and subsequent re-establishment of HAT very unlikely. Maro is close to the border of the Central African Republic, and the area has not been well investigated concerning refugee populations of tsetse, which could contribute to a reinvasion of the Maro zone. The higher level of genetic heterogeneity of the Maro population indicates that invasions from neighboring populations are already ongoing. This immigration could also be the reason for not detecting the bottleneck signature in the Maro population. 

The two HAT areas need different levels of attention while implementing vector eradication programs. While Madoul is relatively safe against reinvasion, Maro needs another type of attention, as frequent and persistent immigration might counteract eradication efforts. Thus, it is recommended that continuous tsetse suppression needs to be implemented in Maro.  

This study shows nicely that an in-depth knowledge of the processes within and between populations is needed to understand how these populations behave. This can be used to extrapolate, make predictions, and inform the organisations implementing vector control programs to include valuable adjustments, as in the case of Maro. Such integrative approaches can help prevent the failure of programs, potentially saving costs and preventing infections of humans and animals who might die if not treated.

References

Ibrahim MAM, Weber JS, Ngomtcho SCH, Signaboubo D, Berger P, Hassane HM, Kelm S (2021) Diversity of trypanosomes in humans and cattle in the HAT foci Mandoul and Maro, Southern Chad- Southern Chad-A matter of concern for zoonotic potential? PLoS Neglected Tropical Diseases, 15, e000 323. https://doi.org/10.1371/journal.pntd.0009323

Ravel S, Mahamat MH, Ségard A, Argiles-Herrero R, Bouyer J, Rayaisse JB, Solano P, Mollo BG, Pèka M, Darnas J, Belem AMG, Yoni W, Noûs C, de Meeûs T (2023) Population genetics of Glossina fuscipes fuscipes from southern Chad. Zenodo, ver. 9 peer-reviewed and recommended by PCI Zoology. https://doi.org/10.5281/zenodo.7763870

WHO (2022) Trypanosomiasis, human African (sleeping sickness). https://www.who.int/news-room/fact-sheets/detail/trypanosomiasis-human-african-(sleeping-sickness), retrieved 17. March 2023