Biomedical subjects

Martin Wikelski

Publications and source records attributed to Martin Wikelski.

At least 19 recordsLinked to original sources

PubMed2007

Going wild: what a global small-animal tracking system could do for experimental biologists.

Tracking animals over large temporal and spatial scales has revealed invaluable and spectacular biological information, particularly when the paths and fates of individuals can be monitored on a global scale. However, only large animals (greater than approximately 300 g) currently can be followed globally because of power and size constraints on the tracking devices. And yet the vast majority of animals is small. Tracking small animals is important because they are often part of evolutionary and ecological experiments, they provide important ecosystem services and they are of conservation concern or pose harm to human health. Here, we propose a small-animal satellite tracking system that would enable the global monitoring of animals down to the size of the smallest birds, mammals (bats), marine life and eventually large insects. To create the scientific framework necessary for such a global project, we formed the ICARUS initiative (www.IcarusInitiative.org), the International Cooperation for Animal Research Using Space. ICARUS also highlights how small-animal tracking could address some of the ;Grand Challenges in Environmental Sciences' identified by the US National Academy of Sciences, such as the spread of infectious diseases or the relationship between biological diversity and ecosystem functioning. Small-animal tracking would allow the quantitative assessment of dispersal and migration in natural populations and thus help solve enigmas regarding population dynamics, extinctions and invasions. Experimental biologists may find a global small-animal tracking system helpful in testing, validating and expanding laboratory-derived discoveries in wild, natural populations. We suggest that the relatively modest investment into a global small-animal tracking system will pay off by providing unprecedented insights into both basic and applied nature. Tracking small animals over large spatial and temporal scales could prove to be one of the most powerful techniques of the early 21st century, offering potential solutions to a wide range of biological and societal questions that date back two millennia to the Greek philosopher Aristotle's enigma about songbird migration. Several of the more recent Grand Challenges in Environmental Sciences, such as the regulation and functional consequences of biological diversity or the surveillance of the population ecology of zoonotic hosts, pathogens or vectors, could also be addressed by a global small-animal tracking system. Our discussion is intended to contribute to an emerging groundswell of scientific support to make such a new technological system happen.

Animal Migration↗

PubMed2006

Navigation: bat orientation using Earth's magnetic field.

Bats famously orientate at night by echolocation, but this works over only a short range, and little is known about how they navigate over longer distances. Here we show that the homing behaviour of Eptesicus fuscus, known as the big brown bat, can be altered by artificially shifting the Earth's magnetic field, indicating that these bats rely on a magnetic compass to return to their home roost. This finding adds to the impressive array of sensory abilities possessed by this animal for navigation in the dark.

Animals↗

PubMed2006

Simple rules guide dragonfly migration.

Every year billions of butterflies, dragonflies, moths and other insects migrate across continents, and considerable progress has been made in understanding population-level migratory phenomena. However, little is known about destinations and strategies of individual insects. We attached miniaturized radio transmitters (ca 300 mg) to the thoraxes of 14 individual dragonflies (common green darners, Anax junius) and followed them during their autumn migration for up to 12 days, using receiver-equipped Cessna airplanes and ground teams. Green darners exhibited distinct stopover and migration days. On average, they migrated every 2.9+/-0.3 days, and their average net advance was 58+/-11 km in 6.1+/-0.9 days (11.9+/-2.8 km d-1) in a generally southward direction (186+/-52 degrees). They migrated exclusively during the daytime, when wind speeds were less than 25 km h-1, regardless of wind direction, but only after two nights of successively lower temperatures (decrease of 2.1+/-0.6 degrees C in minimum temperature). The migratory patterns and apparent decision rules of green darners are strikingly similar to those proposed for songbirds, and may represent a general migration strategy for long-distance migration of organisms with high self-propelled flight speeds.

Animal Migration↗

PubMed2006

Contrasting adaptive immune defenses and blood parasite prevalence in closely related Passer sparrows.

