Imantodes cenchoa (Linnaeus 1758)

Apart from male-male combat that may occur during mating, very little is known about intraspecific interactions between individuals. Blunthead tree snakes rely primarily on their binocular vision and vertically slit, protruding eyes to hunt at night. Like all snakes, they have well developed olfactory, tactile, and hearing systems.

Communication Channels: visual ; tactile

Perception Channels: visual ; tactile ; acoustic ; vibrations ; chemical

Blunthead tree snakes have not yet been evaluated by the IUCN and are not considered threatened by any conservation agency. Their cryptic nature and nocturnal habits make population size estimations difficult, though they are known to have low population densities in various areas throughout their large geographical range.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

Young develop in eggs laid by females, nourished by yolk. Eggs average 30.7 mm in length. Juveniles hatch, appearing as small adults (average SVL 279.7 mm), typically from March through August, and grow approximately 3.5 millimeters per week for their first two years of life. After this time, they will typically have achieved sexual maturity.

Development - Life Cycle: indeterminate growth

Although these snakes are venomous, their bite is not particularly harmful to humans. These snakes are typically docile and are well hidden during the day, limiting any potential human interaction.

Negative Impacts: injures humans (bites or stings)

No economic benefits to humans from this snake are currently known. Due to their small, thin bodies and general fragility, they are not commonly seen in the pet trade. The general lack of knowledge regarding this species, however, does leave the potential for additional research.

Positive Impacts: research and education

These snakes are primarily predators of Anolis lizards and frogs. While no published information exists regarding specific predators of this snake species, it is likely a food source for various bird of prey species.

Commensal/Parasitic Species:

• Eimeria cenchoae (Order Eucoccidiorida, Phylum Apicomplexa)
• Kalicephalus costatus (Order Strongylida, Phylum Nematoda)

Blunthead tree snakes forage at night, feeding on small arboreal lizards (primarily anoles, such as Norops capito, Anolis latifrons, Norops limifrons, Norops mariarum, and Norops tropidogaster). In addition to small lizards, these snakes have been known to target adult frogs (such as Craugastor crassidigitus and Craugastor raniformis) as well as frog (Agalychnis callidryas) and reptile eggs. Female blunthead tree snakes from Panama are known to be capable of consuming larger prey, such as Anolis frenatus due to their larger heads, leading to a difference in feeding trends not only regionally but between males and females in this area.

Animal Foods: amphibians; reptiles; eggs

Primary Diet: carnivore (Eats terrestrial vertebrates, Eats eggs)

Blunthead tree snakes are found in Central America (eastern Mexico to Panama) and throughout the coastal countries of northern and western South America (Venezuela, Columbia, Ecuador, Peru, Bolivia, French Guiana, Brazil, Paraguay, Peru, and Argentina), as well as the islands of Trinidad and Tobago.

Biogeographic Regions: neotropical (Native )

Blunthead tree snakes occupy primary and secondary growth forests as well as plantation areas in lowland moist and wet forests. They also occur in premontane wet forests and rainforests, as well as lower montane wetforests and rainforests. These snakes are arborial, often found in low vegetation including coffee trees and bromeliads, and can be found up to 2000 meters above sea level (most often 1500 meters or below).

Range elevation: 0 to 2000 m.

Habitat Regions: tropical ; terrestrial

Terrestrial Biomes: forest ; rainforest ; mountains

No published information exists regarding the average lifespan of these snakes; it is suspected to be highly variable throughout their broad geographical range.

Blunthead tree snakes have long, slim bodies and very thin necks, with large heads. Their eyes, which have vertical pupils, account for approximately 25% of the length of the head and protrude from the side of the head, enabling the animal to look downward. Snout to vent length for this species is generally around 800 millimeters, but can reach over 901 millimeters; total body length can exceed 1 meter. This snake has an adaptive, enlarged middorsal scale row, which provides stability when climbing. Blunthead tree snakes are primarily white ventrally and pale brown dorsally. The dorsal surface is overlaid with 29-56 dark brown blotches (the number of dorsal blotches varies across this species' range). These snakes have rear fangs. Northern populations of these snakes exhibit sexual dimorphism, with males having slightly longer tails (southern populations do not share this characteristic). Additionally, in some regions head size is variable between males and females, with females typically having greater head lengths and widths.

