Territorial patterns of Dickerson’s collared lizard, Crotaphytus dickersonae

Patrón territorial de la lagartija de collar de Dickerson, Crotaphytus dickersonae

 

Phenotypic traits did not differ between territorial males (> 12 sightings, n = 6) and other males (≤ 6 sightings, n = 10; Mann-Whitney U tests: U > 23.00, p > 0.44; Table 1). Males with higher SVL had longer tails (r_s = 0.83, p = 0.04), wider heads (r_s = 0.90, p = 0.01), and longer legs (r_s = 0.99, p = 0.0003). Strangely, SVL did not relate with body mass (r_s = 0.60, p = 0.21). Heavier males tended to have longer tails (r_s = 0.77, p = 0.07) and wider heads (r_s = 0.75, p = 0.08). When corrected for SVL, body mass, tail length, head width, and leg length were uncorrelated (r_s < 0.66, p > 0.16). For further analyses we used morphology traits corrected for SVL. Bluer males had wider heads (r_s = 0.93, p = 0.01), and tended to be heavier (r_s = 0.77, p = 0.07), but blue chroma did not correlate with SVL (r_s = 0.71, p = 0.11) or tail length (r_s = 0.66, p = 0.16).

Male territory size averaged 2,912.9 m², ranging from 693.5 m² to 6,236.0 m² (n = 6) (Figs. 1, 3). We found no significant correlations between phenotypic traits and the size of the territory (Table 2).

Table 1

Morphological traits of territorial and non-territorial males. Average ± SD are given. Differences were not significant.

	Males
	with territory	without territory
SVL (mm)	84 ± 8	84 ± 8
Weight (g)	23 ± 8	28 ± 11
Head width (mm)	20 ± 5	22 ± 3
Tail length (mm)	187 ± 39	197 ± 14
Hind limb length (mm)	29 ± 5	30 ± 3

The average number of overlapping female home ranges for each male territory was 3 and ranged from 2 to 5 (Figs. 1, 3). Males with longer tails had more overlapping female home ranges (r_s = 0.94, p = 0.005, Fig. 4). Males with longer SVL, wider heads, and higher blue chroma tended to have more overlapping female home ranges (Table 2). However, when correcting for territory size all these correlations became non–significant (r_s < 0.26, p > 0.623). There was no correlation between hind limb length and the number of overlapping female home ranges.

The average of the total number of females observed in a male’s territory (including the females for which we did not calculate home range size) was 3.8 and ranged from 2 to 8. The total number of females observed in a male’s territory was significantly higher for males with longer tails (r_s = 0.94, p = 0.005) and tended to be higher for males with longer SVL, wider heads, and higher blue chroma (Table 2). We found no correlation between the total number of females seen in a male’s territory and the hind limb length or body mass of the male. When correcting for territory size, no morphological measure bore statistical relation to the total number of females observed in the territory (p > 0.20). Interestingly, the additional females, with too few sightings to calculate their home range, were only seen in the territories of the males with the widest heads and the longest tails (Fig. 5).

 

 

 

 

 

 

 

 

 

 

 

 

The number of overlapping male territories averaged 1.33 and ranged from 0 to 2. Males with longer tails had less overlapping male territories (r_s = -0.93, p = 0.008). There was a trend that males with wider heads and higher blue chroma had less overlapping male territories (Table 2). We found no significant correlation between the SVL and the hind limb length of the male and the number of overlapping male territories (Table 2).

The percentage of overlap with other male territories averaged 31.5% and ranged from 0% to 98.7%. Males with longer tails had significantly less overlap with other male’s territories (Table 2; Fig. 6). There was no significant correlation between the other phenotypic traits and the percentage of overlap with other male territories (Table 2).

Males with longer tails, wider heads, and higher blue chroma had a higher number of additional males (males for which we did not calculate territory size) in their territories (Table 2). Males with higher SVL also tended to have more incidences of additional males in their territories (Table 2). When correcting for territory size, head width and blue chroma still related positively with the number of additional males (Table 2). Interestingly, the number of additional males was correlated with the number of females in the territory (r_s = 0.89, p = 0.02).

The number of trees and shrubs in a territory ranged from 150 to 456 and averaged 248. The territories had an average of 0.107 trees and shrubs per m²; this ranged from 0.069 to 0.216. The total number of rocks in a territory ranged from 75 to 526 and averaged 224.5. The number of rocks per m² averaged 0.080 and ranged from 0.042 to 0.108. When corrected for territory size, the number of refuges or basking sites did not correlate with any of the phenotypic traits evaluated (Table 2). Furthermore, the number of refuges or basking sites did not correlate with the number of females present in the territory, whether considering total number of females, percent of overlap with female home ranges, or additional females (p > 0.11).

Table 2

Spearman correlations between territory aspects and morphological traits of the male lizards. Results are given with and without correcting for territory size. Tail length and head width were corrected for SVL. Body mass and hind limb length were excluded from the table, as they did not correlate with any of the territory aspects. · indicates a trend (0.10 < p > 0.05), * indicates p < 0.05, and ** indicates p < 0.01.

