Cas Eikenaar

Maternal androgens in black-headed gull ( Larus ridibundus ) eggs: consequences for chick development

We tested the hypothesis that mother birds counterbalance the negative effects of hatching asynchrony for later–hatched chicks by increasing the yolk androgen concentrations in consecutive eggs of their clutch. In doing so, they may adaptively tune each offsprings competitive ability and, thus, growth and survival. However, evidence in support of this hypothesis is contradictory. The yolk concentrations of maternal androgens in the eggs of black–headed gulls increase significantly with the laying order of the eggs in a clutch. We experimentally tested the functional consequences of this increase on chick development under natural conditions by injecting eggs with either an oil or androgen solution. We created experimental clutches in which androgen levels either stayed constant or increased with laying order while controlling for differences in egg quality by using only first–laid eggs. We then compared development, growth and survival between these broods. Androgen treatment enhanced embryonic development because androgen–treated eggs hatched half a day earlier than controls, while their size at hatching was similar to oil–treated controls. Androgen treatment did not increase chick survival, but it enhanced growth. Androgen–treated, third–hatched chicks had a higher body mass and longer legs than thirdndash;hatched chicks that hatched from oil–treated eggs. At the same time, growth of first chicks (which were all oil treated) was reduced by the presence of two androgen–treated siblings, suggesting that yolk androgens enhance the competitive ability of later–hatched chicks. Our results support the hypothesis that transfer of different amounts of androgens to the eggs of a clutch is a mechanism by which mothers maximize their reproductive output.

Variation in Testosterone and Corticosterone in Amphibians and Reptiles: Relationships with Latitude, Elevation, and Breeding Season Length

Latitudinal variation in life-history traits has been the focus of numerous investigations, but underlying hormonal mechanisms have received much less attention. Steroid hormones play a central role in vertebrate reproduction and may be associated with life-history trade-offs. Consequently, circulating concentrations of these hormones vary tremendously across vertebrates, yet interspecific geographic variation in male hormone concentrations has been studied in detail only in birds. We here report on such variation in amphibians and reptiles, confirming patterns observed in birds. Using phylogenetic comparative analyses, we found that in amphibians, but not in reptiles, testosterone and baseline corticosterone were positively related to latitude. Baseline corticosterone was negatively related to elevation in amphibians but not in reptiles. For both groups, testosterone concentrations were negatively related to breeding-season length. In addition, testosterone concentrations were positively correlated with baseline corticosterone in both groups. Our findings may best be explained by the hypothesis that shorter breeding seasons increase male-male competition, which may favor increased testosterone concentrations that modulate secondary sexual traits. Elevated energetic demands resulting from greater reproductive intensity may require higher baseline corticosterone. Thus, the positive relationship between testosterone and corticosterone in both groups suggests an energetic demand for testosterone-regulated behavior that is met with increased baseline glucocorticoid concentrations.

The Relationship between Plasma Steroid Hormone Concentrations and the Reproductive Cycle in the Northern Pacific Rattlesnake, Crotalus oreganus

We describe the reproductive cycle of Northern Pacific rattlesnakes (Crotalus oreganus) by quantifying steroid hormone concentrations and observing reproductive behaviors in free-ranging individuals. Additionally, we examined reproductive tissues from museum specimens. Plasma steroid hormone concentrations were quantified for both male and female snakes throughout the active season (March-October). We measured testosterone (T), 5alpha-dihydrotestosterone (DHT), and corticosterone (B) concentrations in both sexes and 17beta-estradiol (E2) and progesterone (P) in females only. We observed reproductive behaviors (e.g., consortship, courtship, and copulation) in the field and measured testis and follicle size in male and female snakes from museum collections to relate steroid hormone concentrations to the timing of reproductive events. Our study revealed that C. oreganus in central California exhibits a bimodal pattern of breeding, with most mating behavior occurring in the spring and some incidences of mating behavior observed in late summer/fall. Each breeding period corresponded with elevated androgen (T or DHT) levels in males. Testes were regressed in the spring when the majority of reproductive behavior was observed in this population, and they reached peak volume in August and September during spermatogenesis. Although we did not detect seasonal variation in female hormone concentrations, some females had high E2 in the spring and fall, coincident with mating and with increased follicle size (indicating vitellogenesis) in museum specimens. Females with high E2 concentrations also had high T and DHT concentrations. Corticosterone concentrations in males and females were not related either to time of year or to concentrations of any other hormones quantified. Progesterone concentrations in females also did not vary seasonally, but this likely reflected sampling bias as females tended to be underground, and thus unobtainable, in summer months when P would be expected to be elevated during gestation. In females, P was positively correlated with T and DHT, and E2 was positively correlated with T.

Testosterone, plumage colouration and extra-pair paternity in male North-American barn swallows.

