1
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Gálvez D. Ecology of fear: predator avoidance reduces seed dispersal in an ant. R Soc Open Sci 2023; 10:230530. [PMID: 37476511 PMCID: PMC10354471 DOI: 10.1098/rsos.230530] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/22/2023] [Accepted: 05/23/2023] [Indexed: 07/22/2023]
Abstract
The ecology of fear refers to the non-fatal cost that predators and parasites impose on prey populations. These non-consumptive effects (NCEs) can influence animal-plant interactions, but evidence thereof comes mainly from vertebrate systems with less focus on invertebrates. Here, I investigated whether the foraging behaviour of the ant Ectatomma ruidum was influenced by its primary predator, the forest toad Rhinella alata. In field tests, the probability of seed removal by the ants was 25% for seeds placed with the forest toad compared to 32% for control seeds, suggesting that toads reduce ant foraging rates. A further experiment revealed that ants which had previously encountered the predator and its faeces were more likely (59%) than inexperienced ants (50%) to avoid the exit with the predator faeces. This outcome suggests that ants are capable of learning cues associated with predation risk, possibly leading to NCEs. This indicates that predators can exert NCEs on invertebrate prey with potential cascading effects on seed dispersal, extending results previously seen only in vertebrate seed dispersal systems.
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Affiliation(s)
- Dumas Gálvez
- Coiba Scientific Station, City of Knowledge, Calle Gustavo Lara, Boulevard 145B, Clayton 0843-01853, Panama
- Smithsonian Tropical Research Institute, Panamá PO Box 0843-03092, Balboa, Ancón, Panama
- Programa Centroamericano de Maestría en Entomología, Universidad de Panamá, Estafeta universitaria, Avenida Simón Bolívar, 0824 Panama City, Panama
- Sistema Nacional de Investigación, Panama City, Panama
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2
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Adamo S. The Integrated Defense System: Optimizing Defense against Predators, Pathogens, and Poisons. Integr Comp Biol 2022; 62:1536-1546. [PMID: 35511215 DOI: 10.1093/icb/icac024] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2022] [Revised: 03/04/2022] [Accepted: 04/28/2022] [Indexed: 01/05/2023] Open
Abstract
Insects, like other animals, have evolved defense responses to protect against predators, pathogens, and poisons (i.e., toxins). This paper provides evidence that these three defense responses (i.e., fight-or-flight, immune, and detoxification responses) function together as part of an Integrated Defense System (IDS) in insects. The defense responses against predators, pathogens, and poisons are deeply intertwined. They share organs, resources, and signaling molecules. By connecting defense responses into an IDS, animals gain flexibility, and resilience. Resources can be redirected across fight-or-flight, immune, and detoxification defenses to optimize an individual's response to the current challenges facing it. At the same time, the IDS reconfigures defense responses that are losing access to resources, allowing them to maintain as much function as possible despite decreased resource availability. An IDS perspective provides an adaptive explanation for paradoxical phenomena such as stress-induced immunosuppression, and the observation that exposure to a single challenge typically leads to an increase in the expression of genes for all three defense responses. Further exploration of the IDS will require more studies examining how defense responses to a range of stressors are interconnected in a variety of species. Such studies should target pollinators and agricultural pests. These studies will be critical for predicting how insects will respond to multiple stressors, such as simultaneous anthropogenic threats, for example, climate change and pesticides.