Immune system components differ in their functions and costs, and immune defense profiles are likely to vary among species with differing ecologies. We compared adaptive immune defenses in two closely related species that have contrasting inflammatory immune responses, the widespread and abundant house sparrow (Passer domesticus) and the less abundant tree sparrow (Passer montanus). We found that the house sparrow, which we have previously shown mounts weaker inflammatory responses, exhibits stronger adaptive immune defenses, including antibody responses, natural antibody titers, and specific T-cell memory, than the tree sparrow. Conversely, tree sparrows, which mount strong inflammatory responses, also mount stronger nonspecific inflammatory T-cell responses but weaker specific adaptive responses. Prevalence of avian malaria parasite infections, which are controlled by adaptive immune defenses, was higher in the geographically restricted tree sparrow than in the ubiquitous house sparrow. Together these data describe distinct immune defense profiles between two closely related species that differ greatly in numbers and distributions. We suggest that these immunological differences could affect fitness in ways that contribute to the contrasting abundances of the two species in North American and Western Europe.

Analysis of Variance↗

PubMed2006

How and why do insects migrate?

Countless numbers of insects migrate within and between continents every year, and yet we know very little about the ultimate reasons and proximate mechanisms that would explain these mass movements. Here we suggest that perhaps the most important reason for insects to migrate is to hedge their reproductive bets. By spreading their breeding efforts in space and time, insects distribute their offspring over a range of environmental conditions. We show how the study of individual long-distance movements of insects may contribute to a better understanding of migration. In the future, advances in tracking methods may enable the global surveillance of large insects such as desert locusts.

Animal Migration↗

PubMed2006

Investment in immune defense is linked to pace of life in house sparrows.

The evidence for a relationship between life history and immune defense is equivocal, although the basic premise is intuitively appealing: animals that live short lives and reproduce early and rapidly should not waste resources on defenses they might never use. One possible reason for a lack of strong support for this hypothesis could be the inherent complexity of the vertebrate immune system. Indeed, different components of the vertebrate immune system vary in their relative costs and benefits, and therefore only some defenses may complement variation in species' life history. To address this hypothesis, we compared multiple types of immune activity between two populations of house sparrows (Passer domesticus) with distinct life histories, one from Colon, Panama, which lay small clutches over an extended breeding season (i.e., slow-living) and the other from Princeton, New Jersey, which lay larger clutches in a smaller window of time (i.e., fast-living). We expected (a) that more costly types of immune defenses would be stronger in the slow-living sparrows and (2) that the slow-living sparrows would show a greater increase in whole-body energy expenditure after immune challenge compared to their fast-living counterparts. We found that secondary antibody response to a novel antigen was more rapid and energetic investment in immune activity was greater in slow-living sparrows. However, cell-mediated immune activity was more robust in fast-living sparrows, and other measures of defense were not different between populations. These results provide partial support for a relationship between life history and immune defense in this species, but they also indicate that this relationship is not clear-cut. Further study is necessary to identify the influence of other factors, particular pathogen environment during development, on the architecture of the immune system of wild animals.

Animals↗

PubMed2005

Conservation physiology.

Conservation biologists increasingly face the need to provide legislators, courts and conservation managers with data on causal mechanisms underlying conservation problems such as species decline. To develop and monitor solutions, conservation biologists are progressively using more techniques that are physiological. Here, we review the emerging discipline of conservation physiology and suggest that, for conservation strategies to be successful, it is important to understand the physiological responses of organisms to their changed environment. New physiological techniques can enable a rapid assessment of the causes of conservation problems and the consequences of conservation actions.

Adaptation, Physiological↗

PubMed2005

Diurnal and nocturnal differences in hypothalamic-pituitary-adrenal axis function in Galápagos marine iguanas.

Temporal modulation of the stress response is a ubiquitous characteristic of animals. Here, we investigate possible mechanisms underlying daily changes in corticosterone release in an ectotherm model system. Earlier work indicated that free-living Galápagos marine iguanas (Amblyrhynchus cristatus) have lower corticosterone concentrations during the night than during the day. This could result from: (i) a lower circadian secretion of adrenocorticotropic hormone (ACTH) as seen in mammals; (ii) from an increase in corticosterone negative feedback; or (iii) reflect lower metabolic activity during the night when core body temperature falls (from 35 degrees C during the day to as low as 21 degrees C during the night). To begin to distinguish between these three possibilities, exogenous ACTH was used to compare diel differences in adrenocortical tissue responsiveness, and dexamethasone was used to compare diel differences in the efficacy of corticosterone negative feedback. Low levels of exogenous ACTH (30 IU/kg body weight) potently stimulated both daytime and nighttime corticosterone release. Dexamethasone (1 mg/kg) inhibited only daytime, but not nighttime endogenous corticosterone release. Because the response to ACTH was similar between day and night we suggest that a simple lowering of core body temperature cannot explain the nighttime reduction in corticosterone release. However, the failure of negative feedback at night suggests that the response is not equivalent to the controlled downregulation seen in mammals.