In most monogamous bird species, circulating testosterone concentration in males is elevated around the social females fertile period. Variation in elevated testosterone concentrations among males may have a considerable impact on fitness. For example, testosterone implants enhance behaviours important for social and extra-pair mate choice. However, little is known about the relationship between natural male testosterone concentration and sexual selection. To investigate this relationship we measured testosterone concentration and sexual signals (ventral plumage colour and tail length), and determined within and extra-pair fertilization success in male North American barn swallows (Hirundo rustica erythrogaster). Dark rusty coloured males had higher testosterone concentrations than drab males. Extra-pair paternity was common (42% and 31% of young in 2009 and 2010, respectively), but neither within- nor extra-pair fertilization success was related to male testosterone concentration. Dark rusty males were less often cuckolded, but did not have higher extra-pair or total fertilization success than drab males. Tail length did not affect within- or extra-pair fertilization success. Our findings suggest that, in North American barn swallows, male testosterone concentration does not play a significant direct role in female mate choice and sexual selection. Possibly plumage colour co-varies with a male behavioural trait, such as aggressiveness, that reduces the chance of cuckoldry. This could also explain why dark males have higher testosterone concentrations than drab males.

Corticosterone and migratory fueling in Northern wheatears facing different barrier crossings

Corticosterone, at baseline and moderately elevated levels, is thought to regulate energy mobilization during the predictable life-history cycle. In birds, corticosterone is known to be moderately elevated during migration, and some experiments on captive, but migratory active birds have shown that exogenous corticosterone can positively affect food intake and fat deposition, i.e. fueling. We present observations which indicate that in wild birds endogenous corticosterone does not promote refueling during migratory stopovers. We took a comparative approach and studied two subspecies of Northern wheatears (Oenanthe oenanthe) during their simultaneous spring stopovers on Helgoland, a small island some 50km off the German coast. In spring O. oenanthe have to travel relatively short distances from Helgoland to their next stopover or breeding sites, whereas Oenanthe leucorhoa face a lengthy overseas journey. Consequently, for their next flight bout leucorhoa wheatears deposit more fuel, more rapidly than oenanthe wheatears. Corticosterone levels, however, were lower in leucorhoa than oenanthe wheatears, contradicting the idea that corticosterone promotes migratory refueling. This finding was solidified by the observation that actual fuel deposition rate was negatively correlated with corticosterone level. We also observed a positive correlation between corticosterone level and fuel stores. Together these findings suggest that, rather than promoting migratory refueling, corticosterone may function as a readiness cue, with levels increasing towards departure from the stopover site.

Food availability and fuel loss predict Zugunruhe

AbstractMigrating birds spend most of their time at stopover sites where they replenish the fuel used during flight, termed refueling. The overall time of migration thus largely depends on the duration of stopovers, and factors shaping stopover duration therefore are of interest. A handful of field studies have shown that the likelihood of departure from stopover sites increases with poor feeding conditions. However, food availability and stopover duration are generally difficult to quantify accurately in the field. Results of fasting-refueling experiments on captive birds using migratory restlessness (Zugunruhe) as a proxy for departure likelihood are mixed. Although Zugunruhe usually decreased with refueling, fasting often failed to increase Zugunruhe. In addition, some experiments lacked randomization. In a fasting-refueling experiment on Northern Wheatears (Oenanthe oenanthe), using birds as their own control in a randomized fashion, we found that fasting increased Zugunruhe, whereas refueling decreased Zugunruhe. These results show that the motivation to migrate, quantified by Zugunruhe, is affected by changes in food availability. Furthermore, Zugunruhe during refueling did not depend on fuel reserves left after fasting, but tended to decrease with the amount of fuel lost during fasting. We discuss why extent of fuel loss may be a better predictor of stopover duration than fuel reserves.ZusammenfassungNahrungsverfügbarkeit und Körpermassenverlust bedingen Zugunruhe Zugvögel verbringen in Rastgebieten während des Zuges die meiste Zeit damit, im Flug vorher verbrauchte Energiereserven wieder aufzufüllen („aufzutanken“). Deshalb hängt die Zuggeschwindigkeit insbesondere von der Dauer der Rastaufenhalte ab, weshalb Faktoren, die diese bestimmen, von großem Interesse sind. Eine Handvoll von Arbeiten hat gezeigt, dass die Wahrscheinlichkeit, ein Rastgebiet wieder zu verlassen, ansteigt, wenn die Nahrungsbedingungen schlecht sind. Allerdings ist die Erfassung von Nahrungsverfügbarkeit und Rastdauer im Freiland ganz allgemein schwierig zu quantifizieren. Sog. „Fasten-Auftanken“-Experimente („fasting-refuelling“) an gekäfigten Vögeln, die Zugunruhe als Merkmal für Abflugwahrscheinlichkeit nutzen, ergaben unterschiedliche Ergebnisse. Auch wenn die Zugunruhe üblicherweise abnahm, sobald die Vögel nach Fasten wieder Futter bekamen, erhöhte das Fasten selbst die Zugunruhe meist nicht. Zudem mangelt es manchen dieser Studien an fehlender Randomisierung. In einem „fasting-refuelling“Experiment mit Steinschmätzern (Oenanthe oenanthe), in dem wir den einzelnen Vogel randomisiert als seine eigene Kontrolle nutzten, fanden wir, dass während des Fasten die Zugunruhe zunahm, während sie bei erneuter Massenzunahme („refuelling“) abnahm. Dies zeigt, dass die Motivation zu ziehen, ausgedrückt über die gemessene Zugunruhe, von der Nahrungsverfügbarkeit bestimmt ist. Weiterhin war die Zugunruhe während des „refuelling“nicht von der nach dem Fasten noch restlichen Energiemenge abhängig. Vielmehr nahm sie mit dem Ausmaß des Energieverlustes während des Fastens ab. In der Diskussion erörtern wir, warum der Verlust an „Treibstoff“eine bessere Vorhersage der Rastdauer erlaubt als der Energievorrat selbst.

