Timothy H. Parker

Melanin- versus carotenoid-based sexual signals: is the difference really so black and red?

A large number of coloured sexually selected ornamental traits in the animal kingdom are based on carotenoid and melanin pigments. The biochemical differences between these two classes of pigment, together with their different physiological roles, have led to the general belief that there will be a fundamental difference in the way in which they are used in animal signals. Specifically, it has been argued that carotenoid-based colours will have higher levels of condition dependence and that melanin-based traits will be under tighter genetic control. We present a meta-analysis of studies that have experimentally investigated the signalling quality of the two kinds of colour in birds and show that there is no evidence of a difference between them. Furthermore, we show that the available data are currently very limited, both in the number of studies and in the quality of many of the studies that have attempted to examine this question, and we suggest directions for future work.

Redefine Statistical Significance

We propose to change the default P-value threshold for statistical significance from 0.05 to 0.005 for claims of new discoveries.

Detecting and avoiding likely false‐positive findings – a practical guide

Recently there has been a growing concern that many published research findings do not hold up in attempts to replicate them. We argue that this problem may originate from a culture of ‘you can publish if you found a significant effect’. This culture creates a systematic bias against the null hypothesis which renders meta‐analyses questionable and may even lead to a situation where hypotheses become difficult to falsify. In order to pinpoint the sources of error and possible solutions, we review current scientific practices with regard to their effect on the probability of drawing a false‐positive conclusion. We explain why the proportion of published false‐positive findings is expected to increase with (i) decreasing sample size, (ii) increasing pursuit of novelty, (iii) various forms of multiple testing and researcher flexibility, and (iv) incorrect P‐values, especially due to unaccounted pseudoreplication, i.e. the non‐independence of data points (clustered data). We provide examples showing how statistical pitfalls and psychological traps lead to conclusions that are biased and unreliable, and we show how these mistakes can be avoided. Ultimately, we hope to contribute to a culture of ‘you can publish if your study is rigorous’. To this end, we highlight promising strategies towards making science more objective. Specifically, we enthusiastically encourage scientists to preregister their studies (including a priori hypotheses and complete analysis plans), to blind observers to treatment groups during data collection and analysis, and unconditionally to report all results. Also, we advocate reallocating some efforts away from seeking novelty and discovery and towards replicating important research findings of ones own and of others for the benefit of the scientific community as a whole. We believe these efforts will be aided by a shift in evaluation criteria away from the current system which values metrics of ‘impact’ almost exclusively and towards a system which explicitly values indices of scientific rigour.

Behavior of early life intervals of Klamath River green sturgeon, Acipenser medirostris, with a note on body color

We studied ontogenetic behavior, migration, and wintering behavior of young Klamath River green sturgeon, Acipenser medirostris, in the laboratory to provide insight into likely behavior of wild sturgeon. Hatchling free embryos preferred cover but were poor swimmers and could not move farther than a few centimeters to cover. The poor swimming ability and cover preference of hatchlings suggests evolution for habitat selection of females to place eggs in habitat with cover for eggs (and hatchlings), and for egg characteristics (large, dense, and weakly adhesive) to cause rapid sinking into cover without drifting. A day or so after fish developed into larvae (first life interval feeding exogenously), day-12 larvae initiated a 12-day downstream nocturnal migration. A totally nocturnal migration is unlike other Acipenser migrants yet studied. Migrant larvae had a dark-colored body typical of other Acipenser species that migrate as larvae. Tail color was a dark black (black-tail phenotype) only during the early larva period, suggesting a morphological adaptation for migration, foraging, or both. Post-migrant larvae and early juveniles to day 84 foraged diurnally with a nocturnal activity peak. Day 110–181 juveniles moved downstream at night until water temperature decreased to about 8°C, indicating wild juveniles migrate downstream to wintering habitat. Habitat preference of month 9–10 wintering juveniles suggests wild juveniles are in deep pools with low light and some rock structure. Wintering juveniles were only active at night. Initiation and cessation of daily activity was at dusk and dawn during illumination changes of <1.0lx. This sensitivity to illumination has not been found before in sturgeons. During the first 10months of life, nocturnal activity of early life intervals is a dominant feature of migration, foraging, and wintering.