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Affiliation(s)
- Shelley Adamo
- Department of Psychology and Neuroscience, Dalhousie University, Halifax, NS B3H 4R2, Canada
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3
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Breiner DJ, Whalen MR, Worthington AM. The developmental high wire: Balancing resource investment in immunity and reproduction. Ecol Evol 2022; 12:e8774. [PMID: 35414895 PMCID: PMC8986548 DOI: 10.1002/ece3.8774] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2021] [Revised: 03/03/2022] [Accepted: 03/10/2022] [Indexed: 11/15/2022] Open
Abstract
The strategic allocation of resources into immunity poses a unique challenge for individuals, where infection at different stages of development may result in unique trade‐offs with concurrent physiological processes or future fitness‐enhancing traits. Here, we experimentally induced an immune challenge in female Gryllus firmus crickets to test whether illness at discrete life stages differentially impacts fitness. We injected heat‐killed Serratia marcescens bacteria into antepenultimate juveniles, penultimate juveniles, sexually immature adults, and sexually mature adults, and then measured body growth, instar duration, mating rate, viability of stored sperm, egg production, oviposition rate, and egg viability. Immune activation significantly impacted reproductive traits, where females that were immune challenged as adults had decreased mating success and decreased egg viability compared to healthy individuals or females that were immune challenged as juveniles. Although there was no effect of an immune challenge on the other traits measured, the stress of handling resulted in reduced mass gain and smaller adult body size in females from the juvenile treatments, and females in the adult treatments suffered from reduced viability of sperm stored within their spermatheca. In summary, we found that an immune challenge does have negative impacts on reproduction, but also that even minor acute stressors can have significant impacts on fitness‐enhancing traits. These findings highlight that the factors affecting fitness can be complex and at times unpredictable, and that the consequences of illness are specific to when during an individual's life an immune challenge is induced.
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Affiliation(s)
- Daniel J. Breiner
- Department of Biological Sciences Creighton University Omaha Nebraska USA
| | - Matthew R. Whalen
- Department of Biological Sciences Creighton University Omaha Nebraska USA
- Department of Psychology University of Michigan Ann Arbor Michigan USA
| | - Amy M. Worthington
- Department of Biological Sciences Creighton University Omaha Nebraska USA
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4
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Krams R, Munkevics M, Popovs S, Dobkeviča L, Willow J, Contreras Garduño J, Krama T, Krams IA. Ecological Stoichiometry of Bumblebee Castes, Sexes, and Age Groups. Front Physiol 2021; 12:696689. [PMID: 34721052 PMCID: PMC8548625 DOI: 10.3389/fphys.2021.696689] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2021] [Accepted: 09/22/2021] [Indexed: 11/13/2022] Open
Abstract
Ecological stoichiometry is important for revealing how the composition of chemical elements of organisms is influenced by their physiological functions and ecology. In this study, we investigated the elemental body composition of queens, workers, and males of the bumblebee Bombus terrestris, an important pollinator throughout Eurasia, North America, and northern Africa. Our results showed that body elemental content differs among B. terrestris castes. Young queens and workers had higher body nitrogen concentration than ovipositing queens and males, while castes did not differ significantly in their body carbon concentration. Furthermore, the carbon-to-nitrogen ratio was higher in ovipositing queens and males. We suggest that high body nitrogen concentration and low carbon-to-nitrogen ratio in young queens and workers may be related to their greater amount of flight muscles and flight activities than to their lower stress levels. To disentangle possible effects of stress in the agricultural landscape, further studies are needed to compare the elemental content of bumblebee bodies between natural habitats and areas of high-intensity agriculture.
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Affiliation(s)
- Ronalds Krams
- Chair of Plant Health, Estonian University of Life Sciences, Tartu, Estonia.,Department of Biotechnology, Daugavpils University, Daugavpils, Latvia
| | - Māris Munkevics
- Department of Biotechnology, Daugavpils University, Daugavpils, Latvia.,Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga, Latvia
| | - Sergejs Popovs
- Department of Biotechnology, Daugavpils University, Daugavpils, Latvia
| | - Linda Dobkeviča