Territory quality	SVL	Tail length	Head width	Blue chroma
	rs	p	rs	p	rs	p	rs	p
Territory size:	0.54	0.27	0.60	0.21	0.60	0.21	0.66	0.16
Female presence:
Nr of female’s home ranges overlap	0.76	0.08˙	0.94	0.01**	0.76	0.08˙	0.76	0.08˙
corrected for territory size	-0.03	0.70	-0.26	0.62	-0.26	0.62	-0.26	0.62
Additional females	0.83	0.14	0.93	0.01**	0.74	0.09˙	0.74	0.09˙
corrected for territory size	0.94	0.36	0.76	0.08˙	0.76	0.08˙	0.76	0.08˙
Total nr of females	0.76	0.08˙	0.94	0.01**	0.76	0.08˙	0.76	0.08˙
corrected for territory size	0.60	0.40	0.49	0.33	-0.30	0.96	0.14	0.79
Exclusivity:
Nr of male territories	-0.68	0.14	-0.93	0.01**	-0.74	0.09˙	-0.74	0.09˙
corrected for territory size	-0.60	0.21	-0.77	0.07˙	-0.77	0.07˙	-0.77	0.07˙
% Overlap male territories	-0.54	0.27	-0.83	0.04*	-0.83	0.21	-0.66	0.16
Additional males	0.77	0.07˙	0.83	0.04*	0.93	0.01**	0.93	0.01**
corrected for territory size	0.52	0.30	-0.60	0.12	0.88	0.02*	0.88	0.02*
Habitat:
Refuges (per m2)	-0.49	0.33	-0.43	0.40	-0.60	0.21	-0.54	0.27
Basking sites (per m2)	0.14	0.79	0.31	0.54	-0.20	0.70	-0.14	0.79

Discussion

We studied the relation of phenotypic traits with territory quality in Crotaphytus dickersonae. Results show that males with longer tails had higher female presence in their territory and less overlap with other male territories. Additionally, bluer males and males with wider heads tended to have territories with more females and less overlapping male territories, yet these males had more additional males in their territories.

Tail length and head width are morphological features that can be related to RHP. Longer tails increase agility, allowing the animal to run faster without losing stability (Wheatley et al., 2015; Wynn et al., 2015). Tail length is related to sprint speed in lizards. Experimental reductions in tail length decreased sprint speed in some lizard species (Ballinger et al., 1979; Downes & Shine, 2001; Formanowicz et al., 1990), but increased sprint speed in others (Daniels, 1983; Li et al., 2011). Garland (1985) suggested that longer tails would be especially beneficial for lizards that run bipedally. Crotaphytus species are well known for their bipedal gait (McGuire, 1996), and longer tails may allow them to run faster. Lizards that can run faster have larger territories (Peterson & Husak, 2006) and are more likely to achieve dominance (Garland et al., 1990; Robson & Miles, 2000). This suggests that running speed is important in male-male competition. Accordingly, Husak and Fox (2006) found that male C. collaris use near maximum sprint speed when they respond to other males in their territory. In the interactions between males that we observed, a typical pattern of behavior involved one of the males running quickly to the other male, with the latter subsequently running away. We observed several such interactions between 2 males, which even led to territory replacement of one of the males (see results). The lizard that had chased away the other male was the male with the widest head and longest tail of all males included in this study. Hence, faster and stronger males may be able to maintain exclusivity of their territory. These males might also be better able to expand the boundaries of their territory, possibly increasing the number of females in their territory while still being able to keep other males away from those females.

 

Biting is often used in agonistic encounters (Hall et al., 2010; Lailvaux & Irschick, 2006) and can be considered an important trait determining RHP (Lailvaux & Irschick, 2006). Male lizards that bite harder are more likely to win fights (Husak, Lappin et al., 2006; Huyghe et al., 2005; Lailvaux et al., 2004), have larger territories with more overlap with female home ranges (Lappin & Husak, 2005), and they have a higher reproductive success (Husak et al., 2009; Lappin & Husak, 2005). Head size and shape are strongly correlated with bite force (Anderson et al., 2008). In male lizards, head width is a predictor of bite force; males with wider heads bite harder (Herrel et al., 2006; Husak, Lappin et al., 2006; Lappin & Husak, 2005). The positive relation between head width and territory quality suggests that biting also plays a major role in territory acquisition and defense in C. dickersonae. Head width, however, correlated positively with additional males. Perhaps males are attracted by the higher territory quality of these males, in which more female encounters may be worth the risk of the attack of the strong territory owner.

