Abstract
Informative integumentary coloration plays vital roles in bird
communication. However, determining whether such ornaments on a bearer can be perceived for a receiver is imperative for understanding avian communication. Carotenoid-based ornaments involving a red, orange, or yellow are considered to reflect the bearers’ physical conditions in numerous avian species. According to the “indicator
hypothesis”, females can use these informative traits as the cue for sexual selection. However, studies have revealed that not every species can use these informative traits as sexually selected traits. In the current study, I tested whether informative carotenoid-based bills and tarsi can act as the target traits of female preference to understand the roles of carotenoid bared parts in the Himalayan black bulbul (Hypsipetes leucocephalus nigerrimus). The results indicate that females did not have significant preferences for carotenoid-richer (redder) males. I argue that
carotenoid-based bills and tarsi alone might not be the sexually selected traits for female Himalayan black bulbuls.
Keywords: carotenoid-based trait, female preference, Himalayan black bulbul (Hypsipetes leucocephalus nigerrimus), sexual selected trait.
Introduction
Birds have a complex and diverse integumentary coloration, which generally indicates information about the bearer; such information can be used to determine the species, age (different appearances among different ages, e.g., Inouye et al. 2001), or an individual bird’s quality (e.g., Faivre et al. 2003). Such informative coloration could play essential roles in bird communication, such as being indicators of mate choice (Hill 2006) or assessing opponents (Senar 2006). However, a complete communication involves both a sender and a receiver (Stevens 2013), and determining whether the informative ornament can be perceived by the receiver is imperative to understanding avian communication.
Carotenoid-based ornaments are represented in red, orange, or yellow coloration and are commonly considered to serve as indicators reflecting individual qualities in birds (McGraw 2006). Carotenoids are essential antioxidants and must be ingested by animals (Brush 1990). Through antioxidation, carotenoids can directly or indirectly affect an individual’s immune response and affect multiple life history traits such as molting, migration (McGraw 2006). The amount of carotenoids that an animal can use is also limited by the genotypes and physiological conditions of animals themselves (Olson and Owens 1998); therefore, trade-offs must be made in carotenoid allocation between ornamentation and other somatic needs. Such trade-offs should render to correlation between the expression of carotenoid-based ornaments and animals’ physical
conditions (Svensson and Wong 2011). Studies on birds have revealed that compared with individuals with carotenoid-richer ornaments are typically healthier (e.g., Blount et al. 2003, Faivre et al. 2003, Saks et al.
2003), more agile (e.g., Blount and Matheson 2006, Pryke and Andersson 2003), or more aggressive (e.g., Pryke et al. 2001) compared with those with carotenoid-lesser ornaments; they also provide more favorable resources and better territories for their mates and offspring (e.g., Casagrande et al. 2006, Senar et al. 2002).
Traditionally, according to the “indicator hypotheses” that suggest that females choose mates according to informative traits (Andersson and Simmons 2006), carotenoid-based ornaments are considered to function as sexually selected traits which correlated with male mating success; this has been demonstrated in several avian species (e.g., Blount et al. 2003, Burley and Coopersmith 1987, Faivre et al. 2003). However, other studies have revealed that although carotenoid-based ornaments might reflect an individual’s condition, they might not always be used by females as the targets of mate choice (e.g., Horak et al. 2001, Nolan et al. 2006, Viera et al. 2008). This suggests that the utility of carotenoid-based ornaments in animal communication may be more complicated than conventionally understood.
Himalayan black bulbuls (Hypsipetes leucocephalus nigerrimus) have black plumage and carotenoid-based bills and tarsi. The red parts of these birds are conspicuous because of the dull plumage coloration. Previous studies have indicated that the red bill is the indicator of a black bulbul’s physical condition, including its immunocompetence and level of
oxidative stress (Chapter Three). According to “indicator hypothesis”, I hypothesize that female black bulbuls choose mates on the basis of the coloration of the red bill and tarsi; specifically, females show higher preferences for males with redder bills and tarsi than for other males. To
test this hypothesis, I artificially reduced the redness of the bill and tarsi to determine whether female bulbuls show preferences to males with redder bills and tarsi (i.e., carotenoid-richer individuals). The results of this study can increase understanding of the roles of carotenoid-based integuments in birds.
