The effect of albinism on avian predator attack rates in eastern garter snakes.

Albinism is a conspicuous and distinctive phenotype arising from the absence of melanin in the integument that has been documented in all major vertebrate groups. With few exceptions, albinism is rare in natural populations, suggesting that it incurs significant fitness costs as compared to wild-type phenotypes. One possible explanation for this rarity is that albinos experience higher predation risk as compared to wild-type individuals. We tested this hypothesis by comparing rates of attack by avian predators on immobile clay model proxies that mimicked wild-type and albino eastern garter snakes (Thamnophis s. sirtalis) at Bond Swamp in central Georgia, USA; two mixed pattern treatments provided intermediate phenotypes for comparison. Surprisingly, we found no difference in attack rate across all four model treatments, nor among pairs of treatments (i.e., wild-type-like vs. albino-like models). This indicates that albino garter snakes are not subject to higher predation risk from birds due to coloration alone, and that other factors (correlated pathologies of albinism, thermoregulatory challenges, mate selection) are likely to be more important in explaining the rarity of albinism in snakes and perhaps other vertebrate groups.

Comparative morphology and trophic ecology in a population of the polymorphic lizard Sceloporus minor (Squamata: Phrynosomatidae) from central Mexico.

Polymorphism among individuals of the same population has generally been linked to alternative reproductive tactics, where different morphs can exhibit differences in their morphological, ecological, and behavioral attributes. These differences may result in a divergence in diet between morphs due to differential exploitation of habitat, morphological differences that influence prey selection, or differential energy expenditure that results in different nutritional needs. The present study analyzes the morphology (morphometry and body mass) and diet of red and yellow male morphs in a population (El Enzuelado) of the lizard Sceloporus minor from central Mexico. No differences between morphs were found for any of the morphometric variables analyzed (snout-vent length, tail length, jaw length, jaw width, head length, head width, head height, tibia length, femur length, forearm length and ventral patch length). In both morphs, allometric growth was observed in all body features analyzed, as well as in morphometric features of the head across seasons. Analysis of stomach contents showed that the diet of red males was composed of 12 categories of prey, while that of yellow males was composed of 10 categories; those categories of diet not shared between morphs (e.g., Isoptera, Psocoptera) were consumed by their respective morph in very low proportions. Categories of diet with the highest values of food importance for both groups were Coleoptera, Orthoptera, and leaves; a similar pattern was seen across seasons. This, in turn, is reflected in low niche breadth values for each morph and a very high niche overlap. There were no significant differences between morphs overall, or between morphs per season, in the weight and volume of stomach contents or in the number of prey items found in stomachs; however, differences in these variables across all males (independent of morph) were recorded between seasons. Likewise, no significant correlations were found between body size (snout-vent length) and the volume of stomach contents for either morph or between lizard mandibular dimensions and the volume of stomach contents for red morph males. For the yellow morph, prey volume unexpectedly decreased significantly with jaw size rather than increasing as expected. Overall, this study adds new information about the morphology and feeding of males in this species, and suggests that in this population, color morphs lack the morphological and ecological differences found in some other species of polymorphic lizard.

Amphibian life history in a temperate environment of the Mexican Plateau: dimorphism, phenology and trophic ecology of a hylid frog, Hyla eximia (=Dryophytes eximius).

The study of demographic and life history aspects of an organism provides valuable information for its conservation. Here, we analyze the phenology of the Mountain Treefrog Hyla eximia (= Dryophytes eximius) in a temperate environment of the Mexican Plateau. Females were larger in snout-vent length and body mass than males. The peak period of activity occurred in the rainy season (May-September), with amplexus and egg deposition occurring between June and July, and larval development from July to August. A logistic model best explained observed male growth patterns, while the Von Bertalanffy model better described female growth. Notably, males grew faster than females, although females reached a larger overall body size. The diet of this species is made up of 10 prey categories. The index of diet importance indicated that males feed mainly on Coleoptera and Diptera, while females feed on Coleoptera, Diptera, Hemiptera, and Aranea. Both females and males showed a significant abundance of plant material in their stomachs, suggesting that H. eximia might exhibit highly specialized feeding behavior. Reproduction was seasonal, and both female and male reproductive cycles are synchronized with the rainy season. These natural history characteristics provide information to better understand their responses to environmental conditions.

Atypical reproductive cycles in a population of Sceloporus grammicus (Squamata: Phrynosomatidae) from the Mexican Plateau.

The spiny lizard Sceloporus grammicus (Squamata: Phrynosomatidae) is a small reptile from central México and the southern United States, occurring in a wide geographic area characterized by extensive variation in topographic and climatic regimes. Genetic variation among lineages from central México is substantial, though the extent to which this variation corresponds with life-history traits remains obscure. To address part of this puzzle, we studied a population of S. grammicus from Tepeapulco, Hidalgo, México. Male-biased sexual dimorphism was extensive in this population; males were larger than females overall, and expressed proportionately larger heads and longer limbs. Minimum size at sexual maturity was similar in the sexes (males: 43 mm; females: 42 mm). In contrast to other populations from the Central Plateau, reproductive activity of males and females was synchronous. Testicular recrudescence of adult males was initiated in October-November, and maximum testis size maintained from December to July. Female reproductive activity showed no clear seasonal pattern: females had vitellogenic follicles from October to July, and pregnant females were found throughout the year. Female body size was not related to litter size. Neither male nor female gonadal mass was correlated with any abiotic environmental variable examined. Differences in reproductive characteristics among populations of S. grammicus might be indicative of plasticity in response to local environmental conditions, local adaptation, or complex gene × environment interactions. We consider these results in the context of previously studied populations of S. grammicus from the Central Plateau and elsewhere, and propose directions for future research.