Evolutionary shifts in the melanin-based color system of birds

Abstract

Melanin pigments contained in organelles (melanosomes) impart earthy colors to feathers. Such melanin-based colors are distributed across birds and thought to be the ancestral color-producing mechanism in birds. However, we have had limited data on melanin-based color and melanosome diversity in Palaeognathae, which includes the flighted tinamous and large-bodied, flightless ratites and is the sister taxon to all other extant birds. Here, we use scanning electron microscopy and spectrophotometry to assess melanosome morphology and quantify reflected color for 19 species within this clade. We find that brown colors in ratites are uniquely associated with elongated melanosomes nearly identical in shape to those associated with black colors. Melanosome and color diversity in large-bodied ratites is limited relative to other birds (including flightless penguins) and smaller bodied basal maniraptoran dinosaur outgroups of Aves, whereas tinamous show a wider range of melanosome forms similar to neognaths. The repeated occurrence of novel melanosome forms in the nonmonophyletic ratites suggests that melanin-based color tracks changes in body size, physiology, or other life history traits associated with flight loss, but not feather morphology. We further anticipate these findings will be useful for future color reconstructions in extinct species, as variation in melanosome shape may potentially be linked to a more nuanced palette of melanin-based colors.

Sleep-Related Electrophysiology and Behavior of Tinamous (<b><i>Eudromia elegans</i></b>): Tinamous Do Not Sleep Like Ostriches

“…Even though the fact that tinamous are nested within a group of flightless birds seemingly suggests that they reacquired flight, recent genetic and biogeographic evidence suggests that tinamous actually retained the ancestral ability to fly, and each of the other extant Palaeognath birds independently evolved flightlessness [Mitchell et al, 2014;Yonezawa et al, 2017]. In this case, if the mixed sleep state found in ostriches is somehow linked to physiological changes associated with flightlessness in Palaeognath birds [Withers et al, 1987;Li et al, 2014;Eliason et al, 2016] it is possibile that tinamous retain the primitive sleep patterns for Palaeognath birds and therefore birds in general. Ultimately, in addition to examining other Palaeognath birds, further studies aimed at clarifying the nature of sleep in crocodilians, the closest reptilian relatives to birds, are needed to tease apart the evolution of avian REM sleep.…”

A bony-crested Jurassic dinosaur with evidence of iridescent plumage highlights complexity in early paravian evolution

“…We obtained data on nanostructure or melanosome morphology from the fossil and extant bird feathers (see Supplementary Table 2 for taxa added to the data set of ref. 59 ) using techniques previously described in ref. 34 .…”

“…Length and width for fossil nanostructures were compared to melanosome measurements for extant birds (taxonomic sample of ref. 59 plus new taxa listed in Supplementary Table 2 ). To compare nanostructure diversity in Caihong and other extinct paravian dinosaurs to that in extant birds, we used linear models to test whether the multivariate distance from the group mean centroid varied significantly between groups.…”

“…Previous analyses revealed that melanosome density is not linked to discrete colour classes 34 , 35 , therefore we did not include density as a variable in these analyses. We ran analyses using six separate training data sets: (1) a data set with black, brown, grey and iridescent categories 34 , 35 , 59 in addition to a ‘platelet iridescence’ category (including all new taxa with platelet-shaped melanosomes); (2) a data set similar to data set 1 but with hummingbird platelets as a distinct group; (3) a data set similar to 1 and 2 but with non-hummingbird platelets grouped with other species with rod iridescence; (4) a data set similar to 1 but only classifying Caihong samples not showing evidence for platelet morphology and dense packing (eg, Supplementary Fig. 7 ); (5) a data set where brown-black colours in penguins, known to be linked to novel, large and spherical melanosomes not seen in other extant Aves 45 were assigned to a distinct group and only non-platelet Caihong samples were categorized; and (6) a data set that accounted for 20% taphonomic shrinkage 47 of fossil melanosomes (with the same categories as data set 1).…”

Alternative reproductive strategies and the maintenance of female color polymorphism in damselflies

“…A key aspect in understanding the maintenance of the female‐limited color polymorphism in ischnuran damselflies that has thus far, however, been neglected is the possibility that color morphs could be signaling alternative reproductive strategies (see Roulin, 2004; Roulin & Bize, 2006 and references therein). This idea has recently received considerable support, with increasing evidence suggesting that the pigments necessary to produce alternative colors may have pleiotropic effects on physiological attributes (Armbruster, 2002; Eliason, Shawkey, & Clarke, 2016; Forsman, Ringblom, Civantos, & Ahnesjö, 2002; Merrill, Van Schooten, Scott, & Jiggins, 2011; Roulin, Almasi, Meichtry‐Stier, & Jenni, 2011). Such alternative strategies have only been studied in a handful of male‐polymorphic organisms (Ahnesjo & Forsman, 2003; Hutchings & Myers, 1994; Lank, Smith, Hanotte, Burke, & Cooke, 1995; Tuttle, 2003) and in three female‐polymorphic organisms: butterflies (Ellers & Boggs, 2002), fishes (Craig & Foote, 2001), and recently, reptiles (Galeotti et al., 2013).…”