Copyright (c) 2012 All rights reserved. http://works.bepress.com/etaylor Recent documents in Emily N. Taylor en-us Thu, 06 Dec 2012 01:51:30 PST 3600 http://works.bepress.com/etaylor/15 http://works.bepress.com/etaylor/15 Tue, 04 Dec 2012 15:40:36 PST Parasitism is one of the most notable forms of symbiosis in the biological world, with nearly all organisms hosting parasites. In many vertebrates, males have higher ectoparasite burdens than females, especially when testosterone concentrations are elevated. Furthermore, reproductive females may have higher ectoparasite burdens than non-reproductive females. It is possible that testosterone-stimulated behaviors in males and offspring investment by females incur energetic costs that inhibit immune function. If questing ticks can sense host sex or reproductive condition prior to attachment, they could potentially choose hosts with the poorest immune function, thereby leading to improved feeding success and decreased feeding duration. In this study, we examined the host–parasite relationship between western fence lizards (Sceloporus occidentalis) and the western black-legged tick (Ixodes pacificus) to test the following hypotheses: (1) ticks prefer male lizards to female lizards. (2) Ticks prefer male lizards with higher testosterone. (3) Ticks prefer reproductive female lizards to non-reproductive female lizards. (4) Ticks feed to repletion more rapidly (decreased feeding duration) on reproductive females and males with higher testosterone. In all three experiments, ticks failed to show a preference for one group over another as demonstrated by similar attachment rates between groups. This suggests that observed differences in ectoparasite loads in free-ranging lizards is due to some other factor than host choice. However, tick feeding duration on female lizards was shorter when hosts were reproductive, suggesting that host reproductive condition alters tick feeding, possibly due to a decreased immune response. Interestingly, ticks fed more slowly on male lizards with elevated testosterone, suggesting that testosterone may actually improve immune function against ectoparasites.

]]> Nicholas B. Pollock et al. Articles http://works.bepress.com/etaylor/14 http://works.bepress.com/etaylor/14 Wed, 08 Feb 2012 08:02:34 PST Seasonal variability in intensities of ectoparasites of western fence lizards (Sceloporus occidentalis) was quantified in spring, summer, and autumn 2008. Lizards were suspended over pans of water in the laboratory to enumerate replete western black-legged ticks (Ixodes pacificus) and mites. Intensity of ectoparasites was greatest in spring. On average, mites comprised the greatest proportion of ectoparasites, followed by larval and nymphal ticks, respectively. Mites were on lizards year-round. Overall intensity of ticks was greatest in spring, but low during summer and autumn. Males were more heavily parasitized by ticks than females in spring, whereas females had greater intensities of mites in autumn. There was a slight, positive relationship between size of lizard and intensity of ticks, but not with intensity of mites. We report greater intensities of ectoparasites on S. occidentalis than several other studies, which may be due to differences in habitats and methods.

]]> Anthony S. Lumbad et al. Articles http://works.bepress.com/etaylor/13 http://works.bepress.com/etaylor/13 Fri, 27 Jan 2012 14:57:07 PST The hippocampus of birds and mammals plays a crucial role in spatial memory and navigation. The hippocampus exhibits plasticity in adulthood in response to diverse environmental factors associated with spatial demands placed on an animal. The medial and dorsal cortices of the telencephalon of squamate reptiles have been implicated as functional homologues to the hippocampus. This study sought to experimentally manipulate the navigational demands placed on free-ranging northern Pacific rattlesnakes (Crotalus o. oreganus) to provide direct evidence of the relationship between spatial demands and neuroplasticity in the cortical telencephalon of the squamate brain. Adult male rattlesnakes were radio-tracked for 2 months, during which time 1 of 3 treatments was imposed weekly, namely 225-meter translocation in a random direction, 225-meter walk and release at that day’s capture site (handling control) or undisturbed (control). Snakes were then sacrificed and the brains were removed and processed for histological analysis of cortical features. The activity range was larger in the translocated (Tr) group compared to the handled (Hd) and undisturbed control (Cn) groups when measured via 95% minimum convex polygon (MCP). At the 100% MCP level, Tr snakes had larger activity ranges than the Cn snakes only. The volume of the medial cortex (MC) was larger in the Tr group compared to the Cn group. The MC of Hd snakes was not significantly different from that of either of the other groups. No differences in dorsal cortex (DC) or lateral cortex volumes were detected among the groups. Numbers of 5-bromo-2′-deoxyuridine (BrdU)-labeled cells in the MC and DC 3 weeks after BrdU injection were not affected by treatment. This study establishes a causal relationship between navigational demands and greater MC volume in a free-ranging reptile.