Adrenal Cortex↗

PubMed2005

Experimentally increased social competition compromises humoral immune responses in house finches.

Although social behavior can substantially influence an individual's physiology, few studies have examined whether intraspecific competition compromises individual immunocompetence. We experimentally manipulated the intensity of social competition in captive non-breeding house finches (Carpodacus mexicanus) by supplying few (high competition) or many (low competition) feeding sites. We tested whether elevated levels of social competition caused individual changes in aggression rates, humoral immunity, body mass, and baseline and stress-induced corticosterone concentrations. We also examined whether physiological responses to social competition were related to an individual's social status. We found that house finches under high social competition had significantly higher aggression rates, lower antibody responses, and lost more body mass. Within flocks, dominant individuals mounted stronger immune responses in both competition treatments. Our statistical power to detect differences in circulating corticosterone concentrations was low, but we did not find any support for the hypothesis that corticosterone concentrations mediate immunosuppression among or within flocks: baseline and stress-induced corticosterone concentrations did not differ under high and low social competition, were unrelated to individual social status, and did not predict the extent of immunosuppression among individuals. Overall, we documented that two universal components of social behavior, intraspecific competition and social status, modulated the strength of a humoral immune response in house finches.

Adaptation, Physiological↗

PubMed2005

Evolution of body size in Galapagos marine iguanas.

Body size is one of the most important traits of organisms and allows predictions of an individual's morphology, physiology, behaviour and life history. However, explaining the evolution of complex traits such as body size is difficult because a plethora of other traits influence body size. Here I review what we know about the evolution of body size in a group of island reptiles and try to generalize about the mechanisms that shape body size. Galapagos marine iguanas occupy all 13 larger islands in this Pacific archipelago and have maximum island body weights between 900 and 12 000g. The distribution of body sizes does not match mitochondrial clades, indicating that body size evolves independently of genetic relatedness. Marine iguanas lack intra- and inter-specific food competition and predators are not size-specific, discounting these factors as selective agents influencing body size. Instead I hypothesize that body size reflects the trade-offs between sexual and natural selection. We found that sexual selection continuously favours larger body sizes. Large males establish display territories and some gain over-proportional reproductive success in the iguanas' mating aggregations. Females select males based on size and activity and are thus responsible for the observed mating skew. However, large individuals are strongly selected against during El Niño-related famines when dietary algae disappear from the intertidal foraging areas. We showed that differences in algae sward ('pasture') heights and thermal constraints on large size are causally responsible for differences in maximum body size among populations. I hypothesize that body size in many animal species reflects a trade-off between foraging constraints and sexual selection and suggest that future research could focus on physiological and genetic mechanisms determining body size in wild animals. Furthermore, evolutionary stable body size distributions within populations should be analysed to better understand selection pressures on individual body size.

Animals↗

PubMed2005

Stress responses and disease in three wintering house finch (Carpodacus mexicanus) populations along a latitudinal gradient.

In laboratory studies, stress hormones have been shown to impair immune functions, and increase susceptibility to diseases. However, the interactions between stress hormones and disease have rarely been studied in free-ranging populations. In this study, we measured concentrations of the avian stress hormone corticosterone across four winter months (December-March) over two years in three eastern North American house finch populations (Carpodacus mexicanus) along a latitudinal gradient. Because Mycoplasma gallisepticum infections appear in these populations in late winter, we hypothesized that the timing of the disease outbreaks could be mediated by changes in corticosterone concentrations. We found a significant increase in baseline and stress-induced plasma corticosterone concentrations in house finches without Mycoplasma symptoms in late winter; when the prevalence of Mycoplasma infection peaks. We also found that house finches with Mycoplasma symptoms had elevated stress-induced corticosterone concentrations. High baseline concentrations were associated with a low body condition and a high fat load. We found that the relationship between corticosterone concentrations and the latitude of the study population changed between years. The first year, corticosterone concentrations were lowest in the southern latitude, but became higher in the second year when average winter temperatures were low. A causal understanding of the implications for this variation in corticosterone concentrations for Mycoplasma disease dynamics awaits further studies.

