Inter-Individual Variation in Anti-Parasitic Egg Rejection Behavior: A Test of the Maternal Investment Hypothesis.

Hosts of avian brood parasites may reduce or forego the costs of caring for foreign young by rejecting parasitic eggs from the nest. Yet, many host species accept parasitic eggs and, even among rejecter species, some individuals go on to incubate and hatch them. The factors explaining the variation in egg rejection between species have received much theoretical and empirical attention, but the causes of intraspecific variation in different individuals' propensity for accepting parasitic eggs are less well understood. Here we tested the maternal investment hypothesis, which predicts that hosts with costlier clutches will be more likely to reject parasitic eggs from their nest. We studied variation in the egg rejection responses of American robins (Turdus migratorius), a robust egg-rejecter host of the brood parasitic brown-headed cowbird (Molothrus ater), to 3D-printed cowbird-sized eggs which were painted dark blue, a color known to induce variable and repeatable egg rejection responses in individual robins. Costlier clutch investment was estimated by earlier laying date, larger clutch size, heavier unincubated yolk mass, and variable yolk steroid hormone concentrations. There was no statistical support for most of our predictions. However, we detected more concentrated and greater overall amount of deoxycorticosterone deposited in egg yolks of rejecters relative to acceptors, although this accounted for no more than 14% of variance in the data. Future work should test experimentally the potential physiological linkage between maternal egg yolk steroid investment and egg rejection propensity in this and other host species of avian brood parasites.

Egyedek közti variáció az antiparazitikus tojás-diszkriminációban: az anyai befektetés hipotézis tesztelése Kivonat A költésparazita madárfajok gazdái csökkenthetik vagy megszüntethetik az idegen fióka nevelésének költségeit úgy, hogy eltávolítják a fészekbol a paraziták tojásait. Ennek ellenére több gazdafaj is elfogadja a parazita tojásokat, sot az elutasító gazdafajok körében is akadnak olyan egyedek, amelyek elfogadják és kikeltik az idegen tojást. Bár az eddigi elméleti és kísérleti kutatások nagy hangsúlyt fektettek a különbözo gazdafajok közti, költésparazita tojások diszkriminacióját befolyásoló tényezok feltárására, a parazita tojások elfogadásának gazdafajon belüli varianciája kevésbé tisztázott. Jelen kutatásunkban az anyai befektetés hipotézisét vizsgáltuk, melynek értelmében azok az egyedek, amelyek több energiát fektetnek fészekaljukba, nagyobb valószínuséggel távolítják el a költésparazita tojásait a fészekbol. A vándorrigónak (Turdus migratorius) a parazita tojást elutasító viselkedését vizsgáltuk meg a költésparazita barnafeju gulyajáró (Molothrus ater) tojásához hasonló méretu, de sötétkék színu, 3 D nyomtatóval készített mutojásokat használva. A költési periódusban korábban letojt, több tojásból álló, nagyobb inkubálatlan tömegu és változatos szteroid hormonkoncentrácijú sárga szíku tojásokkal rendelkezo fészekaljakat tekintettük a magasabb anyai befektetésu fészekaljaknak. Feltevéseink nagyrészét statisztikai eredményeink nem támasztották alá. Mindezek ellenére, a parazita tojásokat elfogadó egyedekéhez képest, az idegen tojást elutasító egyedek tojásai sárga szíkanyagában nagyobb koncentrációban és mennyiségben volt jelen a dezoxikortikoszteron, bár ez az adatainkra jellemzo varianciának csupán 14%-át magyarázta. További kísérletes vizsgálatok szükségesek ahhoz, hogy feltárhassuk az esetleges fiziológiás kapcsolatot az anyai tojas szteroid hormone befektetés és a parazita tojások elutasításának gyakorisága között ennél a gazdafajnál, és más költésparazita gazdáknál egyaránt. Translated kindly by Attila Marton, Debrecen University.

The Devil is in the Details: Identifying Aspects of Temperature Variation that Underlie Sex Determination in Species with TSD.