Transparency in ecology and evolution: Real problems, real solutions

To make progress scientists need to know what other researchers have found and how they found it. However, transparency is often insufficient across much of ecology and evolution. Researchers often fail to report results and methods in detail sufficient to permit interpretation and meta-analysis, and many results go entirely unreported. Further, these unreported results are often a biased subset. Thus the conclusions we can draw from the published literature are themselves often biased and sometimes might be entirely incorrect. Fortunately there is a movement across empirical disciplines, and now within ecology and evolution, to shape editorial policies to better promote transparency. This can be done by either requiring more disclosure by scientists or by developing incentives to encourage disclosure.

DO MELANIN- OR CAROTENOID-PIGMENTED PLUMAGE ORNAMENTS SIGNAL CONDITION AND PREDICT PAIRING SUCCESS IN THE KENTUCKY WARBLER?

Abstract Yellow and red feathers pigmented by carotenoids can signal a birds condition and are important to both female choice and male-male competition. Melanin-based ornaments are thought to be less effective than carotenoid ornaments as indicators of condition, though their signal function is still relatively unstudied. We examined both carotenoid and melanin-pigmented feather ornaments in a single species, the Kentucky Warbler (Oporornis formosus). Both males and females of this species have melanin-pigmented ornamentation (black crown, black face) and carotenoid-pigmented ornamentation (lemon-yellow undersides). Melanin-pigmented plumage patches were larger in males than females, and males had brighter breast plumage, with a larger ultraviolet component, than females. Among males in the wild, individuals in better condition had more extensive black caps and faces, but not brighter-yellow breasts. Males with larger black caps were more likely to attract mates. This work demonstrates that, in a species with both melanin- and carotenoid-pigmented plumage, melanin-pigmented ornaments can signal condition, and that a melanin-based signal can explain variance in mating success. ¿Funcionan Como Indicadores de la Condición Física y Predicen del Éxito de Apareamiento los Ornamentos del Plumaje Pigmentados con Melanina o Carotenoides en Oporornis formosus? Resumen. Las plumas amarillas y rojas pigmentadas por carotenoides pueden indicar la condición de un ave y son importantes tanto para la elección de machos por parte de las hembras como para la competencia entre machos. Se cree que los ornamentos a base de melanina son menos efectivos como indicadores de condición que los ornamentos carotenoides, aunque su función como señal ha sido menos estudiada. Examinamos plumas ornamentales pigmentadas con carotenoides y melanina en la especie Oporornis formosus. Tanto el macho como la hembra presentan ornamentación pigmentada con melanina (corona negra, rostro negro) y ornamentación pigmentada con carotenoides (flancos inferiores amarillo-limón). Los parches de plumaje pigmentados con melanina fueron mayores en los machos que en las hembras y el plumaje del pecho de los machos fue más brillante y con un componente ultravioleta mayor que el de las hembras. Entre los machos en ambiente natural, los individuos con mejor condición tuvieron coronas y rostros negros más extensos, pero no tuvieron pechos amarillos más brillantes. Los machos con coronas negras más extensas tuvieron una mayor probabilidad de atraer parejas. Este trabajo demuestra que en una especie con plumaje pigmentado con melanina y carotenoides, los ornamentos pigmentados con melanina pueden señalar la condición de los individuos, y que una señal con base en melanina puede explicar la varianza en el éxito reproductivo.

Dominant male red junglefowl (Gallus gallus) test the dominance status of other males

Abstract. In sexual selection, honest signals are maintained by a variety of mechanisms. In red junglefowl (Gallus gallus), health, condition and social status affect comb size, a well-documented predictor of female choice. The comb size of subordinate male junglefowl appears to be suppressed when in the company of other males. One hypothesis for how social status could affect ornament expression in this way involves punishment of cheaters. Under this scenario, dominant males periodically challenge similar males signalling putative high status. For subordinate males, the risk of fighting a high-ranked male could make it prohibitively costly to develop ornamentation signalling dominance. We asked if dominance signals influenced the direction of aggression by dominant males. To address this issue, we conducted experiments in which 19 dominant-acting, large-combed male junglefowl were allowed to choose to fight one of two opponents. The two potential fight opponents differed in comb size, dominance behaviour, or in both traits. In 15 of 19 trials, dominant-acting males chose to fight large-combed, dominant-acting opponents rather than small-combed, subordinate-acting opponents. This is the first demonstration that aggression of dominant male birds is directed at other males based on the display of an ornament known to be attractive to females. However, males did not discriminate between fight opponents when potential opponents differed in only one of the two status indicators (large-combed males chosen in 11 of 19 trials, dominant-acting males chosen in 10 of 19 trials).

