THE EVOLUTION OF TARSAL SPURS IN GALLIFORMES Emily Griffith e.g firstname.lastname@example.org University of Florida Under the Supervision of Rebecca Kimball Biology/Zoology Department s Submitted March 30 th 2018
Page 1 Abstract Animal weaponry has long been of interest to biologists. While most birds lack structures that likely evolved specifically as weapons, birds in the order Galliformes (chickens, pheasants, and turkeys) are unique in possessing tarsal spurs. These horn l ike structure located on the back of the tarsus vary vastly in size, shape and number between species and although spurs are known worldwide for their role in male male competition (see: cockfighting) spurs are also present in the females of many species Using data collected from mu seum skins and published sources and a recent phylogeny of Galliformes, I found that tarsal spurs originate d from a common ancestor in both males and females, and that there has been a rapid loss of tarsal spurs in females whereas males have largely retained them. I also found evidence supporting the hypothesis that spurs in both males and females may have been favored by natural selection for reasons other than intraspecific fighting for mates (i.e. defense) and that ma le tarsal spur presence in Phasianidae is likely influenced by sexual selection. Introduction One of the few recorded instances of specialized weapons in birds are found within the Galliformes a group of ground feeding birds including Megapodiidae (brush turkeys), Cra cidae (guans and curassows), Numididae (guinea fowl), Odontophoridae (New W o rld quail), and Phasianida e (Old World quail, peafowl, and partridges, pheasants, turkeys, and other game birds) Many g alliform species bear a horn like structures found approximately 2/3 of the way down the posterior side of the tarsomata tarsus known as the tarsal spur. These spurs may be any combination of sharp (Figure 1A) or dull (Figure 1B), and long (Figure 1 A ) or short (Fig ure 1 B ), and in some species, multiple spurs can be found on a single leg (Figure 1 C ). And as recorded by archeologists
Page 2 (Sa dl er, 1991; d e Cupere et al 2005; Serjeantson, 2009; Doherty, 2013) cockfighters (Dundes, 1994) and domestic farmers, (Darwin, 1 871) tarsal spurs can cause significant damage to an opponent. As with most animal weapons (E ml en, 2008) tarsal spurs are hypothesized to have been selected upon for use in the intrasexual fights of males competing for access to mating opportunities (e.g., Darwin 1871, Davison 1985, and Sullivan and Hillgarth 1991). However, the inconsistent pattern of spur presence and absence within within the two Galliformes families that tarsal spurs are found in (Numididae and Phasianidae) challenges the theory that intrasexual competition is their sole function. Spurs are found in the females of numerous species and the males of many monogamous species (both groups lack ing the need for excessive or frequent intraspecific or intrasexual combative behavior ), and are absent in many of the polygamous species that display highly aggressive competitive behavior, such as those found in Tetraoninae (grouse) (Davison, 198 5 ; Crowe et al. 2006). Despite these confound ing observations the literature focuses on the hypothesis that the basic function of tarsal spurs is intraspecific fighting though there are some alternative hypotheses to explain the variability of presence in males as well as their presence in females In the Descent of Man (1871), Darwin suggested spurs originated in males in response for the need for male male competition, and suggested that female spurs were nothing more than a byproduct of their presence in males stating the following : [T ] he presence and absence of spurs in the females result from different laws of inheritance having prevailed, independently of natural selection. With the many females in which spurs appear as rudiments, we may conclude that some few of the successive
Page 3 variatio ns, through which they were developed in the males, occurred very early in life, and were as a consequence transferred to the females, i n the other and much rarer cases, in which the females possess fully developed spurs, we may conclude that all the succe ssive variations were transferred to them; and that they gradually acquired the inherited habit of not disturbing their nests Alternatively, Davison ( 198 5 ) provides a different hypothesis to the origin of female spurs. In a study examining over 2500 museum skins to assess spur presence, Davison concluded that spurs originated together at one time in both males and females and that their condition was blunt, (i.e. not that natural selection may have originally favored spurs that stunned their opponent instead of fatally wounding them .) He mentioned in this and a subsequent 1986 paper that spurs could have a function in both males and females due to predator or territory defense, and noted the differences in spur characteristics of specie s living in different habitat B ut ultimately Davison also concluded that their original function was for intraspecific fighting, offering the hypothesis t hat over evolutionary time spurs were either lost if the need for fighting in both males and female s disappeared, or selected upon to become longer and sharper as intense male male competition increased Davison (198 5 1986) also emphasized the potential role between mating system and spur presence in both males and females with higher rate of male spur presence being found in polygynous systems, and a higher rate of female spur presence being found in monogamous systems. Subsequently, the re has been much discussion on the potential role of sexual selection on male spur presence and physical variability in phasianid species (von Schantz et al., 1989, Kirkpatrick 1989; Hillgarth 1990; GÂšransson et al., 1990; Ligon et al., 1990; Wittzel l 1991; Mateos and Carranza, 1996; Buchholz 1997; Badyaev et al., 1998; Hill 2003 ; Papeschi and
