Martín Alejandro Serrano-Meneses

Sexual selection, sexual size dimorphism and Rensch’s rule in Odonata

Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male‐biased (males > females) or female‐biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male‐biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch’s rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch’s rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male‐biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male‐biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.

Male mating tactics in the American rubyspot damselfly: territoriality, nonterritoriality and switching behaviour

Odonates exhibit a wide range of territorial and nonterritorial mating tactics and are ideal for investigating alternative reproductive behaviours. We studied male mating tactics in the American rubyspot damselfly, Hetaerina americana, a species that exhibits red wing spots that have been suggested to have evolved as a consequence of male–male contests. In this species mating success is enhanced by the ability of males to defend territories along streams and rivers, which depends on the amount of thoracic fat reserves available. Previous studies on this species have distinguished between territorial and nonterritorial males, in which the former obtain significantly more matings than the latter. In our study, however, we found a third reproductive tactic: switching. Switcher males exhibit both territorial and nonterritorial tactics and a mating success similar to that of territorial and nonterritorial males, although this result may be confounded by the small sample size used for this analysis. We suggest that the different mating tactics may be condition determined: territorial males contained the highest fat reserves, nonterritorial males had the least fat content and switchers had intermediate fat loads. We also show that there were no age differences between males using these tactics. Our results suggest that territorial behaviour is extremely plastic in this species. Finally, we discuss the implications of our study and directions for future work on territorial and nonterritorial reproductive tactics in odonates.

The lek mating system of Hetaerina damselflies (Insecta: Calopterygidae).

Summary We investigated whether territorial males of Hetaerina damselflies show lekking behaviour using experimental techniques and observations: (i) we altered potential vegetation substrates to determine whether this affected the number of female visitations and matings; (ii) by removing territorial males and allowing other males to occupy the territory, we determined whether females changed their visitation and mating number; (iii) we observed whether vegetation substrates were present and used, and whether lighting conditions affected male territorial behaviour; (iv) we documented female pre- and post-copulatory behaviour to examine whether female choice occurred; and (v) we investigated whether male traits were linked to mating success. Our results revealed that (1) vegetation substrates were rarely found in territories and even when vegetation was present, it did not affect female visitation and mating number; (2) males constantly moved to more illuminated places and females had little opportunity to exert choice due to harassment from males; (3) females oviposited outside territories; and (4) males with larger wing pigmentation and body size obtained a larger mating number because they were more likely to acquire a territory and/or displace other males while in tandem. This is the first documented evidence that odonate males display a lek mating system.

Adult sex ratio and operational sex ratio exhibit different temporal dynamics in the wild

Lay Summary Sex ratios vary within wild populations, but these variations and the relationship of different sex ratio indices are rarely explored. Using data from a well-monitored polygamous bird population, we show that adult sex ratio (ASR, proportion of males in adult population) and operational sex ratio (OSR, ratio of sexually active males to females) fluctuate over the breeding season and these fluctuations are not correlated, suggesting that OSR is a poor predictor of ASR under polygamy.

Camouflage and Clutch Survival in Plovers and Terns

Animals achieve camouflage through a variety of mechanisms, of which background matching and disruptive coloration are likely the most common. Although many studies have investigated camouflage mechanisms using artificial stimuli and in lab experiments, less work has addressed camouflage in the wild. Here we examine egg camouflage in clutches laid by ground-nesting Snowy Plovers Charadrius nivosus and Least Terns Sternula antillarum breeding in mixed aggregations at Bahía de Ceuta, Sinaloa, Mexico. We obtained digital images of clutches laid by both species. We then calibrated the images and used custom computer software and edge detection algorithms to quantify measures related to three potential camouflage mechanisms: pattern complexity matching, disruptive effects and background color matching. Based on our image analyses, Snowy Plover clutches, in general, appeared to be more camouflaged than Least Tern clutches. Snowy Plover clutches also survived better than Least Tern clutches. Unexpectedly, variation in clutch survival was not explained by any measure of egg camouflage in either species. We conclude that measures of egg camouflage are poor predictors of clutch survival in this population. The behavior of the incubating parents may also affect clutch predation. Determining the significance of egg camouflage requires further testing using visual models and behavioral experiments.