Range length: 232 to 901 mm.

Average length: 800 mm.

Other Physical Features: venomous

Sexual Dimorphism: female larger

While no information regarding specific predators of blunthead tree snakes currently exists, main predators of other snakes in Latin America include birds of prey such as laughing falcons (Herpetotheres cachinnans) and crane hawks (Geranospiza caerulescens). These nocturnal snakes hide during the day and their coloration acts as camouflage, helping them to avoid predation.

Anti-predator Adaptations: cryptic

Mating may occur in this species year round throughout its range, although the mating season may be correlated with the rainy season in some regions. Areas with long rainy seasons tend to produce longer reproductive seasons, whereas areas with shorter rainy seasons call for a much more rapid reproduction and development process. A few instances of mating related male-male fighting have been recorded for this species, both in the Brazilian Amazon and in a Peruvian rainforest. This is particularly notable, as no other record of ritualistic male-male combat has been reported for any other species within the dipsadine clade. One account of such behavior records a male of a mating pair approaching another nearby male and using the anterior portion of his body to force away this potential rival.

Mating System: polygynandrous (promiscuous)

Blunthead tree snakes generally exhibit continuous reproduction. However, in areas that have seasonal rainfall, egg laying and hatching is found to correlate positively with local wet seasons; for example, in Guatemala and Mexico, eggs are laid in June and July, with hatchlings appearing in July and August, corresponding with the wet seasons of these countries. In Brazilian rainforests, continuous reproduction occurs; vitellogenesis (yolk production) takes place from September through November or December, eggs are laid from November through January, and hatchlings appear from March through August. These snakes are oviparous. An average clutch will contain 2-3 eggs; clutch size is dependent on factors such as female body size, habitat and feeding habits. Female blunthead tree snakes reach sexual maturity at about 620 mm SVL; males mature around the same size, typically about two years after hatching.

Breeding interval: Blunthead tree snakes may breed multiple times throughout the year, depending on the region where they live.

Breeding season: Some populations of blunthead tree snakes breed year round while other have breeding seasons correlating to rainy seasons.

Range number of offspring: 1 to 3.

Average age at sexual or reproductive maturity (female): 2 years.

Average age at sexual or reproductive maturity (male): 2 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; sexual ; fertilization ; oviparous

There is no literature currently available that describes the parental care in these snakes. Brooding is uncommon in all species of snakes apart from those in the family Pythonidae. Therefore, it is a reasonable assumption, as this species is oviparous, that the female most likely leaves her eggs after laying them and that neither the male nor female looks after the hatchlings.

Parental Investment: no parental involvement

Imantodes cenchoa

NATURAL HISTORY

Imantodes cenchoa is a slender, elongate (snout-vent length to 901 mm but adults usually less than 800 mm), arboreal snake. It occurs in primary and secondary forests mainly below 1500 m but up to 2000 m. Frequenting low vegetation, it is commonly found in bromeliads (Stuart, 1948; Taylor, 1951) and coffee trees (Slevin, 1939; Landy, et al., 1966) during the day. In a simulated natural environment, captive specimens spent 90 percent of daylight hours coiled in bromeliads (Henderson and Nickerson, 1976). One adaption for arboreal living, an enlarged middorsal scale row, provides rigidity while spanning gaps between branches (Schmidt and Inger, 1957) as much as one-half its body length (Gans, 1974). It forages for food at night, often feeding on sleeping anoles (Henderson and Nickerson, 1976). Small arboreal lizards (mainly Anolis sp.) appear to be the principal prey (Henderson and Nickerson, 1976; Landy, et al., 1966; Stuart, 1948, 1958; Wehekind, 1955), although reptile eggs have also been reported as stomach contents (Landy, et al., 1966). Beebe (1946) reported an individual pursuing a tree frog (Ololygon rubra). In captivity, frogs (Colostethus and Eleutherodactylus) are accepted as food (Test, et al., 1966).

