Javier Manjarrez

Predation risk is associated with the geographic variation of a sexually selected trait in a viviparous fish (Xenotoca variata)

Male secondary sexual traits may increase the risk of predation because mating signals make them conspicuous to predators and hamper evasive manoeuvres. Males of Xenotoca variata, a viviparous freshwater fish, show on their flanks bright and colourful spots (speckles), the number of which varies geographically. In this study, the association of this variation with the presence of the piscivorous snake Thamnophis melanogaster, which co-occurs with X. variata, was investigated. A test was also done to establish whether the snake distinguishes between male fish with contrasting numbers of speckles and if the perception of speckles is influenced by water turbidity. The amount of speckling and the prevalence of snakes in key localities was assessed by using one-way mirrors, and the effect of speckles on the predatory responsiveness of snakes was evaluated by presenting them with pairs of male fish in clear and in turbid water. In localities where snakes were infrequent there was a tendency for male fish to have many speckles. The snakes preferentially approached the males with more speckles than the males with fewer speckles. The direction of the preference did not change with the conditions of the water, but the magnitude was stronger in clear water than in turbid water. The snakes also approached first the males with more speckles. These findings indicate that predation risk by T. melanogaster may select against speckles and produce population differentiation.

ECOLOGY OF THE MEXICAN ALPINE BLOTCHED GARTER SNAKE (THAMNOPHIS SCALARIS)

Abstract We report the first information on seasonal abundance, sex ratio, body size, reproduction, and diet for a population of Thamnophis scalaris from the State of Mexico. Data were collected during 1992 and from 1998 to 2000 at Toluca. Thamnophis scalaris showed a bimodal peak of activity in spring-early summer and in autumn. The sex ratio of adults was significantly different from 1:1, but can be misleading because of the unknown natural history of T. scalaris. Sexual size dimorphism was only apparent in relative tail length of adults. Four litters were born from July 1 to 17. Mean litter size was 7.0 ± 1.4 snakes per litter. Male and female neonates were of similar snout-vent length (SVL) and mass at birth. The sex ratio of neonates was 1:1. Of the stomachs examined, 81% contained earthworms and 19% contained vertebrates. Vertebrates (lizards and viperine snakes) were ingested only by T. scalaris >40.0 cm SVL. This suggests an ontogenetic shift in the diet of T. scalaris. The mean vertebrate prey mass was 21.4 ± 10.0% of snake mass. No differences were found in prey type between the sexes.

Variation in the Diet of the Mexican Black-bellied Gartersnake Thamnophis melanogaster: Importance of Prey Availability and Snake Body Size

Abstract We describe the diet of Thamnophis melanogaster on the Mexican plateau, including geographic variation between populations in the Lerma and Tula drainages (represented by 39 localities), annual and seasonal variation in an isolated population in the Nazas drainage, and sexual and size-related variation in all three drainages. The Mexican Black-bellied Gartersnake inhabits banks of streams, rivers, canals, ponds, and lakes, where it captures fishes, leeches, crayfishes, tadpoles, earthworms, and frogs by foraging underwater. There was no geographic variation between the Lerma and Tula populations in proportions of prey taxa consumed, but whereas primary and secondary tadpole stages were consumed by snakes in Lerma, only secondary stages were consumed by snakes in Tula. Crayfishes, novel prey for this genus, were consumed over a restricted zone in the contiguous headwaters of the Lerma and Tula drainages. Interannual and seasonal variation in proportions of annelids and anurans ingested was documented in the Nazas population, and attributed to temporal change in relative abundance of prey taxa and snake size classes. Snake size was a major determinant of diet in all three drainages, affecting proportions of different prey taxa consumed and the upper limits on the mass of ingested prey. In the three drainages, sexes were similar in body size and consumed the same prey taxa in similar proportions and masses. We discuss proximate and functional determinants of diet and suggest that the observed interannual and seasonal variation observed in Mexican Black-bellied Snakes is due mostly to temporal variation in prey availability and proportions of snake size classes.

Female vibration discourages male courtship behaviour in the Amarillo fish (Girardinichthys multiradiatus)

Amarillo fish females (Girardinichthys multiradiatus) vibrate when conspecific males approach them; the reason behind this behaviour is unclear. Hypotheses are that females vibrate either to avoid aggression from males or to court them. We prevented females from vibrating by temporarily blocking their lateral line organs and eyes, on the assumption that they rely on these senses to detect approaching males. Females with the lateral line organs obstructed vibrated less frequently than females with the lateral line intact, indicating that the mechanosensory lateral line system is necessary for perceiving approaching males. Males displayed more courtship behaviour to sighted females with the lateral line organs obstructed than to sighted females with the lateral line intact. A general tendency indicated that the less the females vibrated the more the males courted them. These findings indicate that female vibration discourages male courtship behaviour.