- Department of Environmental Science, Faculty of Geography and Earth Sciences, University of Latvia, Riga, Latvia
| | - Jonathan Willow
- Chair of Plant Health, Estonian University of Life Sciences, Tartu, Estonia
| | - Jorge Contreras Garduño
- Escuela Nacional de Estudios Superiores Unidad Morelia, Universidad Nacional Autónoma de México, Morelia, Mexico
| | - Tatjana Krama
- Chair of Plant Health, Estonian University of Life Sciences, Tartu, Estonia.,Department of Biotechnology, Daugavpils University, Daugavpils, Latvia
| | - Indrikis A Krams
- Department of Biotechnology, Daugavpils University, Daugavpils, Latvia.,Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga, Latvia.,Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
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5
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Neuhäuser M, Mackowiak MM, Ruxton GD. Unequal sample sizes according to the square‐root allocation rule are useful when comparing several treatments with a control. Ethology 2021. [DOI: 10.1111/eth.13230] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Markus Neuhäuser
- Department of Mathematics and Technology RheinAhrCampus Koblenz University of Applied Sciences Remagen Germany
| | - Malwina M. Mackowiak
- Department of Mathematics and Technology RheinAhrCampus Koblenz University of Applied Sciences Remagen Germany
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6
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Sievert T, Ylönen H, Blande JD, Saunier A, van der Hulst D, Ylönen O, Haapakoski M. Bank vole alarm pheromone chemistry and effects in the field. Oecologia 2021; 196:667-677. [PMID: 34173057 PMCID: PMC8292297 DOI: 10.1007/s00442-021-04977-w] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2021] [Accepted: 06/19/2021] [Indexed: 11/29/2022]
Abstract
Chemical communication plays an important role in mammalian life history decisions. Animals send and receive information based on body odour secretions. Odour cues provide important social information on identity, kinship, sex, group membership or genetic quality. Recent findings show, that rodents alarm their conspecifics with danger-dependent body odours after encountering a predator. In this study, we aim to identify the chemistry of alarm pheromones (AP) in the bank vole, a common boreal rodent. Furthermore, the vole foraging efficiency under perceived fear was measured in a set of field experiments in large outdoor enclosures. During the analysis of bank vole odour by gas chromatography–mass spectrometry, we identified that 1-octanol, 2-octanone, and one unknown compound as the most likely candidates to function as alarm signals. These compounds were independent of the vole’s sex. In a field experiment, voles were foraging less, i.e. they were more afraid in the AP odour foraging trays during the first day, as the odour was fresh, than in the second day. This verified the short lasting effect of volatile APs. Our results clarified the chemistry of alarming body odour compounds in mammals, and enhanced our understanding of the ecological role of AP and chemical communication in mammals.
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Affiliation(s)
- Thorbjörn Sievert
- Department of Biological and Environmental Science, Konnevesi Research Station, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland.
| | - Hannu Ylönen
- Department of Biological and Environmental Science, Konnevesi Research Station, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
| | - James D Blande
- Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, 70211, Kuopio, Finland
| | - Amélie Saunier
- Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, 70211, Kuopio, Finland
| | - Dave van der Hulst
- Environmental Sciences Department, Resource Ecology Group, Wageningen University, 6700 AA, Wageningen, Netherlands
| | - Olga Ylönen
- Department of Biological and Environmental Science, Konnevesi Research Station, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
| | - Marko Haapakoski
- Department of Biological and Environmental Science, Konnevesi Research Station, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
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7
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Krams R, Krama T, Munkevics M, Eichler S, Butler DM, Dobkeviča L, Jõers P, Contreras-GarduÑo J, Daukšte J, Krams IA. Spider odors induce stoichiometric changes in fruit fly Drosophila melanogaster. Curr Zool 2020; 67:127-129. [PMID: 33654497 PMCID: PMC7901749 DOI: 10.1093/cz/zoaa070] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2020] [Accepted: 10/21/2020] [Indexed: 12/28/2022] Open
Affiliation(s)
- Ronalds Krams
- Plant Health, Estonian University of Life Sciences, Tartu 51006, Estonia.,Department of Biotechnology, Daugavpils University, Daugavpils 5401, Latvia
| | - Tatjana Krama
- Plant Health, Estonian University of Life Sciences, Tartu 51006, Estonia.,Department of Biotechnology, Daugavpils University, Daugavpils 5401, Latvia.,Institute of Ecology and Earth Sciences, University of Tartu, Tartu 51010, Estonia