BODY SIZE

HATCHLINGS.—The largest snake with a yolk-sac scar was 327 mm (snout-vent length). Out of 17 individuals of this length or less, five (232, 255, 259, 272, and 327 mm long) possessed yolk-sac scars. The average snout-vent length of hatchlings is 279.7 ± 27.2 mm; range 232–327.

GROWTH AND SEXUAL MATURITY.—Sexually mature females of Imantodes (all localities) had a mean snout-vent length of 715.9 ± 67.7 mm and a range of 621–901 mm (N = 33). Females (5) with oviducal ova and class V follicles have an average snout-vent length of 789.4 ± 96.7 mm (637–901). The sexually mature females from Chiapas (2) have a mean length of 708.5 (693–724), from Honduras (3) 640.7 ± 21.7 mm (621–664), from Costa Rica (1) 639.0 mm, from Panama (7) 734.3 ± 83.4 mm (662–901), from Ecuador (15) 742.5 ± 59.6 mm (641–818), and from Peru (5) 674.0 ± 47.6 mm (628–732). From these data, we infer that female Imantodes reach sexual maturity at about 620 mm. Again, as in both Coniophanes and Dipsas, we can only speculate that males mature at about the same size or less.

A size-month matrix for Imantodes (Figure 11) exhibits weak segregation of size classes. We propose that most hatchlings appear from March through August. After one year, they have grown to an average of 450 mm, and after two years of growth, their average body length approaches 640 mm. This suggests that most of the snakes have attained sexual maturity by the end of two years. From these speculations, an approximate growth rate of 3.5 mm/week (first two years) can be estimated.

SEXUAL DIMORPHISM

BODY AND TAIL LENGTH.—In the more northern samples (Chiapas; Yoro, Honduras; Costa Rica) adult males on the average have longer tails than equal-sized females (Figure 12) but only slightly so. Snakes from southern localities (Panama, Ecuador, and Peru) do not show sexual dimorphism in tail length (Figure 12). The Ulúa River, Honduras, sample (N = 20) females possess longer tails.

GONAD POSITION.—The relative position of the gonads in Imantodes shows little sexual dimorphism. F values show borderline significance for our two larger samples; 8.02, df 1/27 for Ecuador (Napo), and 1.78, df 1/13 for Peru (Loreto). The gonads are situated 6 to 8 percent of snout-vent length from the vent.

TAIL BREAKAGE.—Tail break frequency in Imantodes is low but substantial. Combining localities, the incidence is 6 percent in females and 10 percent in males of all sizes, 9 percent in mature females, and 14 percent in mature males. Males of all sizes have a higher incidence of tail breaks in five of nine local samples, females in only one of nine; mature males have a higher incidence of breaks in four of the nine samples, females in two of nine. These differences are not significant (x2 = 0.95 for all sizes of all localities; x2 = 0.40 for adults of all localities); hence there is no evidence of differential prédation of the sexes.

SEX RATIO

In six of the nine local samples for all sizes, males outnumber females = 3.64 for combined nine samples). The same trend is observed when considering only mature snakes (x2 = 3.52). The Panamanian sample differs in having a preponderance of females (13 females: 4 males, juveniles and adults; 7:2, only adults). Only the Chiapas and Rio Bobonaza samples show an equality of sexes.