Corticosterone, food intake and refueling in a long-distance migrant.

Elevated baseline corticosterone levels function to mobilize energy in predictable life-history stages, such as bird migration. At the same time, baseline corticosterone has a permissive effect on the accumulation of fat stores (fueling) needed for migratory flight. Most migrants alternate flight bouts with stopovers, during which they replenish the fuel used during the preceding flight (refueling). The role of corticosterone in refueling is currently unclear. In a fasting-re-feeding experiment on northern wheatears (Oenanthe oenanthe) in autumn we found that baseline total and free corticosterone levels were negatively related with both food intake and the rate of fuel deposition after fasting. This confirms our earlier findings in wild conspecifics in spring and indicates that corticosterone does not stimulate stopover refueling. Whether the negative relationship between baseline corticosterone level and fuel deposition rate is causal is questionable, because within-individual comparison of corticosterone metabolite levels in droppings did not reveal differences between refueling and control periods. In other words, corticosterone does not appear to be down-regulated during refueling, which would be expected if it directly hampers refueling. We discuss possible correlates of corticosterone level that may explain the negative association between corticosterone and stopover refueling. Additionally, we found that fasting decreases total corticosterone level, which contrasts with previous studies. We propose that the difference is due to the other studies being conducted outside of the migration life-history stage, and provide a possible explanation for the decrease in corticosterone during fasting in migrating birds.

Natal dispersal patterns are not associated with inbreeding avoidance in the Seychelles Warbler

In this study, we test whether patterns of territory inheritance, social mate choice and female‐biased natal dispersal act as inbreeding avoidance mechanisms in the cooperatively breeding Seychelles warbler. Our results show that Seychelles warblers do not reduce the likelihood of inbreeding by avoiding related individuals as mates. The occurrence of natural and experimentally induced territory inheritance did not depend on whether the remaining breeder was a parent of the potential inheritor or an unrelated breeder. Furthermore, dispersing individuals were no less related to their eventual mates than expected given the pool of candidates they could mate with. The female bias in natal dispersal distance observed in the Seychelles warbler does not facilitate inbreeding avoidance because, contrary to our prediction, there was no sex difference in the clustering of related opposite sex breeders around the natal territories of dispersers. As a result, the chance of females mating with relatives was not reduced by their greater dispersal distance compared with that of males.

How do energy stores and changes in these affect departure decisions by migratory birds? A critical view on stopover ecology studies and some future perspectives

In birds, accumulating energy is far slower than spending energy during flight. During migration, birds spend, therefore, most of the time at stopover refueling energy used during the previous flight. This elucidates why current energy stores and actual rate of accumulating energy are likely crucial factors influencing bird’s decision when to resume migration in addition to other intrinsic (sex, age) and extrinsic (predation, weather) factors modulating the decision within the innate migration program. After first summarizing how energy stores and stopover durations are generally determined, we critically review that high-energy stores and low rates of accumulating energy were significantly related to high departure probabilities in several bird groups. There are, however, also many studies showing no effect at all. Recent radio-tracking studies highlighted that migrants leave a site either to resume migration or to search for a better stopover location, so-called “landscape movements”. Erroneously treating such movements as departures increases the likelihood of type II errors which might mistakenly suggest no effect of either trait on departure. Furthermore, we propose that energy loss during the previous migratory flight in relation to bird’s current energy stores and migration strategy significantly affects its urge to refuel and hence its departure decision.

Sex biased natal dispersal is not a fixed trait in a stable population of Seychelles warblers

We observed a change in the sex-specific rate of delayed natal dispersal in a stable population of Seychelles warblers over a period of 20 years. At first, females were more likely to delay dispersal in their first year of life than were males, whereas later there was no sex bias in the rate of delayed natal dispersal. Similarly, the female-bias in helping-at-the-nest and the male-bias in floating have also weakened over time. These changes may have resulted from the decrease in variation in territory quality observed in the population over the study period. Our findings strengthen the view that natal dispersal is a highly plastic response to local ecological and social circumstances, and clearly show that rates of sex-biased dispersal cannot be considered a species or population constant. Our study also highlights the importance of collecting long-term datasets to understand complex behaviour such as natal dispersal.