Page 4 DessÂ“ Fulgheri, 2013 ) But experimental evidence does not consistently support the hypothesis that spurs are used as an ornament displayed female preferenc e, and a publication using phylogenetic controls (Sullivan and Hillgarth, 1993) was unable to conclude that either spur length or spur number is significantly related to mating system within phasianid species. Lastly contrary to Davison's 198 5 conclusion that spurs evolved once in all Galliformes, Crowe et al.'s 2006 study examining spur presence/absence in a phylogenetic context and concluded that spurs have been gained at least twice for males within Galliformes, once in Numididae and once in Phasianidae. But since the time of that publication, our understanding of the relationships within Galliformes has changed ( revie wed in Wang et al. 2013 ) and Crowe et al. did not conduct a separate test for the evolution of female spurs. Thus, several questions about the origin and evolution of tarsal spurs remain unanswered : 1 In the context of recent phylogenetic data, is it more likely that tarsal spurs originate d one or multiple times within Galliformes? ; 2 Is spur presence in males or females dependent upon mating system or other potential correlates of sexual selection such as size dimorphism ? ; 3 Is spur presence in male s or females dependent upon factors that would indicate a more defensive use (e.g. natural selection) ? ; and, 4 Do patterns of spur evolution differ within just Phasianidae as compared to Galliformes overall ? This study aims to answer these questions usin g a recent phylogeny of Galliformes and a thorough review of available data. In addition to providing a comprehensive overview of the rates of spur gains and losses in both Galliformes and Phasianidae, I have tested the dependence of spur presence upon a variety of non spur variables, such as mating system ( base d on the discussion s by Davison ( 198 5) and Sullivan and Hillgarth ( 1993) ) parental care system, habitat, and nesting location ( to represent a potential selective pressure related to a need for d efense), and
Page 5 body size dimorphism (to represent a selective press ure related to sexual selection). An o verview of the predictions for dependent relationships between these variables and spur presence in under the forces of eith er natural or sexual selectio n is provided in Table 1. Methods Spur presence or absence in either sex was coded for each species in the tree based on data from Davison (198 5 ), Madge and McGowan (2002), and supplementary visits to museum collections (Florida Museum of Natural History, Louisiana State University Museum of Natural Science, and the Smithsonian National Museum of Natural History). Spur data was not obtained for 60 species, and there were 20 cases in which Davison and Madge and McGowan disagreed on the presence of female spurs, (15 where Madge and McGowan suggested females had spurs, and five where Davison suggested females had spurs ). In species that have spurs in both sexes, spurs typically show up far less frequently in females (i.e. presence may be found in less than 10% of females in the population) than in males, and are usually much smaller in females than in males (Davison, 198 5 ). Consequentially, their presence may not be identified if a relatively small number of specime ns may be examined. As such, I coded any recorded instance of a spur presence as a "yes" for the species. Illustrating the challenges with identifying which species had spurs, I identified a female Pternistis leucoscepus with spurs, a species which nei ther Davison nor McGowan recorded as having female spurs. In addition to spur presence and absence, to test for alternate explanations for tarsal spur origin, I coded the following variables into binary categories based upon data from previous publications (Davison, 198 5 ; Dunning Jr., 1992; Cockburn, 2006; del Hoyo et al., 2018):
Page 6 1. Mating System: monogamous (species suspected to be monogamous or largely monogamous) or non monogamous 2. Parental Care System : evidence of bi parental care, or no evidence of bi parental care 3. Habitat: mostly open (grassland, desert, rocky areas, savanna, wetlands) or mostly closed (shrubland and forest) 4. Nesting Location: in/on/close to ground (not higher than 1m) or above ground (higher than 1m) 5. Body Si ze Dimorphism: coded as "yes" when [weight(male) weight(female)]/weight(female) > .10, and "no" when the difference in weights was < .10 Analyses 1. Test for Ancestral Reconstruction : I used Mesquite (Version 3.4, Maddison and Maddison 2017) to reconstru ct the ancestral condition for spur presence and absence in both males and females based on a phylogeny of Galliformes consisting of 26 4 species (Kimball et al., in prep) constructed using a super matrix of mitochondrial and nuclear DNA Using a likelihood ratio test incorporating b ranch lengths, I found the best model to explain the rates of gain and loss (Asymmetric 2 paramater model or MK1 model) within all Galliformes and Phasianidae only. To determine if spurs originated once or multiple times I used the mode l which best explained spur evolution within all Galliformes and recorded likelihood for the presence or absence of spurs at key nodes