Materials and methods
Captive and molecular sex typing
I purchased 47 black bulbuls from a pet-shop in Taiwan in the winter of 2008. Four to five individuals of the same sex were kept in the same cage measuring 40 cm x 40 cm x 60 cm. Different sexes were separated by opaque plastic veils, but acoustic contacts were allowed. All birds were fed with a mixed diet of commercial bird food and fresh fruits daily. The birds were housed in the outdoor and experienced the natural light cycle.
I drew 150 μL of blood from each individual and used for molecular sex-typing. Genomic DNA was extracted from blood samples with traditional proteinase K digestion followed by LiCl extraction (Gemmell and Akiyama 1996). The detailed programing of polymerase chain reactions (PCRs) for molecular sex-typing (2550F / 2718R, Fridolfsson and Ellegren 1999) was the same as that of Hung et al. (2015a in revision). A total of 20 males and 27 females were identified.
Color measurements
The reflectance values of the red bill and tarsi were measured using a USB2000 spectrometer (Ocean Optics Inc., Dunedin, U.S.A.) with illumination from an HL2000 halogen light source. The measurement procedures were the same as those of Hung et al. (2015b in revision).
Redness (Hue550-700nm) was calculated for the bill and tarsi by deriving the wavelength of the mean of maximum and minimum reflectance values in the wavelength range of 550 - 700 nm (Thomas and Gausman 1977). A higher hue indicates higher redness.
Female preferences experiments
Female preference experiments were conducted at the beginning of the reproductive season (March to April) in 2009. For each trial, I randomly selected two males. One as the experimental male, the bill and tarsi of which were artificially painted with orange acrilan, and one as the control male, the bill and tarsi of which were painted with transparent acrilan.
I measured the proximity preference of females that were given a choice of a red-billed (control group) or an orange-billed male (orange group) in a dichotomous choice chamber (Fig. 5.1). I quantified the proximity preference of a female according to the amount of time the female spent in front of a respective male. In each trial, I randomly selected a female bird to be placed in the central cage, and two males to be placed in each of the cages next to the central one. The birds were placed in the apparatus for 15 min to acclimate to the new environment, while water and food were provided ad lib. During the acclimatization period, the birds were visually separated by the opaque plastic veils. After acclimatization, the water, food and veils were removed, and the position of the female was then recorded for 1h with a digital video recorder (Sony DCR-SB62). The amount of time a female spent perched in front of the male’s compartment was defined as the preference time. The amount of time a female spent perched in the neutral zone was defined as the neutral time, indicating no preference to either male. I assumed a trial
was successful when the neutral time was less than 30 min. After excluding the neutral time, I calculated the percentages of time females stayed close to the control and orange males, respectively. . The female preference toward each male in each trial was arcsine transformed then tested using the paired t-test.
I also measured the lengths of the bills, heads, wings and tails and the body weight of tested males to assess the correlation between these traits and female preferences. The precision of the length is to 0.1mm. The statistical analysis was conducted using SAS JMP 7.0.
Results
In the male birds, the average redness (Hue550-700nm) of the bills was 594.5
± 1.6 nm (mean ± SE) and that of the tarsi was 589.8 ± 1.9 nm; in the females, the averaged redness of the bills was 593.5 ± 1.4 nm and that of the tarsi was 591.5 ± 1.7 nm. The redness between the control and orange groups was significantly different after treatment (Hueorange group= 579.0 ± 5.2; Huecontrol groups= 601.8 ± 4.4; paired t -test, t 23= 3.35, p = 0.0004). The experimental coloration ranged within the coloration variability of the males (572.9 - 612.7 nm).
I conducted a total of 24 experimental trials, 16 of the 24 trials which were considered successful. Among the 16 successful trials, the females showed preferences for the red males in seven trials, and showed preferences on orange males in nine trials. Moreover, on average, the females spent 1085.4 ± 184.5 s near control males (red males), 995.3 ± 155.7 s near the experimental males (orange males) and 1451.3 ± 188.2 s
in the neutral zones. I determined that the females did not show higher preferences for the control males (0.51 ± 0.06) than they did for the orange males (0.49 ± 0.06; Fig. 5.1, paired t -test, t 15= -0.17, p = 0.87).