Niche partitioning among three tree-climbing bird species in subtropical mountain forest sites with different human disturbance

BackgroundClosely related and ecologically similar species that overlap in ranges can coexist through resource partitioning without one pushing the others to extinction through competition. Understanding resource partitioning among species is essential to predicting how species decline can affect the functioning of communities and ecosystems. In this study, we analyzed niche overlap and resource partitioning of three tree-climbing bird species in disturbed and undisturbed forest sites at La Malinche National Park, Tlaxcala, Mexico. From January to December 2008, resource partitioning between the three species was examined through the frequency of sightings of individuals foraging in different sites in the trees of both forest types. We characterized the pattern of resource utilization by niche breadth and niche overlap. Finally, we tested if these birds divide tree space differentially according to forest type.ResultsOur results indicate that in undisturbed sites, pygmy nuthatches had a high niche breadth (foraging significantly more on the thin branches), while white-breasted nuthatches and brown creepers had similar medium niche breadth values (more often foraged in thick branches and lower trunk, respectively), causing the last two to have a high niche overlap. In contrast, in disturbed sites, niche breadth and overlap values were similar for all three tree-climbing species. All observed overlaps for both forest types were higher than expected, but expected values in disturbed sites suggest the possibility of competition on these sites. Pygmy nuthatches were more sighted than white-breasted nuthatches and brown creepers in both forest types.ConclusionsOur study shows that differences in tree use suggest resource partitioning among the three bird species, particularly at undisturbed sites. However, activities of excessive logging and grazing in the disturbed sites resulted in niche overlap among species. This could lead to changes in competitive dynamics among them.

Wing Pigmentation in Males of a Territorial Damselfly: Alternative Reproductive Tactics, Allometry and Mating Success

Alternative reproductive tactics (ARTs) evolve to maximise fitness by favouring alternative phenotypes when high variance in relative fitness occurs amongst individuals. In the damselfly Hetaerina vulnerata males occur as either territorial or nonterritorial, depending on whether males acquire and defend an area to which females are attracted for copulation. Territorial males are usually larger, more pigmented and more successful in obtaining copulations than nonterritorial males. Several studies further suggest that territorial males are in overall better condition than nonterritorial ones. Other studies have investigated whether wing pigmentation, a sexual trait in damselflies, scales hyperallometrically with body size, and asked whether this pattern is related to fitness—nonetheless, a clear answer to this question remains elusive. Here we investigate whether i) territorial and nonterritorial males differed in body size and wing pigmentation; ii) body size, wing pigmentation and/or male status (male ART) predicted male mating success; and iii) the allometry of wing pigmentation in territorial and nonterritorial males, and amongst mated and unmated males. We first found that territorial and nonterritorial males did not differ in body size. Second, contrary to what occurs in other damselflies, territorial and nonterritorial males exhibited similar amounts of wing pigmentation. Third, only territory tenure, but not body size or wing pigmentation, predicted male mating success. Finally, with the exception of the relationship exhibited by mated males, which exhibited isometry, wing pigmentation was hyperallometric in all groups of males tested. The latter result suggests that hyperallometry of the sexual trait in this damselfly may not be selectively advantageous.

Superciliums in white-eared hummingbirds as badges of status signaling dominance

The role of badges as indicators of contest ability has been previously described. In hummingbirds, the exhibition of a badge is expected to save energy expenditure in agonistic interactions and to favor energy intake. Here, we investigate whether variable supercilium size in the white-eared hummingbird has a role in dominance status signaling. Firstly, 45 hummingbird males were captured and their superciliums were photographed to investigate variation in size and any possible allometric relationships. Secondly, 42 male birds were used to analyze whether the supercilium has a role in dominance status signaling in a dyadic contest. We found that supercilium size varied continuously but that despite variability between individuals, there was no relationship between supercilium size and body size. However, our dyad experiment indicated that birds with larger badges were able to make more visits to the feeders than individuals with smaller badges. We suggest a status signaling function of the supercilium.

Assortative Mating by Size in the American Rubyspot Damselfly (Hetaerina americana)

Assortative mating refers to the non-random nature of mating patterns between certain males and females. Thus, males and females may associate negative- or positively, based on different traits. Amongst these associations, assortative mating by size is one of the most common patterns found in natural populations of animals. Two main hypotheses have been proposed to account for the occurrence of assortative mating by size. First, it may be the result of mechanical, temporal, or physiological constraints. Second, it may occur in response to direct or indirect selection on mating preferences. Here we investigate whether the American rubyspot damselfly exhibits true assortative mating by size. Males of this species exhibit high levels of male-male competition, as they compete over territories, to which females are attracted for copulation. There is a documented large male body size advantage: the largest males are better able to hold their territories and thus secure more copulations. Our major results show that i) mated males are more likely to be larger than unmated males, whereas mated and unmated females tend to have similar body sizes; ii) H. americana exhibits true assortative mating by size; as such, this pattern is not driven by seasonal changes in the body sizes of males and females. We suggest that this mating pattern occurs in this species given the advantages of large male size, and the advantages of large female body size (i.e. higher fecundity). We believe that males may be able to evaluate a female’s reproductive value and exert mate choice.