Page 7 2. Correlates of S pur P resence : To determine whether spurs were dependent upon the variables in Table 1, I used a test for correlated evolution of discrete characters ( Pagel 1994 ) as implemented in Mesquite which measure s the likelihood of dependence between two variables within a phylogenetic context In all cases, spurs were considered the dependent variable. Any species in which trait information on could not be found were removed from that individual analysis. I measured the p values from the output of 1000 simulations, and used a criterion of p < 0.05 to denote significance Separate analyses were conduc ted for male spur presence and female spur presence B ecause there has been discussion about the potential role of sexual selection just with in Phasianidae analyses were done both in the context of all Galliformes and of Phasianidae Results Out of the 264 galliform species in the phylogeny, 113 had spurs and 151 lacked spurs Of those 113 spurred species 44 had spurs in both sexes. No species had spurs present in females alone. Spurs were only present in Numididae and Phasianidae, and were absent in Megapodiidae Cracidae and Od ontophoridae (Figure 2). The results of ancestral reconstruction test (Table 2) indicates that for all Galliformes, the Asymmetric 2 Paramater model best represents the data for both male (p < 0.05) and female (p < 0.01) spur presence. In both sexes, the rate of loss was higher than the rate of gain, with a gain:loss ratio of approximately 1:6 .0 for males and 1:8.5 for females. For Phasianidae the Asymmetric 2 Para mater model again best represented the data (p < 0.01), for females with a gain:loss ratio of approximately 1:3. 3 However, t he data for males was best described by the MK1 model, which indicated no significant difference between the rates of gains and losses of
Page 8 spurs within that family. This result suggests that spurs in Phasianidae were largely maintained in males and successively lost in females over time. When I examined the likelihoods of spur presence or absence at each key ancestral node ( reported in Table 3), resu lts indicated that a single gain of spurs in both males and females was more likely followed by a los s in the ancestor o f Odnotophoridae. The results of the test for correlates of spur presence are listed in Table 4 Across all Galliformes, female spur presence was correlated with habitat, nesting location, and body size dimorphism though there were no correlates of male spur pres ence However, f or Phasianidae only, male spur presence was correlated with body size dimorphism but not mating system These results support the hypo these s that females may have originally evolved (or at least maintained) spurs for a functional purpose such as nest or territory defense and that sexual selection (at least as evidenced by size dimorphism) has likely played an important role in males within Phasianidae but possibly not in the origin of spurs Discussion I examined the presence and absence of tarsal spurs in a phylogenetic framework to answer questions about their origin and provide insights as to their functional history. The results support the conclusion of Davison (1985) that spurs likely evolved only once in Galliformes in the ancestor of Numididae, Odontophoridae, and Phasianidae These results contradict Crowe et. al (2006) who suggest spurs evolved independently in Numididae and Phasianidae I did not find any eviden ce supporting the hypothesis that spur presence in males or females is dependent upon mating system. This result supports a previous phylogenetic regression study by Sullivan and Hillgarth (1991) which found no significant relationship
Page 9 between mating syst em and spur number and length. However, the mating system in many of these species is not well understood (Sullivan and Hillgarth, 1991) and it may be that incorrect classification of mating system may have obscured my ability (and that of Sullivan and Hillgarth 1991) to identify a link between these traits. In males, spur presence was correlated body size dimorphism in phasiands indicating that forces of sexual s election may be important. Sexual selection is known to be strong in many phasianid species ( Darwin, 187 1 ), but may not be as important outside of that family where many taxa are monomorphic. Thus, i t is possible that the strong influence of sexual selec tion on male spur presence in Phasianidae combined with a lack of a role for sexual selection and spur presence outside of Phasianidae, may explain why there was no clear correlates of spur presence in males for all Galliformes. For females, it is likely that spurs may have evolved for a functional use related to defense, but it is still plausible that they could have also been used in intraspecific competition for other resources, or for males during assortative mating within flocks as suggested by Davison (198 5 ). As no significant correlations with female spur presence were found within Phasianidae I am unable to provide an explanation as to why females may have retained spurs in only select species within that family It is possible that for som e species, spurs are vestigial structures. In my observations of specimens, spurs in females were typically blunt, even when males of the same species had sharp spurs. F uture analyses may want to survey and quantitatively score variables indicating the relative utility of spurs in a species (e.g. number, length in relation to body size, and "sharpness"), and also measure the relative abundance of tarsal spur presence with in that species for both adult males and females to help answer questions about their retention in some species.