Animals↗

PubMed2005

Corticosterone suppresses immune activity in territorial Galápagos marine iguanas during reproduction.

Individuals that display elaborate sexually selected characters often show reduced immune function. According to the immunocompetence handicap hypothesis, testosterone (T) is responsible for this result as it drives the development and maintenance of sexual characters and causes immunosuppression. But glucocorticoids also have strong influences on immune function and may also be elevated in reproductively active males. Here, we compared immune activity using the phytohemagglutinin (PHA) skin test in three discrete groups of male marine iguanas (Amblyrhynchus cristatus): territorials, satellites, and bachelors. Males of these three reproductive phenotypes had indistinguishable T concentrations during the height of the breeding season, but their corticosterone (cort) concentrations, body condition and hematocrit were significantly different. Territorial males, the animals with the most elaborate sexual ornaments and behaviors, had lower immune responses and body condition but higher cort concentrations and hematocrit than satellites or bachelors. To test directly cort's immunosuppressive role, we elevated cort by either restraining animals or additionally injecting cort and compared their PHA swelling response with the response of free-roaming animals. Such experimental elevation of cort significantly decreased immune activity in both restrained and cort-injected animals. Our data show that cort can induce immunosuppression, but they do not support the immunocompetence handicap hypothesis in its narrow sense because T concentrations were not related to immunosuppression.

Animals↗

PubMed2005

House sparrows (Passer domesticus) adjust their social status position to their physiological costs.

For group-living animals, the maintenance of a position in the social hierarchy may be associated with physiological costs such as increased stress and energy expenditure or suppressed immune functions. In this study, we experimentally manipulated the social status of house sparrows so that each bird experienced two social environments in random sequence: being dominant and subordinate. For 14 males, we investigated how corticosterone concentrations, energy expenditure and immune functions were affected by these changes in social status position. We found that the cost of maintaining a social status position differed between individuals and were related to individual body size. Birds with small body size had increased costs in terms of increased stress responses and reduced cell-mediated immune responses while being experimentally kept as dominants, while birds with large body size had increased costs while they were subordinates. We also found that birds with increased energetic and immunological costs as dominants obtained a low status position in the large group, while birds with increased costs as subordinates obtained a high status position in the large group. In summary, we found that the costs associated with the maintenance of social status position differed between individuals and was related to the individuals' body size. Furthermore, in a large group, individuals maintained a social status position that minimized energetic and immunological costs.

Adaptation, Physiological↗

PubMed2005

Steroid hormones and aggression in female Galápagos marine iguanas.

We studied steroid hormone patterns and aggression during breeding in female Galápagos marine iguanas (Amblyrhynchus cristatus). Females display vigorously towards courting males after copulating (female-male aggression), as well as fight for and defend nest sites against other females (female-female aggression). To understand the neuroendocrine basis of this aggressive behavior, we examined changes in testosterone (T), estradiol (E2), corticosterone (CORT), and progesterone (P4) during the mating and nesting periods, and then measured levels in nesting females captured during aggressive interactions. Testosterone reached maximal levels during the mating stage when female-male aggression was most common, and increased slightly, but significantly, during the nesting stage when female-female aggression was most common. However, fighting females had significantly lower T, but higher E2 and P4, than non-fighting females. It remains unclear whether these changes in hormone levels during aggressive interactions are a cause or a consequence of a change in behavior. Our results support the "challenge hypothesis", but suggest that E2 and/or P4 may increase in response to aggressive challenges in females just as T does in males. Females may be rapidly aromatizing T to elevate circulating levels of E2 during aggressive interactions. This hypothesis could explain why non-fighting females had slightly elevated baseline T, but extremely low E2, during stages when aggressive interactions were most common. Although P4 increased rapidly during aggressive encounters, it is unclear whether it acts directly to affect behavior, or indirectly via conversion to E2. The rapid production and conversion of E2 and P4 may be an important mechanism underlying female aggression in vertebrates.