Feeding Ecology of Nerodia rhombifera in a Veracruz Swamp

In Mexico, the niche of non-venomous water snakes is occupied by two genera of thamnophiine snakes, Thamnophis (garter snakes) and Nerodia (water snakes). Both genera are widely distributed in Mexico, but are sympatric only in the coastal slopes (Conant, 1963, 1969). Nerodia rhombifera and T. proximus are found throughout the Gulf coast both at freshwater and brackish ponds, marshes, and swamps, mainly near river estuaries (Rossman, 1963; Conant, 1969). Recent ecological studies of these genera in Mexico have reported on single or mixed populations of garter snakes (T. eques, T. melanogaster, and T. rufipunctatus; Sosa, 1982; Drummond, 1983; Macias Garcia and Drummond, 1988; Drummond and Macias Garcia, 1989). The ecology of Mexican populations of Nerodia, whether allopatric or sympatric with Thamnophis, is largely unknown. Several studies have reported ecological differences between N. rhombifera and sympatric water snakes and queen snakes (Regina spp.) on the Gulf coast of the United States (Mushinsky and Hebrard, 1977a, b; Hebrard and Mushinsky, 1978; Mushinsky et al., 1980, 1982) related to diet, use of time, and microhabitat. Nerodia rhombifera is regarded mainly as a fish-eater (Clark, 1949; Mushinsky and Hebrard, 1977a; Mushinsky et al., 1982; Plummer and Goy, 1984), but also feeds on anurans (Clark, 1949). Our aim was to describe the use of microhabitats, daily activity patterns, and diet in Nerodia rhombifera werleri in Alvarado, Ve-

Chemical and physical defense traits in two sexual forms of Opuntia robusta in Central Eastern Mexico.

Sexually dimorphic plants provide an excellent opportunity for examining the differences in the extent of their defense against herbivores because they exhibit sex-related differences in reproductive investment. Such differences enable comparison of the sex with high reproduction expenses with the sex that expends less. The more costly sex is usually also better defended against herbivores. Generally, females are considered more valuable than hermaphrodites in terms of fitness; however, hermaphrodites are more valuable if they can produce seed by autonomous selfing, provided that the inbreeding depression is low and pollen is limited. We studied a gynodioecious population of Opuntia robusta from Central-Eastern Mexico, which has been reported to be trioecious, dioecious, or hermaphrodite, and addressed the following questions: 1) Is the hermaphrodites reproductive output higher than the females, and are hermaphrodites thus better defended? 2) Are plant tissues differentially defended? 3) Do trade-offs exist among different physical defense traits? and 4) among physical and chemical defense traits? We found that 1) hermaphrodites had a higher seed output and more spines per areola than females and that their spines contained less moisture. Non-reproductive hermaphrodite cladodes contained more total phenolic compounds (TPCs) than female ones. In addition, 2) hermaphrodite reproductive cladodes bore more spines than female cladodes, and 3) and 4) we found a negative relationship between spine number per areola and areola number per cladode and a positive relationship between spine number per areola per plant and TPC concentration per plant. Non-reproductive hermaphrodite cladodes contained a higher concentration of TPCs than female cladodes, and parental cladodes contained fewer TPCs than both reproductive and empty cladodes.

Present and future ecological niche modeling of garter snake species from the Trans-Mexican Volcanic Belt

Land use and climate change are affecting the abundance and distribution of species. The Trans-Mexican Volcanic Belt (TMVB) is a very diverse region due to geological history, geographic position, and climate. It is also one of the most disturbed regions in Mexico. Reptiles are particularly sensitive to environmental changes due to their low dispersal capacity and thermal ecology. In this study, we define the important environmental variables (considering climate, topography, and land use) and potential distribution (present and future) of the five Thamnophis species present in TMVB. To do so, we used the maximum entropy modeling software (MAXENT). First, we modeled to select the most important variables to explain the distribution of each species, then we modeled again using only the most important variables and projected these models to the future considering a middle-moderate climate change scenario (rcp45), and land use and vegetation variables for the year 2050 (generated according to land use changes that occurred between years 2002 and 2011). Arid vegetation had an important negative effect on habitat suitability for all species, and minimum temperature of the coldest month was important for four of the five species. Thamnophis cyrtopsis was the species with the lowest tolerance to minimum temperatures. The maximum temperature of the warmest month was important for T. scalaris and T. cyrtopsis. Low percentages of agriculture were positive for T. eques and T. melanogaster but, at higher values, agriculture had a negative effect on habitat suitability for both species. Elevation was the most important variable to explain T. eques and T. melanogaster potential distribution while distance to Abies forests was the most important variable for T. scalaris and T. scaliger. All species had a high proportion of their potential distribution in the TMVB. However, according to our models, all Thamnophis species will experience reductions in their potential distribution in this region. T. scalaris will suffer the biggest reduction because this species is limited by high temperatures and will not be able to shift its distribution upward, as it is already present in the highest elevations of the TMVB.