| | - Māris Munkevics
- Department of Biotechnology, Daugavpils University, Daugavpils 5401, Latvia.,Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga 1004, Latvia
| | - Sarah Eichler
- Department of Biological Sciences, Kent State University, Ohio, Salem 44460, USA
| | - David M Butler
- Department of Plant Sciences, University of Tennessee, Knoxville 37996-4561, USA
| | - Linda Dobkeviča
- Department of Environmental Science, Faculty of Geography and Earth Sciences, University of Latvia, Riga 1004, Latvia
| | - Priit Jõers
- Department of General and Microbial Biochemistry, University of Tartu, Tartu 51010, Estonia
| | - Jorge Contreras-GarduÑo
- Escuela Nacional de Estudios Superiores Unidad Morelia, Universidad Nacional Autónoma de México, Morelia 58190, Mexico
| | - Janīna Daukšte
- Institute of Food Safety, Animal Health and Environment BIOR, Riga 1076, Latvia
| | - Indrikis A Krams
- Institute of Ecology and Earth Sciences, University of Tartu, Tartu 51010, Estonia.,Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga 1004, Latvia.,Institute of Food Safety, Animal Health and Environment BIOR, Riga 1076, Latvia.,Department of Psychology, University of Tennessee, Knoxville 37996-4561, USA
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8
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Adamo SA. Animals have a Plan B: how insects deal with the dual challenge of predators and pathogens. J Comp Physiol B 2020; 190:381-90. [PMID: 32529590 DOI: 10.1007/s00360-020-01282-5] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2020] [Revised: 04/08/2020] [Accepted: 04/27/2020] [Indexed: 12/15/2022]
Abstract
When animals are faced with a life-threatening challenge, they mount an organism-wide response (i.e. Plan A). For example, both the stress response (i.e. fight-or-flight) and the immune response recruit molecular resources from other body tissues, and induce physiological changes that optimize the body for defense. However, pathogens and predators often co-occur. Animals that can optimize responses for a dual challenge, i.e. simultaneous predator and pathogen attacks, will have a selective advantage. Responses to a combined predator and pathogen attack have not been well studied, but this paper summarizes the existing literature in insects. The response to dual challenges (i.e. Plan B) results in a suite of physiological changes that are different from either the stress response or the immune response, and is not a simple summation of the two. It is also not a straight-forward trade-off of one response against the other. The response to a dual challenge (i.e. Plan B) appears to resolve physiological trade-offs between the stress and immune responses, and reconfigures both responses to provide the best overall defense. However, the dual response appears to be more costly than either response occurring singly, resulting in greater damage from oxidative stress, reduced growth rate, and increased mortality.
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9
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Cinel SD, Hahn DA, Kawahara AY. Predator-induced stress responses in insects: A review. J Insect Physiol 2020; 122:104039. [PMID: 32113954 DOI: 10.1016/j.jinsphys.2020.104039] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/08/2017] [Revised: 04/26/2018] [Accepted: 02/26/2020] [Indexed: 06/10/2023]
Abstract
Predators can induce extreme stress and profound physiological responses in prey. Insects are the most dominant animal group on Earth and serve as prey for many different predators. Although insects have an extraordinary diversity of anti-predator behavioral and physiological responses, predator-induced stress has not been studied extensively in insects, especially at the molecular level. Here, we review the existing literature on physiological predator-induced stress responses in insects and compare what is known about insect stress to vertebrate stress systems. We conclude that many unrelated insects share a baseline pathway of predator-induced stress responses that we refer to as the octopamine-adipokinetic hormone (OAH) axis. We also present best practices for studying predator-induced stress responses in prey insects. We encourage investigators to compare neurophysiological responses to predator-related stress at the organismal, neurohormonal, tissue, and cellular levels within and across taxonomic groups. Studying stress-response variation between ecological contexts and across taxonomic levels will enable the field to build a holistic understanding of, and distinction between, taxon- and stimulus-specific responses relative to universal stress responses.
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Affiliation(s)
- Scott D Cinel
- Department of Biology, University of Florida, Gainesville, FL 32611, USA; Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA.