Most organisms experience thermal variability in their environment; however, our understanding of how organisms cope with this variation is under-developed. For example, in organisms with temperature-dependent sex determination (TSD), an inability to predict sex ratios under fluctuating incubation temperatures in the field hinders predictions of how species with TSD will fare in a changing climate. To better understand how sex determination is affected by thermal variation, we incubated Trachemys scripta eggs using a "heat wave" design, where embryos experienced a male-producing temperature of 25 ± 3°C for the majority of development and varying durations at a female-producing temperature of 29.5 ± 3°C during the window of development when sex is determined. We compared the sex ratios from these incubation conditions with a previous data set that utilized a similar heat wave design, but instead incubated eggs at a male-producing temperature of 27 ± 3°C but utilized the same female-producing temperature of 29.5 ± 3°C. We compared the sex ratio reaction norms produced from these two incubation conditions and found that, despite differences in average temperatures, both conditions produced 50:50 sex ratios after ∼8 days of exposure to female-producing conditions. This emphasizes that sex can be determined in just a few days at female-producing conditions and that sex determination is relatively unaffected by temperatures outside of this short window. Further, these data demonstrate the reduced accuracy of the constant temperature equivalent model (the leading method of predicting sex ratios) under thermally variable temperatures. Conceptualizing sex determination as the number of days spent incubating at female-producing conditions rather than an aggregate statistic is supported by the mechanistic underpinnings of TSD, helps to improve sex ratio estimation methods, and has important consequences for predicting how species with TSD will fare in a changing climate.

Short heatwaves during fluctuating incubation regimes produce females under temperature-dependent sex determination with implications for sex ratios in nature.

Patterns of temperature fluctuations in nature affect numerous biological processes, yet, empirical studies often utilize constant temperature treatments. This can limit our understanding of how thermally sensitive species respond to ecologically relevant temperatures. Research on turtles with temperature-dependent sex determination (TSD) provides good examples of this, since nest temperatures from many populations rarely exceed those necessary to produce females under constant laboratory conditions. We hypothesized that exposure to brief periods of warm temperatures (i.e., heat waves) are integral to sex determination in species with TSD, which requires tests that move beyond constant temperatures. We exposed Trachemys scripta embryos from multiple populations and across the nesting season to heat waves of variable durations and quantified sex ratios. We found that embryos from all populations were highly sensitive to brief exposures to female producing temperatures; only 7.9 days of exposure produced a 50:50 sex ratio, but the response varied across the nesting season. From these findings, a model was developed to estimate sex ratios from field temperature traces, and this model outperformed traditional methods. Overall, these results enhance our understanding of TSD and emphasize the importance of using biologically relevant temperatures when studying thermally sensitive processes.

Turtle hatchlings show behavioral types that are robust to developmental manipulations.

There can be substantial variation among individuals within a species in how they behave, even under similar conditions; this pattern is found in many species and across taxa. However, the mechanisms that give rise to this behavioral variation are often unclear. This study investigated the influence of environmental manipulations during development on behavioral variation in hatchlings of the red-eared slider turtle (Trachemys scripta). First, we examined the effects of three manipulations during incubation (estrone sulfate exposure, corticosterone exposure, and thermal fluctuations) on hatchling righting response and exploration. Second, we determined whether hatchlings showed consistent differences (i.e. behavioral types) in their righting response and exploration across days and months, and whether these behaviors were correlated with one another. Finally, we examined whether righting response was predictive of ecologically relevant behaviors such as habitat choice and dispersal. Hatchling behavior was robust to our early manipulations; none of the pre-hatch treatments affected later behavior. There were significant clutch effects, which due to the split-clutch design suggests genetic underpinnings and/or maternal effects. We found evidence for behavioral types in turtles; both righting response and exploration were strongly repeatable and these behaviors were positively correlated. Righting response was not predictive of dispersal ability in the field, necessitating a revision in the general interpretations of righting response as a proxy for dispersal ability in turtles. Thus, turtle hatchlings show consistent behavioral differences that are robust to early developmental manipulations, and while not necessarily predictive of dispersal, these behavioral types can have important consequences throughout ontogeny.