]]> Matthew L. Holding et al. Articles http://works.bepress.com/etaylor/12 http://works.bepress.com/etaylor/12 Fri, 08 Apr 2011 03:09:44 PDT In lizards, the medial cortex and dorsal cortex are thought to be homologous to the hippocampus in birds and mammals, in that these regions are functionally involved with spatial memory and learning. In birds and mammals, neurogenesis continues into adulthood in these regions and is associated with ability of spatial memory tasks. Little is known about whether adult neurogenesis contributes to memory formation or spatial memory in lizards. In some lizards, territory size is correlated with testosterone (T) levels. If T affects these brain regions to help with spatial navigation, manipulating T levels may influence the medial or dorsal cortex volumes and/or the incorporation of new neurons into these regions. We investigated the effects of castration and T replacement on neurogenesis in adult male S. occidentalis. These animals were in captivity for approximately two months. We also captured a group of adult males from the wild shortly before sacrifice to determine if captivity affects the sizes of these regions and/or neurogenesis. The density of new neurons in the medial and dorsal cortex was determined using immunohistochemistry for doublecortin (DCX), a marker of immature neurons. Numerous DCX-immunoreactive cells were found in the medial cortex, however, there were few to no DCX-immunoreactive cells found in the dorsal cortex. Captivity has negative effects on medial cortex volume and possibly neurogenesis within this region. The functional significance of the presence new neurons in the medial cortex of lizards remains to be determined. However if captivity does decrease neurogenesis, determining the effects of other variables on neurogenesis in captive animals may be difficult to determine.

]]> D. R. Pfau et al. Abstracts http://works.bepress.com/etaylor/11 http://works.bepress.com/etaylor/11 Fri, 08 Apr 2011 03:09:39 PDT The study of the hormonal regulation of reproduction in snakes is in its infancy. Studies have disproportionately examined temperate zone viperid and colubrid snakes, especially the red-sided garter snake (Thamnophis sirtalis parietalis). Indeed, extensive observational and experimental studies on T. s. parietalis form the basis for our understanding of the hormonal regulation of reproduction in snakes. This review focuses on seasonal hormone concentrations in snakes in relation to events in the reproductive cycle; the limited data available on hypothalamic hormones, gonadotropins (GTHs), hormone receptors, and binding globulins; the neuroendocrinology of reproduction; and the environmental, social, physiological, and embryonic aspects of hormonal regulation of reproduction in snakes. The review ends with suggestions for future research, including studies of a more diverse ecological and taxonomic representation of snakes, experimental studies on the effects of hormones on reproductive tissues and behaviors, and further research into the neuroendocrinology of reproduction in this highly diverse group of animals.

]]> Emily N. Taylor et al. Articles http://works.bepress.com/etaylor/10 http://works.bepress.com/etaylor/10 Tue, 02 Nov 2010 16:10:56 PDT Emily N. Taylor et al. Articles http://works.bepress.com/etaylor/9 http://works.bepress.com/etaylor/9 Tue, 02 Nov 2010 16:10:55 PDT Some organisms rely on stored energy to fuel reproductive expenditure (capital breeders) whereas others use energy gained during the reproductive bout itself (income breeders). Most species occupy intermediate positions on this continuum, but few experimental data are available on the timescale over which food intake can affect fecundity. Mark–recapture studies of free-ranging female aspic vipers Vipera aspis have suggested that reproductive output relies not only on the energy in fat bodies accumulated in previous years, but also on food intake immediately before ovulation. A simple experiment was conducted to test this hypothesis, maintaining female snakes in captivity throughout the vitellogenic period and controlling their food intake. The energy input of a female strongly influenced the amount of mass that she gained and the number of ova that she ovulated. Multiple regression showed that litter size in these snakes was affected both by maternal body condition in early spring (an indicator of foraging success over previous years) and by food intake in the spring before ovulation. Our experimental data thus reinforce the results of descriptive studies on free-ranging snakes, and emphasize the flexibility of energy allocation patterns among vipers. Reproducing female vipers may combine energy from ‘capital’ and ‘income’ to maximize their litter sizes in the face of fluctuating levels of prey abundance.