We found that bluer males tended to have more exclusive territories. The intensity of the blue color relates to running speed and bite force in C. dickersonae and could thus be an honest signal of RHP (Plasman et al., 2015). Morphological features may not be easily distinguished from long distances, whereas bright coloration can be easily detected (Losos, 1985; Whiting et al., 2003), especially in the open habitat of the Sonoran desert. The blue color may facilitate the assessment of the opponent’s RHP over long distances (preliminary opponent assessment), whereas morphological traits may be assessed in a later stage when animals are nearer (secondary opponent assessment; Kokko, 2013). The blue color may thus dissuade other males to invade their territory. Contrarily, however, we observed more additional males in the territories of bluer males. Likewise, more additional males were observed in territories of males with longer tails and wider heads. These additional males may temporarily sneak into these territories with the intention to steal copulations. This strategy is frequently seen in species where part of the population is not able to obtain a territory (Gross, 1996; López-Sepulcre & Kokko, 2005) and has been found to have equal fitness benefits as that of territory owners in the C. collaris (York et al., 2014). The big size of the territories of C. dickersonae prevent males from guarding the entire area simultaneously, leaving parts open to opportunistic males. Further, the number of additional males correlated with female abundance. Interestingly, the strong correlation of number of additional males with blue chroma of the resident male may suggest that the sneaker males perhaps uses the information broadcasted by the territorial male to estimate female presence.

More females were found in the territories of males with longer tails, wider heads and higher blue chroma. Higher female abundance may occur due to high territory quality. Although we did not find correlations with abundance of basking sites and refuges, or the presence of predators, we cannot exclude other territory characteristics that were not measured here (e.g., food abundance). On the other hand, female abundance may be higher, because females prefer the territory owner as mate. Females may be attracted by traits used in male-male competition, as these traits may relate to general condition or quality (Berglund et al., 1996) and provide the female with good genes for her offspring (Hamilton & Zuk, 1982; Zahavi, 1975) or give direct benefits in the form of a high quality territory and protection from other males (Emlen & Oring, 1977). Female choice has rarely been documented in lizards (Olsson & Madsen, 1995; Olsson et al., 2013) and generally the female mates with the territory owner with which her home range overlaps (Kiester, 1979; Stamps, 1983, 1987; Tokarz, 1998). Nevertheless, females may temporarily leave their home range to mate with a high quality male (Calsbeek & Sinervo, 2002; Vitousek et al., 2007). In the collared lizard, C. collaris, almost 40% of the offspring was from a male other than the owner of the territory with which their home range overlapped (Husak et al., 2008) and multiple mating may be common (York & Baird, 2015). However, the lack of difference between offspring fitness from territorial and non-territorial males and the thermoregulation costs associated to fleeing from males, suggests that females may accept copulations to avoid male harassment (York & Baird, 2015). On the other hand, in an earlier study with C. collaris it was found that females discriminated against young males (Baird et al., 1996). Furthermore, we found that only males with long tails and wide heads had additional female visitors, which may suggest female choice. To what extent female choice determines overlap of male territories with female home ranges or additional female visitors remains unclear and is an interesting topic for future studies.

Although we assumed basking sites and refuges to be important factors for survival in C. dickersonae, their quantity did not relate to any male trait or to female presence. Possibly refuges and basking sites are not considered when selecting a territory, due to the high abundance of rocks and bushes in our study site. As such, territory quality was determined primarily by its size and the presence of conspecifics, and obtaining mates is likely the most important motive for territory defense in this species (Baird, 2013).

Our results indicate that, in male Crotaphytus dickersonae, head width (i.e., bite force) and especially tail length (i.e., running speed) are important traits for the intra-sexual contest for territory acquisition and determine access to females for mating. The blue coloration of the males probably signals male dominance status (Plasman et al., 2015) and helps to estimate the opponent’s RHP over long distances and possibly attracts females. The conspicuous sexual signals can thus increase fitness benefits (e.g., repel opponents and attract mates). Yet, the higher number of additional males in territories of bluer males suggests that opponents may exploit the signal when intercepting the mates attracted by the signal (Perril et al., 1978). Further, our results indicate that territories of C. dickersonae males are large compared to those of other animals of similar size (Candolin & Voigt, 2001; McLoughlin & Ferguson, 2000), other lizards (Molnár et al., 2016; Stehle et al., 2017), or even other collared lizard species (Schwartz et al., 2007; although this may depend on population, see McCoy et al., 2003). Due to their size, the number of territories that can be established is probably limited, whereas costs for patrolling and defending this big area are likely high, and low female density may result in low reproductive payoff (Briffa & Sneddon, 2007; Lappin & Husak, 2005). Hence, it could be concluded that C. dickersonae is a species with relatively few territorial males and many floaters (López-Sepulcre & Kokko, 2005). It needs to be determined if these 2 mating strategies form an ESS (evolutionary stable strategy) with equal success or that being a floater is ‘making the best of a bad job’.

Acknowledgments

We thank G. E. Castro and A. B. Fleishman for help with fieldwork, G. W. Heil and A. B. Fleishman for help with GIS ArcView, H. de Vries for help with statistics, V. H. Reynoso for logistical and technical support, and Prescott College for facilitating lodging. We thank R. Torres for lending equipment and also for arranging the capture-release permits for C. dickersonae, which were obtained from the Secretaría del Medio Ambiente y Recursos Naturales with permit number SGPA/DGVS/02084/11. This study was performed according to Mexican regulations of animal welfare.

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