Showing posts with label body size. Show all posts
Showing posts with label body size. Show all posts

Monday, July 25, 2011

Why Are Some Snakes More Common Than Others?

Ringnecked Snakes, Diadophis punctatus 
reach some of the most dense populations 
found in snakes. JCM

There is a reason why more people study lizards than snakes, snakes are notorious for being difficult to find. Undoubtedly their cryptic nature and in some cases low population densities are likely contributors to this situation. A paper in Science this week by Hechinger et al. examined parasites (there are no known parasitic snakes), but the authors produced two general rules that they suggest can be applied to animal abundance – for any species. The following quotes were taken from a press release associated with this article. 

First they said that, "In addition to body size, the general rule for animal abundance must factor in the food chain and let both small and large animals be top consumers." The second rule was, "that the amount of biomass produced by a population does not depend on the body size of the animals in the population, or on what type of animal -- bird, fish, crab, or parasite. So, "If this rule is general, it means an aphid population can produce the same amount of biomass as a deer population," said Lafferty. "Furthermore, tapeworms that feed on the deer population produce less biomass than the deer, but can produce the same as a mountain lion population that also feeds on the deer. Predicting animal abundance is one of the most basic and useful things ecological science can provide for management and basic research," said Hechinger. "This simple rule helps with that because it may apply to all life forms and can easily be applied to complex ecosystems in the real world." So, does this apply to snakes? Snakes are all predators or facultative scavengers. Does it explain why many snakes are very difficult to find, while other species seem quiet abundant in their habitats?

Parker and Plummer (1987) compiled a table of population densities of snakes, undoubtedly many more have been published since - but this information was readily available for this post. I added column for size.

In looking at the table below, Hechinger et al seem to be correct, the snakes with the most dense populations are small species (Carphophis and Diadophis), species feeding on invertebrates that are close to their food source - a short food chain; or they are piscivorous species that are probably taking prey that are feeding on autotrophic protists, scavenging decomposing material, or have other abundant food resources. The rarer snakes tend to feed on mammals, have larger body sizes, or live in environments with low productivity (Lampropeltis, Pantherophis, Eryx).

Size (m)
Acrochordus arafurae
Eryx tataricus
Carphophis amoenus
Coronella austriaca
Diadophis punctatus
Elaphe dione
Pantherophis obsoleta
Elaphe quadrivirgata
Pantherophis vulpina
Heterodon nasicus
Heterodon platirhinos
Lampropeltis calligaster
Lampropeltis triangulum
Lycodonomorphus bicolor
Protobothrops flavoviridis
Vipera berus
Vipera ursinii

Hechinger, R. F., K. D. Lafferty,. A. P. Dobson, J. H. Brown, A. M. Kuris. 2011. A Common Scaling Rule for Abundance, Energetics, and Production of Parasitic and Free-Living Species. Science, 2011; 333 (6041): 445-448 DOI: 10.1126/science.1204337.

Parker, W. S. and M. V. Plummer. 1987. Population Ecology. Pages 253-301 In: R. Seigel et al. (eds.) Snakes, Ecology and Evolutionary Biology. New York: McMillian Publishing.

Friday, December 17, 2010

Crocodile Skinks of the South Pacific

Tribolonotus gracilis. JCM
Skinks come in a plethora of shapes and sizes, with about 1500 species they have invaded a huge variety of habitats and evolved a great diversity of life styles. They are found in almost all landscapes that support squamates and are perhaps the most successful lineage of living reptiles – if the measure is by the number of species. Among the most bizarre skinks are the South Pacific Crocodile Skinks of the genus Tribolonotus. Currently 8 species are recognized and they inhabit northern New Guinea and the Admiralty, Bismarck and Solomon Archipelagoes. They are semi-fossorial lizards, often found under vegetation and in the vicinity of water. At least two species are known to vocalize, and they demonstrate parental care. Hartdegen et al. (2001) described defensive vocalizations and parental care in captive specimens. They observed females curled around their egg and when eggs were gently handled by the observer the female exhibited defensive open-mouth lunges. When eggs were left uncovered by human observers they were reburied by the female. Hatchlings stayed near their mother (within 2 cm) and on occasion they were observed resting on the female's dorsum for two weeks after hatching. Their overall appearance is distinctive; they have unusually heavily keeled or spinose scales and two unique characters, abdominal glands and volar pores (pores on the plantar and palmer surfaces).

Recently, Austin et al. (2010) used molecular techniques and found evidence that Tribolonotus originated on either Greater Bougainville Island or in New Guinea, and subsequently dispersed to surrounding islands multiple times. Maximum body size ranged from 40 mm in T. blanchardi (and T. schmidti was a close second at 41 mm) to 125 mm in T. ponceleti. The authors did not find a phylogenetic explanation for differences in body size, and suggest that it evolved as the result of character displacement and ecological factors.

Exactly what the sister of Tribolonotus is remains a point of contention. They have been considered lygosomine skinks, allied with the genera Sphenomorphus, Mabuya, and Egernia by various authors. However,  Donnellan (1991) found Tribolonotus gracilis, has 32 chromosomes, a similar karyotype, to Egerina but there were differences that did not allowed a firm conclusion.

Austin, C. C., E. N. Rittmeyer, S. J. Richards and G. R. Zug. 2010. Phylogeny, historical biogeography and body size evolution in Pacific Island Crocodile skinks Tribolonotus (Squamata; Scincidae). Molecular Phylogenetics and Evolution, 57:227-236.

Donnellan, S. C. 1991. Chromosomes of Australian lygosomine skinks (Lacertilia: Scincidae).Genetica 83:207-222.