]]> Olivier Lourdais et al. Articles http://works.bepress.com/etaylor/8 http://works.bepress.com/etaylor/8 Tue, 02 Nov 2010 16:10:54 PDT Sexual dimorphism in size (sexual size dimorphism; SSD) is nearly ubiquitous, but the relative importance of genetic versus environmental control of SSD is not known for most species. We investigated proximate determinants of SSD in several species of squamate reptiles, including three species of Sceloporus lizards and the diamond‐backed rattlesnake (Crotalus atrox). In natural populations of these species, SSD is caused by sexual differences in age‐specific growth. Males and females, however, may often share similar potentials for growth: growth is strongly responsive to the availability of food, and sexual differences in growth can be greatly suppressed or completely absent under common environmental conditions in the laboratory. Sexually divergent growth is expressed in natural environments because of inherent ecological differences between males and females and because of potential epigenetic effects of sex‐specific growth regulators. In field‐active Sceloporus, sexual differences in growth rate are associated with sexual divergence in plasma testosterone. Experiments confirm that testosterone inhibits growth in species in which females are larger (for example, S. undulatus and S. virgatus) and stimulates growth in those in which males are larger (for example, S. jarrovii). Interestingly, however, sexual divergence in plasma testosterone is not accompanied by divergence in growth in S. jarrovii or in male‐larger C. atrox in the laboratory. Furthermore, experimental effects of castration and testosterone replacement on growth are not evident in captive S. jarrovii, possibly because growth effects of testosterone are superseded by an abundant, high‐quality diet. In female‐larger S. undulatus, growth may be traded‐off against testosterone‐induced reproductive costs of activity. In male‐larger species, costs of reproduction in terms of growth are suggested by supplemental feeding of reproductive female C. atrox in their natural environment and by experimental manipulation of reproductive cost in female S. jarrovii. Growth costs of reproduction, however, do not contribute substantially to the development of SSD in male‐larger S. jarrovii. We conclude that the energetic costs of testosterone‐induced, male reproductive behavior may contribute substantially to the development of SSD in some female‐larger species. However, despite strong evidence that reproductive investment exacts a substantial cost in growth, we do not support the reproductive cost hypothesis as a general explanation of SSD in male‐larger species.

]]> Henry B. John‐Alder et al. Articles http://works.bepress.com/etaylor/7 http://works.bepress.com/etaylor/7 Tue, 02 Nov 2010 16:10:52 PDT We describe the reproductive cycle of Northern Pacific rattlesnakes (Crotalus oreganus) by quantifying steroid hormone concentrations and observing reproductive behaviors in free-ranging individuals. Additionally, we examined reproductive tissues from museum specimens. Plasma steroid hormone concentrations were quantified for both male and female snakes throughout the active season (March–October). We measured testosterone (T), 5α-dihydrotestosterone (DHT), and corticosterone (B) concentrations in both sexes and 17β-estradiol (E2) and progesterone (P) in females only. We observed reproductive behaviors (e.g., consortship, courtship, and copulation) in the field and measured testis and follicle size in male and female snakes from museum collections to relate steroid hormone concentrations to the timing of reproductive events. Our study revealed that C. oreganus in central California exhibits a bimodal pattern of breeding, with most mating behavior occurring in the spring and some incidences of mating behavior observed in late summer/fall. Each breeding period corresponded with elevated androgen (T or DHT) levels in males. Testes were regressed in the spring when the majority of reproductive behavior was observed in this population, and they reached peak volume in August and September during spermatogenesis. Although we did not detect seasonal variation in female hormone concentrations, some females had high E2 in the spring and fall, coincident with mating and with increased follicle size (indicating vitellogenesis) in museum specimens. Females with high E2 concentrations also had high T and DHT concentrations. Corticosterone concentrations in males and females were not related either to time of year or to concentrations of any other hormones quantified. Progesterone concentrations in females also did not vary seasonally, but this likely reflected sampling bias as females tended to be underground, and thus unobtainable, in summer months when P would be expected to be elevated during gestation. In females, P was positively correlated with T and DHT, and E2 was positively correlated with T.