Page 10 Lastly there is reason to believe that spur development is different in females due to the failure of the bony core to completely ossify to th e hypotarsal ridge ( Figure 3) or the lack of a bony core entirely. Though information and discussion on spur development has drastically increased in the last century thanks to researchers such as Goodale (1918, 1925 ), Kozelka (1929 1933a, 1933b) Do mm (1931) Ju h n (1946, 1952 ); Quigley and J uh n (1951) Holman, (1964), Washburn and Smyth (1971), Lucas and Stettenheim (1972) Christmas and Harms (1981) West (1985) Sa dl er (1991) Briganti et al. (2002), Ohlsson et al. (2002), and Serajeston (2009) with the majority of research has been conducted on males within Phasianidae ( and restricted to a very limited number of species.) Increasing our understanding of the development of tarsal spurs in females would also be valuable in discussing the potenti al funct ion and utility of female spurs particularly. Acknowledgements This work would not have been made possible without the guidance of Rebecca Kimball, my mentor on this project. I would like to thank her and the rest of the Kimball Braun lab, whose comments, edits, and encouragement has helped me each step of the way. Thanks is also due to the Florida Museum of Natural History, Louisiana State University Museum of Natural Science, and the Smithsonian National Museum of Natural History which opened their doors to data collection for this study Lastly, I would like to thank t he N ational S cience F oundation who provided an REU supplement for DEB 1118823 (RTK and EL Braun) and the University of Florida's Center for Undergraduate Research (U niversity S cholars Program) whose financial contributions helped to fund this resea rch.
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Page 15 Table 1 : Predicted Relationships Between Variables Non Spur Variable Natural Selection on Male Spur Presence Natural Selection on Female Spur Presence Sexual Selection on Male Spur Presence Mating System No No Yes especially within Phasianidae Parental Care System Yes potential use in in bi parental care systems for nest defense No No Habitat Yes potential use as a defensive weapon in more open habitats Yes, potential use as a defensive weapon in more open habitats No Nesting Location Yes, potential use in bi parental care systems for nest defense of ground nesters Yes, potential use for nest defense of ground nesters No Body Size Dimorphism No No Yes especially within Phasianidae
Page 16 Table 2 : Ancestral Reconstruction Output s Output Male Spur Presence in all of Galliformes Female Spur Presence in all of Galliformes Male Spur Presence in Phasianidae Only Female Spur Presence in Phasianidae Only MK1 Model Log Likelihood 45.414 102.894 32.284 84.741 Asymmetric 2 Paramater Model Log Likelihood 43.313 92.587 31.925 80.522 Difference 2.100 10.307 0.35 9 4.21 9 Chi square Statistic 4.200 20.615 0.71 8 8.43 7 P value < 0.05 < 0.01 < 0.9 < 0.01 Favored Model Asymmetric 2 Parameter Asymmetric 2 Parameter MK1 Asymmetric 2 Parameter Asymmetric Model Forward Rate 0.324 1.414 N/A 4.229 Asymmetric Model Backward Rate 1.974 12.040 N/A 13.186 Table 3 : Likelihood Values for Spur Absence/Presence at Important Nodes Ancestral Node of Likelihood of Spur Absence in Males Likelihood of Spur Presence in Males Likelihood of Spur Absence in Females Likelihood of Spur Presence in Females All Galliformes 0.91 0.09 0.83 0.17 Numididae, Odontophoridae, and Phasianidae 0.30 0.70 0.16 0.84 Numididae 0.07 0.93 0.04 0.96 Odontophoridae and Phasianidae 0.31 0.69 0.15 0.85 Odontophoridae 0.77 0.23 0.45 0.55 Phasianidae 0.24 0.76 0.09 0.91
Page 17 Table 4 : P values for Pagel's 1994 Corre lation Tests* Non Spur Variable Male Spur Presence in all of Galliformes Female Spur Presence in all of Galliformes Male Spur Presence in Phasianidae Only Female Spur Presence in Phasianidae Only Mating System 0.353 0.067 0.557 0.484 Parental Care System 0.223 0.223 0.733 0.067 Habitat 0.187 0.008 0.312 0.134 Nesting Location 0.189 0.017 0.054 0.115 Body Size Dimorphism 0.138 0.030 0.001 0.226 significant results are bolded Figure 1 : E xamples of variability in spur morphology. A. Gallus g allus ; B. Francolinus adspersus ; C Franco linus after All specimens photographed were male. Photographs were taken at the Florida Museum of Natural History.
Page 18 Figure 2 : Tree showing t arsal s pur p resence in males and females of galliform species Blue represents spur presence in males of the species only. Orange represents spur presence in both males and females. Black/grey stripes indicate spur absence in both mal es and females. Figure 3 : Photograph s howing the a bsence of an o ssified b ony c ore in f emale t arsal s purs of Pavo m uticus Photograph taken at the United States Museum of Natural History. Megapodiidae Cracidae Numididae Odontophoridae Phasianidae