Female mating preferences in red junglefowl: a meta-analysis

Numerous experiments have been conducted in an effort to understand the mating preferences of female red junglefowl (Gallus gallus). Males of this species have both colourful ornamental plumage and fleshy head ornaments, including the comb. Previous studies produced conflicting results regarding the responses of females to these different ornaments. Several studies indicated that the comb influences female choice, but others failed to support this finding. Similarly, feather ornamentation correlated with female choice in some studies, whereas in a number of others it did not. Our meta-analyses show that when all mate choice experiments involving combs are analysed together, female preference is significantly related to male comb morphology. Subsets of these data (non-manipulation experiments, comb length [not colour] experiments, or manipulation experiments that leave one comb unaltered) make this point more strongly. This evidence is consistent with current understanding of the signalling value of the comb of male red junglefowl. Interestingly, when the combs of both males in each choice trial were altered (by painting, tine removal, or covering with latex false comb), we detected no female preference for comb morphology, which suggests that females prefer larger, brighter combs only when these combs appear natural. Our meta-analysis of feather experiments indicated that feather ornamentation was not associated with female mate choice. There was only a weak, non-significant indication that feather ornaments influence female choice in our meta-analysis of non-manipulation studies. In contrast, studies in which male plumage was experimentally manipulated provided no evidence that females utilize feathers in mate choice.

Maternal Condition, Reproductive Investment, and Offspring Sex Ratio in Captive Red Junglefowl (Gallus gallus)

Abstract Maternal condition can affect reproductive investment in one or more ways. Captive female Red Junglefowl (Gallus gallus) that were heavier, controlling for tarsus length, produced larger eggs and initiated laying earlier, but did not produce more eggs per clutch. Maternal mass was also a positive predictor of offspring mass. That appears to result from the strong positive relationships between egg size and offspring mass. When the relationship between egg size and offspring mass was statistically controlled, maternal mass no longer predicted offspring mass. Mothers in better condition produced male-biased broods, as predicted when offspring condition depends on maternal condition and when male reproductive success is more dependent than female reproductive success on condition.

GENETIC BENEFITS OF MATE CHOICE SEPARATED FROM DIFFERENTIAL MATERNAL INVESTMENT IN RED JUNGLEFOWL (GALLUS GALLUS)

Abstract Females may choose more attractive mates to obtain better viability or attractiveness genes for their offspring. A number of studies have demonstrated a positive relationship between paternal attractiveness and offspring quality. However, this pattern could be due to inheritance of paternal genes and/or it could be due to increased maternal investment in the offspring of more attractive males. To isolate female responses to male appearance from paternal genetic effects, I housed female red junglefowl (Gallus gallus) with vasectomized (sterile) males and artificially inseminated them. Male junglefowl with larger combs are more attractive to females. Females laid more eggs when housed with a large‐combed, as opposed to a small‐combed, vasectomized mate. Neither egg volume nor offspring body condition was associated with comb size of the mothers vasectomized mate. Paternal genetics appeared important. Body condition and comb size were greater for the sons of large‐combed sperm donor males. This is consistent with the hypothesis that genetic benefits to offspring maintain female preference for the most ornate males. It is possible that greater body condition and comb size in sons of large‐combed sires was not caused by genetic differences, but instead was due to compounds in the ejaculate of large‐combed sperm donors inducing greater reproductive investment from females. However, females artificially inseminated by large‐combed males did not produce more or larger eggs than females artificially inseminated by small‐combed males, and thus there is no other evidence consistent with ejaculate‐induced differential investment. Furthermore, only in older chicks was body condition significantly related to sire comb size, suggesting genetic rather than differential investment mechanisms.