]]> Craig M. Lind et al. Articles http://works.bepress.com/etaylor/6 http://works.bepress.com/etaylor/6 Tue, 02 Nov 2010 16:10:51 PDT Food availability is an important factor in the life histories of organisms because it is often limiting and thus can affect growth, mass change, reproduction, and behaviors such as thermoregulation, locomotion, and mating. Experimental studies in natural settings allow researchers to examine the effects of food on these parameters while animals are free to behave naturally. The wide variation among organisms in energy demands and among environmental food resources suggest that responses to changes in food availability may vary among organisms. Since most supplemental feeding field experiments have been conducted on species with high energy demands, we conducted a supplemental feeding study on free‐ranging, female Western diamond‐backed rattlesnakes (Crotalus atrox), a species with low energy demands and infrequent reproductive investment. Snakes were offered thawed rodents 1‐4 times per week. Over two active seasons, we collected data on surface activity, home range size, growth, mass change, and reproduction of supplementally fed and control snakes. Fed and control snakes did not differ in surface activity levels (proportion of time encountered above versus below ground) or home range size. Fed snakes grew and gained mass faster, and had a dramatically higher occurrence of reproduction than control snakes. Also, fed snakes were in better body condition following reproduction than snakes that were not fed. However, litter characteristics such as offspring number and size were not increased by feeding, suggesting that these characteristics may be fixed. These data experimentally demonstrate that food availability can directly impact some life history traits (i.e., growth and reproduction for C. atrox), but not others (i.e., surface activity and home range size for C. atrox). The relationship between food availability and life history traits is affected in a complex way by ecological traits and physiological constraints, and thus interspecific variation in this relationship is likely to be high.

]]> Emily N. Taylor et al. Articles http://works.bepress.com/etaylor/5 http://works.bepress.com/etaylor/5 Tue, 02 Nov 2010 16:10:50 PDT Accurately assessing free-ranging animals’ patterns of surface activity and refuge use is critical, yet fundamentally challenging for biologists and wildlife managers. We evaluate the accuracy of an automated technique—temperature-based activity estimation (TBAE)—in estimating surface activity and refuge use patterns of two sympatric reptiles, the western diamond-backed rattlesnake (Crotalus atrox) and the Gila monster (Heloderma suspectum) in the Sonoran Desert. TBAE derived from a comparison of body temperature to shaded air temperature was effective in estimating the overall percent surface activity for both rattlesnakes (observed surface activity 51.8%, TBAE estimated surface activity 48.2%) and Gila monsters (observed 22.3%, TBAE 24.5%). There was, however, considerable interspecific difference in the effectiveness of TBAE in predicting surface activity at specific time points; TBAE was far more accurate for Gila monsters than for rattlesnakes (96% vs. 66% time point-specific accuracy, respectively). We assert that, when validated, TBAE can be used to yield concurrent and accurate body temperatures and activity estimates for multiple free-ranging animals, particularly in arid environments, which improves our understanding of animal biology and can be used to inform management decisions.

]]> J R. Davis et al. Articles http://works.bepress.com/etaylor/4 http://works.bepress.com/etaylor/4 Tue, 02 Nov 2010 16:10:48 PDT Recent field studies on the reproductive ecology of western diamond-backed rattlesnakes (Crotalus atrox) from populations in southern Arizona showed significant differences in the concentration of plasma sex steroids (testosterone, T; 5α-dihydrotestosterone, DHT; and 17β-estradiol, E2) throughout the active season (March–October), and peak levels were coincident with the two mating periods (late summer and early spring). There is, however, no information on levels of sex steroids during winter. Similar to most snakes, hibernating individuals of C. atrox are typically inaccessible, but in southern Arizona, where environmental conditions are typically mild during winter, adult males frequently bask at or near the entrances of communal dens. Basking activity, therefore, offers a unique logistical opportunity to assess the complete annual profile of plasma sex steroid levels in males of a temperate reptile in nature. From November to February, we measured levels of plasma T, DHT, and E2 in adult male C. atrox that were located basking at communal dens. Additionally, cloacal, core body, and ambient air temperatures were obtained to investigate potential relationships between body temperatures and levels of sex steroids. Mean levels of T, DHT, and E2 were relatively high, and the concentration hierarchy was T > DHT > E2. Mean levels of T, DHT, and E2 showed no significant variation across the four months of sampling; however, E2 levels decreased progressively. In the annul cycle, sex steroid levels during winter were not basal when compared to values obtained during the active season. Mean cloacal temperatures of basking males were significantly higher than core body temperatures of non-basking males (inside dens) from November–December, and in February, which suggests that one function of winter basking is to elevate body temperatures. Steroid levels, nonetheless, were not significantly correlated with cloacal temperatures. We suggest that future field studies of male C. atrox should: (a) investigate sex steroid levels in non-basking individuals and (b) test whether elevated levels of sex steroids during winter facilitate the large increases that occur in early spring, which are coincident with the second mating season. Our findings on the reproductive biology of C. atrox and other viperids are discussed in the context of the associated–dissociated model of reproduction.