Hartdegen, R. W., M. J. Russell, B. Young, and R. D. Reams. 2001. Vocalization of the Crocodile Skink, Tribolonotus gracilis (De Rooy, 1909), and evidence of parental care. Current Herpetology 2001(2). On-line.

McCoy, M., 2006. Reptiles of the Solomon Islands. Pensoft Publishing, Sofia-Moscow.

Sunday, December 5, 2010

The Strike of a Snake and Body Size

Body size in animals plays a major role in the animal's biology. John Tyler Bonner wrote that "...size is an aspect of the living that plays a remarkable overreaching role that affects life's matter in all its aspects." In a recent paper Herrell et al (2010) investigate size-related changes in defensive strike performance in the White-lipped Green Pit Viper, Trimeresurus (Crypteletrops) albolabris. T. albolabris is an arboreal pit viper from Southeast Asia this is an ambush specialist. However, in some species juveniles select higher, more open foraging sites, possibly giving them a thermal foraging advantage.Thus juvenile arboreal pit vipers may be more exposed while foraging, and consequently subjected to a larger number of predators. Larger individuals tend to forage more in terrestrial situations and usually eat larger prey. The sexes may also be expected to differ in strike performance capacity if they select different prey types, use different types of habitats, or are simply different in size. Striking fast and far is likely of importance in both (defensive and feeding. Fast, long strikes can deter potential predators as well as allow snakes to capture elusive prey.  The authors used 18 female and 17 male albolabris, housed at the Faculty of Science, University of Zagreb, Croatia. They used a digital high speed  camera set at 400 frames per second and filmed 129 strikes made by 29 individual snakes and examined  how defensive strike performance changes with body size in both male and female. Their data show a significant negative allometry in the scaling of head dimensions and head mass to body mass. However, strike velocity and strike distance are independent of body mass, with juveniles in the sample striking as fast and as far as adults. Contrary to model predictions suggesting that acceleration capacity should decrease with increasing body mass, acceleration capacity increases with snake body mass, and that this is the result of a negative allometric scaling of head mass combined with an isometric scaling of the dorsal epaxial musculature. Finally, the data showed a significant sexual dimorphism in body size and strike velocity with females being heavier and striking faster, independent of the dimorphism in body size. Mean strike velocity was 1.5–1.6ms for males and females, respectively.

Bonner, J. T., 2006. Why Size Matters. Princeton University Press. 161 pp.

Herrel, A., K. Huyghe, P. Okovic´, D. Lisicˇic´, Z. Tadic. 2010. Fast and furious: effects of body size on strike performance in an arboreal viper Trimeresurus (Cryptelytrops) albolabris. Journal of Experimental Zoology. 313A. DOI: 10.1002/jez.645

Wednesday, December 1, 2010

A Giant in a Genus of Small Snakes

Atractus gigas
Perhaps the most specious genus of snakes is Atractus (Family Dipsididae) with about 130 species.  Atractus are commonly known as ground snakes, and tend to be small to medium snakes that feed on earthworms, arthropods, and mollusks. They are distributed from Panama to Argentina and overall knowledge about their natural history is at best very incomplete. They occur from sea level to 4,500 m in elevation and many, if not most of them, are known only from the type specimens. They are closely related to the Middle American and northern South American fossorial and cryptozoic genera Adelphicos and Geophis. Atractus range in size from snakes that are less than 200 mm at maturity to the giant, Atractus gigas that exceeds 1000 mm. Myers and Schargel (2006) described Atractus gigas from a single specimen collected on the west side of the Ecuadorian Andes, Tolhurst et al. (2010) recently reported a second specimen of A. gigas about 50 km from the type locality on the basis of photographs.  Now, Passos et al (2010) discovered additional specimens of this poorly known snake in museum material and collected new specimens during fieldwork in the northeastern Peruvian Andes. The largest female was 1040 mm in body length, while the largest male was 255 mm in body length (presumably this was a subadult). The female’s body size makes this species the largest in the genus. The authors describe the juvenile and adult color patterns and detail the sexually dimorphic scale counts. They encountered this snake at Santuario Nacional Tabaconas Namballe, San Ignacio, Peru, on a coffee plantation and in nearby montane forest. Individuals were observed from early morning to late afternoon. Thus, Atractus gigas inhabits primary and secondary cloud and montane forest as well as coffee plantations between 600 and 2300 m on both sides of the Andes. The diurnal activity of this snake is also unusual for the genus, since many others ground snake species are known to be nocturnal. One female contained 12 oviductal eggs, this is an exceptionally large clutch size for an Atractus [other Atractus with known clutch sizes, i.e. A. reticulatus and A. trilineatus, have 1-6 eggs]. Many Atractus show sexual dimorphism with large females and smaller males. Thus this clade of snakes may make an excellent study group to test hypotheses about sexual dimorphism in snakes - given the number of species it is likely one or more has males that are larger than females.

Myers, C. W. and W. E. Schargel. 2006. Morphological extremes – Two new snakes of the genus Atractus from northwestern South America (Colubridae: Dipsadinae). American Museum Novitates, 3532: 1‑13.

Passos, P., M. Dobiey, and P. J. Venegas  2010. Variation and Natural History Notes on Giant Groundsnake, Atractus gigas (Serpentes: Dipsadidae). South American Journal of Herpetology 5(2):73-82.

Tolhurst, B.; M. Peck; J. N. Morales; T. Cane and, I. Recchio. 2010. Extended distribution of recently described dipsadine colubrid snake: Atractus gigas. Herpetology Notes, 3: 73‑75.