Aggression↗

PubMed2004

Corticosterone suppresses cutaneous immune function in temperate but not tropical House Sparrows, Passer domesticus.

Levels of corticosterone (CORT), the primary avian stress hormone, tend to vary over space and time in passerines, but why this is so remains unclear. One reason may be differential need for immune defense. Typically, sustained high levels of CORT suppress immune activity in vertebrates. Thus, animals living where parasite threats are high might maintain low levels of CORT and mount weak CORT stress responses to ensure that their immune defenses are in a high state of readiness at all times. Here, we addressed this hypothesis by comparing CORT levels in two populations of House Sparrows (Passer domesticus), one from the tropics (Colon, Panama) where parasite threats are high and one from the North-temperate zone (New Jersey, USA) where they are lower. Indeed, we found that House Sparrows from Panama had lower baseline and stress-induced CORT levels than House Sparrows from New Jersey. To more directly test our hypothesis, we artificially elevated CORT (via implant) in both populations of birds, expecting that cutaneous immune activity (induced by phytohemagglutinin (PHA)) would be suppressed as it is in most vertebrates studied to date. Surprisingly, we found that CORT implants did not affect immune function in Panamanian sparrows, while immune function in (non-breeding) New Jersey sparrows was suppressed. This suggests that Panamanian House Sparrows may be immunologically insensitive to CORT, in addition to maintaining low baseline and stress-induced levels of this hormone. We propose that other animals living where disease threats are high may use CORT in a similar way.

Adaptation, Physiological↗

PubMed2004

Immunological investments reflect parasite abundance in island populations of Darwin's finches.

The evolution of parasite resistance can be influenced by the abundance of parasites in the environment. However, it is yet unresolved whether vertebrates change their investment in immune function in response to variation in parasite abundance. Here, we compare parasite abundance in four populations of small ground finches (Geospiza fuliginosa) in the Galapagos archipelago. We predicted that populations exposed to high parasite loads should invest more in immune defence, or alternatively use a different immunological defence strategy. We found that parasite prevalence and/or infection intensity increased with island size. As predicted, birds on large islands had increased concentrations of natural antibodies and mounted a strong specific antibody response faster than birds on smaller islands. By contrast, the magnitude of cell-mediated immune responses decreased with increasing parasite pressure, i.e. on larger islands. The data support the hypothesis that investments into the immune defence are influenced by parasite-mediated selection. Our results are consistent with the hypothesis that different immunological defence strategies are optimal in parasite-rich and parasite-poor environments.

Analysis of Variance↗

PubMed2004

Migratory sleeplessness in the white-crowned sparrow (Zonotrichia leucophrys gambelii).

Twice a year, normally diurnal songbirds engage in long-distance nocturnal migrations between their wintering and breeding grounds. If and how songbirds sleep during these periods of increased activity has remained a mystery. We used a combination of electrophysiological recording and neurobehavioral testing to characterize seasonal changes in sleep and cognition in captive white-crowned sparrows (Zonotrichia leucophrys gambelii) across nonmigratory and migratory seasons. Compared to sparrows in a nonmigratory state, migratory sparrows spent approximately two-thirds less time sleeping. Despite reducing sleep during migration, accuracy and responding on a repeated-acquisition task remained at a high level in sparrows in a migratory state. This resistance to sleep loss during the prolonged migratory season is in direct contrast to the decline in accuracy and responding observed following as little as one night of experimenter-induced sleep restriction in the same birds during the nonmigratory season. Our results suggest that despite being adversely affected by sleep loss during the nonmigratory season, songbirds exhibit an unprecedented capacity to reduce sleep during migration for long periods of time without associated deficits in cognitive function. Understanding the mechanisms that mediate migratory sleeplessness may provide insights into the etiology of changes in sleep and behavior in seasonal mood disorders, as well as into the functions of sleep itself.

Animal Migration↗

PubMed2004

Migrating songbirds recalibrate their magnetic compass daily from twilight cues.

Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. We studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. We exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. We suggest that birds orient with a magnetic compass calibrated daily from twilight cues. This could explain how birds cross the magnetic equator and deal with declination.

Animal Migration↗