In addition, the preferences were not correlated with males’
morphometric traits (multiple regression, r2 = 0.25, p = 0.73, supplementary table 5.1).
Discussion
The results reveal that the females did not significantly prefer the redder males, indicating that the carotenoid-based bared parts might not be the sexually selected trait for female Himalayan black bulbuls. I propose two possible explanations for the failure to detect the preference of females for redder males.
My results are consistent with those of a recent study (Dey et al.
2015), suggesting that carotenoid-based bared parts in birds, like bills, tarsi, eye rings, crests or wattles, may serve as informative cues about physical conditions but not sexually selected cues. Dey et al. (2015) reported a positive correlation between the occurrence of carotenoid bill coloration and two ecological factors associated with high rates of agonistic encounters (colonial breeding and nonbreeding sociality), but not with sexual dichromatism. Previous studies have also suggested that bill coloration can change faster (Ardia et al. 2010, Faivre et al. 2003) than plumage coloration (Ardia et al. 2010) and the bill coloration reflects a bird’s real-time quality. Therefore, the carotenoid-based bared parts in
birds are more likely to be selected for social interactions than for mate choice. Because individual birds experience intense competition with numerous unknown birds in nonbreeding flocks and in breeding colonies, the carotenoid-based bared parts of such birds may serve as signals of their physical conditions or fighting ability, enabling them to settle contests quickly without resorting to physical conflicts (West-Eberhard 1983). Therefore, Dey et al. (2015) suggest that carotenoid-based bills might primarily evolve from social interaction, and then subsequently evolve into sexual selection functions through other mechanisms.
Although Himalayan black bulbuls are socially monogamous, they live in groups during nonbreeding seasons (Severinghaus et al. 2012), and they can be extremely aggressive when in groups (personal observation).
Therefore, it is imperative for these species to develop honest quality traits reflecting their physical conditions or fighting ability -to avoid physical conflicts. Furthermore, female black bulbuls may not yet be able to discern these traits as a cue for mate choice.
The other potential reason of showing no female preferences on red male is the possibility of using multiple cues in female Himalayan black bulbuls for mate choice. Results from a previous data (Hung et al 2015 in revision) suggested that several black (melanin-based) parts of the
plumage are sexually dichromatic and could reflect individuals’ oxidative stress, which are the indirect indicators of sexually selected traits
(Andersson and Simmons 2006, Andersson 1994). Several avian species, such as Northern cardinals (Cardinalis cardinalis, Jawor et al. 2004), have been reported to use multiple cues in mate selections. The benefit of multiple sexual ornaments is that these traits can provide females
different types of information at various stages of the mate choice process (Borgia 1995), or function as redundant signals to improve the accuracy of mate assessment (Johnstone 1994, Moller and Pomiankowski 1993).
Females can also use diverse characteristics in different breeding seasons as the current environmental conditions vary (Chaine and Lyon 2008, Qvarnström et al. 2000). Because I manipulated only carotenoid-based bared parts in our experiments, it is possible that females could use other informative characteristics to assess their potential mates.
Studies on the roles of carotenoid-based ornaments in avian species have typically assessed the correlations between the expression of the traits and the bearers’ qualities. However, it is necessary to conduct experiments to ensure that the receivers do use these traits. I thus conducted this study to fill the gap regarding the functions of carotenoid-based ornaments in Himalayan black bulbuls’
communications. Nevertheless, although my results do not reveal the role of carotenoid-based ornaments in the sexual selection of Himalayan black bulbuls, they can serve as the basis for determining the functions of carotenoid-based traits in other aspects, such as signals of fighting ability (Pryke et al. 2001) or social status (Pryke et al. 2002).
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Fig. 5.1 Plain view of the mate choice apparatus (adapted from (Griggio et al. 2007). The dashed lines in the scheme indicate the position of the perches. The bold lines indicate the opaque dividers.
Area marked in gray represents the neutral zone. A female’s
preference for a male was calculated as the percentage of duration of the total time the female stayed in the vicinity of the preferred male (i.e., preference time)
20 cm
Male Male
Female