Temporal stability of an endemic Mexican treefrog

The demographic characteristics of an amphibian population fluctuate independently over time, mainly in response to the temporal variation of environmental factors, especially precipitation and temperature. These temporal fluctuations may contribute to the size of an amphibian population and could be used to determine the current conservation status of a species. During a five year (2004–2008) period, we studied the relative abundance, sex ratio, and age-sex structure of a population of metamorphosed individuals of the endemic treefrog Hyla eximia in Central Mexico. We also studied the species’ relationship with climatic variables such as temperature and precipitation. We found an interannual constant abundance during the study period. However, interannual differences were observed in the population structure by age-sex category (males, females, or juveniles), with decreased abundance of males and juveniles during the rainy months (August–November). The annual abundance of H. eximia was positively correlated with rainfall, but negatively with monthly temperature. We found the sex ratio was male-biased (2:1), except for year 2008. Also, differences in snout-vent length (SVL) were found between years, suggesting changes in recruitment of new individuals. We conclude that variations in abundance, and frequencies by age-sex category, of H. eximia are related to seasonal variations in temperature and precipitation characteristics of temperate zones. However, this temporal stability may suggest that anurans have an unusual capacity to persist even in the face of human-induced habitat change.

Intraspecific variation in the diet of the Mexican garter snake Thamnophis eques

The Mexican Garter Snake (Thamnophis eques) is a terrestrial-aquatic generalist that feeds on both aquatic and terrestrial prey. We describe size-related variation and sexual variation in the diet of T. eques through analysis of 262 samples of identifiable stomach contents in snakes from 23 locations on the Mexican Plateau. The snake T. eques we studied consumed mostly fish, followed in lesser amounts by leeches, earthworms, frogs, and tadpoles. Correspondence analysis suggested that the frequency of consumption of various prey items differed between the categories of age but not between sex of snakes, and the general pattern was a reduction of prey item diversity with size of snake. Snake length was correlated positively with mass of ingested prey. Large snakes consumed large prey and continued to consume smaller prey. In general, no differences were found between the prey taxa of male and female snakes, although males ate two times more tadpoles than females. Males and females did not differ in the mass of leeches, earthworms, fishes, frogs and tadpoles that they ate, and males and females that ate each prey taxon were similar in length. We discuss proximate and functional determinants of diet and suggest that the observed intraspecific variation in T. eques could be explored by temporal variation in prey availability, proportions of snake size classes and possible sexual dimorphism in head traits and prey dimensions to assess the role of intersexual resource competition.

Morphological convergence in a Mexican garter snake associated with the ingestion of a novel prey

Abstract Morphological convergence is expected when organisms which differ in phenotype experience similar functional demands, which lead to similar associations between resource utilization and performance. To consume prey with hard exoskeletons, snakes require either specialized head morphology, or to deal with them when they are vulnerable, for example, during molting. Such attributes may in turn reduce the efficiency with which they prey on soft‐bodied, slippery animals such as fish. Snakes which consume a range of prey may present intermediate morphology, such as that of Thamnophiine (Natricinae), which may be classified morphometrically across the soft–hard prey dietary boundary. In this study, we compared the dentition and head structure of populations of Thamnophis melanogaster that have entered the arthropod–crustacean (crayfish)‐eating niche and those that have not, and tested for convergence between the former and two distantly related crayfish specialists of the genus Regina (R. septemvittata and R. grahamii). As a control, we included the congener T. eques. Multivariate analysis of jaw length, head length, head width, and number of maxillary teeth yielded three significant canonical variables that together explained 98.8% of the variance in the size‐corrected morphological data. The first canonical variable significantly discriminated between the three species. The results show that head dimensions and number of teeth of the two Regina species are more similar to those of crayfish‐eating T. melanogaster than to non‐crayfish‐eating snakes or of T. eques. It is unclear how particular head proportions or teeth number facilitates capture of crayfish, but our results and the rarity of soft crayfish ingestion by T. melanogaster may reflect the novelty of this niche expansion, and are consistent with the hypothesis that some populations of T. melanogaster have converged in their head morphology with the two soft crayfish‐eating Regina species, although we cannot rule out the possibility of a morphological pre‐adaptation to ingest crayfish.