| | - Daniel A Hahn
- Department of Entomology & Nematology, University of Florida, Gainesville, FL 32611, USA
| | - Akito Y Kawahara
- Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA
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10
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Sievert T, Haapakoski M, Palme R, Voipio H, Ylönen H. Secondhand horror: effects of direct and indirect predator cues on behavior and reproduction of the bank vole. Ecosphere 2019. [DOI: 10.1002/ecs2.2765] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Affiliation(s)
- Thorbjörn Sievert
- Department of Biological and Environmental Science Konnevesi Research Station University of Jyväskylä P.O. Box 35 40014 Jyväskylä Finland
| | - Marko Haapakoski
- Department of Biological and Environmental Science Konnevesi Research Station University of Jyväskylä P.O. Box 35 40014 Jyväskylä Finland
| | - Rupert Palme
- Department of Biomedical Sciences University of Veterinary Medicine Veterinärplatz 1 Vienna Austria
| | - Helinä Voipio
- Faculty of Biological and Environmental Sciences University of Helsinki Viikinkaari 1, P.O. Box 65 Helsinki Finland
| | - Hannu Ylönen
- Department of Biological and Environmental Science Konnevesi Research Station University of Jyväskylä P.O. Box 35 40014 Jyväskylä Finland
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11
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Trakimas G, Krams R, Krama T, Kortet R, Haque S, Luoto S, Eichler Inwood S, Butler DM, Jõers P, Hawlena D, Rantala MJ, Elferts D, Contreras-Garduño J, Krams I. Ecological Stoichiometry: A Link Between Developmental Speed and Physiological Stress in an Omnivorous Insect. Front Behav Neurosci 2019; 13:42. [PMID: 30906256 PMCID: PMC6419478 DOI: 10.3389/fnbeh.2019.00042] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2018] [Accepted: 02/14/2019] [Indexed: 12/01/2022] Open
Abstract
The elemental composition of organisms belongs to a suite of functional traits that may adaptively respond to fluctuating selection pressures. Life history theory predicts that predation risk and resource limitations impose selection pressures on organisms’ developmental time and are further associated with variability in energetic and behavioral traits. Individual differences in developmental speed, behaviors and physiology have been explained using the pace-of-life syndrome (POLS) hypothesis. However, how an organism’s developmental speed is linked with elemental body composition, metabolism and behavior is not well understood. We compared elemental body composition, latency to resume activity and resting metabolic rate (RMR) of western stutter-trilling crickets (Gryllus integer) in three selection lines that differ in developmental speed. We found that slowly developing crickets had significantly higher body carbon, lower body nitrogen and higher carbon-to-nitrogen ratio than rapidly developing crickets. Slowly developing crickets had significantly higher RMR than rapidly developing crickets. Male crickets had higher RMR than females. Slowly developing crickets resumed activity faster in an unfamiliar relative to a familiar environment. The rapidly developing crickets did the opposite. The results highlight the tight association between life history, physiology and behavior. This study indicates that traditional methods used in POLS research should be complemented by those used in ecological stoichiometry, resulting in a synthetic approach that potentially advances the whole field of behavioral and physiological ecology.
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Affiliation(s)
- Giedrius Trakimas
- Institute of Biosciences, Vilnius University, Vilnius, Lithuania.,Department of Biotechnology, Daugavpils University, Daugavpils, Latvia
| | - Ronalds Krams
- Department of Biotechnology, Daugavpils University, Daugavpils, Latvia.,Department of Plant Protection, Estonian University of Life Sciences, Tartu, Estonia
| | - Tatjana Krama
- Department of Biotechnology, Daugavpils University, Daugavpils, Latvia.,Department of Plant Protection, Estonian University of Life Sciences, Tartu, Estonia
| | - Raine Kortet
- Department of Environmental and Biological Sciences, University of Eastern Finland, Joensuu, Finland
| | - Shahi Haque
- Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
| | - Severi Luoto
- English, Drama and Writing Studies, University of Auckland, Auckland, New Zealand.,School of Psychology, University of Auckland, Auckland, New Zealand
| | - Sarah Eichler Inwood
- The Bredesen Center, Energy Science and Engineering, University of Tennessee, Knoxville, TN, United States
| | - David M Butler
- Department of Plant Sciences, University of Tennessee, Knoxville, TN, United States
| | - Priit Jõers
- Department of General and Microbial Biochemistry, University of Tartu, Tartu, Estonia
| | - Dror Hawlena
- Department of Ecology, Evolution and Behavior, the Alexander Silberman Institute of Life Sciences, the Hebrew University of Jerusalem, Jerusalem, Israel
| | - Markus J Rantala
- Department of Biology and Turku Brain and Mind Centre, University of Turku, Turku, Finland
| | - Didzis Elferts
- Department of Botany and Ecology, Faculty of Biology, University of Latvia, Riga, Latvia
| | - Jorge Contreras-Garduño
- Ecuela Nacional de Estudios Superiores Unidad Morelia, Universidad Nacional Autónoma de México, Morelia, Mexico
| | - Indrikis Krams
- Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia.,Department of Psychology, University of Tennessee, Knoxville, TN, United States.,Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga, Latvia
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12
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Abstract
Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold.
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