]]> Gordon W. Schuett et al. Articles http://works.bepress.com/etaylor/3 http://works.bepress.com/etaylor/3 Tue, 02 Nov 2010 16:10:47 PDT We report seasonal variation in steroid hormone levels in blood samples from free-ranging Western Diamond-backed Rattlesnakes (Crotalus atrox), and the relationship between these hormones and events in the reproductive cycle. At a field site in the Sonoran Desert of south-central Arizona, we collected monthly blood samples over the course of two active seasons from 17 radiotelemetered females, and over three active seasons from 103 randomly encountered males. We used radioimmunoassay to measure plasma levels of 17β-estradiol, progesterone, corticosterone, and testosterone in samples from females, and corticosterone and testosterone in samples from males. Non-reproductive females have consistently low levels of circulating 17β-estradiol, progesterone, and testosterone throughout the year. In reproductive females, 17β-estradiol levels increase dramatically and testosterone levels increase modestly during vitellogenesis in April and May, while progesterone levels increase dramatically at ovulation in June and then steadily decline until parturition in August. Corticosterone levels appear relatively constant in non-reproductive females, whereas reproductive females show increased levels at the end of gestation. Plasma testosterone levels in males are low in early summer and are elevated during spring and late summer, corresponding to the two mating periods of C. atrox. Plasma corticosterone levels in males did not vary seasonally and were not related to testosterone levels.

]]> Emily N. Taylor et al. Articles http://works.bepress.com/etaylor/2 http://works.bepress.com/etaylor/2 Tue, 02 Nov 2010 16:10:46 PDT We used intracoelomically implanted temperature dataloggers to obtain semi-continuous body temperature data and establish monthly thermal profiles for free-ranging rattlesnakes. We mimicked random and non-random point-sampling methods by selecting a single daily data point from all values or from restricted times of day to reflect common point-sampling constraints. Thermal profiles generated from point-sampling differed from those generated from semi-continuous sampling, and this difference was more apparent when point-sampling was non-random. We conclude that semi-continuous sampling provides a better estimate of thermal profiles, and that point-sampling methods are highly sensitive to deviations from true randomness.

]]> Emily N. Taylor et al. Articles http://works.bepress.com/etaylor/1 http://works.bepress.com/etaylor/1 Tue, 02 Nov 2010 16:10:44 PDT We conducted an experiment to examine the effects of sex and food intake on growth, mass gain, and attainment of sexual maturity in Western Diamond‐backed Rattlesnakes (Crotalus atrox). We also measured testosterone levels to determine whether testosterone might be involved in the male‐biased sexual size dimorphism observed in this species. We collected neonate rattlesnakes and raised them in the laboratory for 2 years on either a high‐intake diet (fed one mouse per week) or a low‐intake diet (fed one mouse every 3 weeks). High‐intake snakes grew and gained mass more rapidly than low‐intake snakes, but males did not grow or gain mass more rapidly than females in either treatment group. High‐intake snakes attained reproductive maturity earlier than low‐intake snakes, indicating that size, not age, is the critical determinant of reproductive maturity. Males had higher levels of testosterone than females but did not grow more quickly, suggesting that testosterone may not affect growth in this species and may therefore not be the proximate determinant of sexual size dimorphism.

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