REPRODUCTION

CLUTCH SIZE.—The modal clutch size for oviducal eggs in I. cenchoa is two. Classes IV and V follicles show a modal clutch size of three in the two northern samples (Chiapas and Honduras), whereas the three southern samples (Ecuador and Peru) have modal sizes of two (Table 6). The range for all samples combined is one to three. The single instance of three oviducal eggs occurred in the largest female (snout-vent length 901 mm; Panama). Despite the inadequate sample size, this may suggest, as in Coniophanes, a positive correlation between body size and clutch size (Figure 13), although available data provide a regression slope of nearly zero. Potential clutch size in classes II and III ranges from 1–14 with 4–6 occurring most frequently in II and 1–3 most often in III.

Fitch (1970) reports a gravid female from Iquitos, Peru, containing two oviducal eggs. Egg size (oviducal) in our sample (9) ranges from 21.2 to 37.2 mm long, with a mean of 30.7 ±5.13 mm.

REPRODUCTIVE CYCLE.—In the Ecuadorian and Peruvian samples (Figure 11), the occurrence of hatchlings throughout the year probably indicates continuous reproduction. Although the northern sample (Mexico to Panama) collectively shows similar trends, individual localities lack a sufficient number of hatchlings to draw conclusions (Table 7). Owing to the seasonal nature of rainfall at some of the sample localities, e.g., Panama Canal Zone, a seasonal reproductive cycle might be found in these populations. The presence of gravid females during late April and mid-June in Guatemala (Stuart, 1948) and of hatchlings during July and August in adjoining Chiapas, Mexico, indicates an extended reproductive season. It correlates well with the long wet season of that area, May through November (Stuart, 1948).

Discussion and Summary

Our two primary questions on reproduction—regional variation in clutch size and seasonality of reproduction—remain inadequately answered. The opportunistic nature of museum collections does not currently permit us to obtain large and year-round samples from single localities. A few old and large collections of single snake species are available, but all too often they lack collecting dates or the viscera have been removed. Surprisingly and disappointingly, we have discovered that, all too frequently, snakes from recent collections have been poorly prepared and preserved and not uncommonly held in captivity before being preserved. Both practices lead to inaccuracies in the initial data gathering and the final interpretations. In spite of these difficulties, museum collections with or without recently collected specimens can contribute to an improved understanding of snake reproduction.

Coniophanes fissidens, D. catesbyi, and I. cenchoa are small or, at most, moderate-sized snakes. Their modal clutch sizes of three, two, and two eggs, respectively, are a reflection of their short and/or narrow -body cavities. Perhaps the possession of small clutch sizes makes them poor candidates for demonstrating regional variation in clutch size. We suspect, however, that life history adaptations, such as growth rate, size at sexual maturity, and average adult life span, will produce locally different reproductive patterns. Differences in clutch sizes will be as apparent in “small clutch” species as in “large clutch” species owing to the lower variance of “small clutch” species. We think our inability to recognize the presence or absence of regional variation results from our small sample sizes.

Coniophanes fissidens is the smallest of the three species examined, yet it has the largest clutch. This is accomplished by having the shortest egg length, mean of 22.4 mm. In D. catesbyi and I. cenchoa, mean egg length is 27.7 and 30.7 mm, respectively; egg length increases directly with body length. This suggested trend is provocative. How closely are egg length and volume associated with body size, intra-and interspecifically? Presumably egg (fertile) size is relatively constant within a species but increases with increasing body size in species comparisons. The larger egg will result in a larger hatchling, whose chance of survival is presumably enhanced by its larger size. Nonetheless, egg size must have an upper limit (plateau) where the advantages of large hatchling size are outweighed by the female's need to limit energy expenditure in egg production and/or to invest energy in egg number rather than size. Fitch's data (1970, figs. 12, 14) show that few subtropical or tropical snakes invest heavily in large clutch sizes (80 percent of snakes with clutch size of 12 or less) no matter what the adult body size of the species. The indication is that there is selective pressure for larger eggs or fewer eggs per clutch but more clutches per year.

Of the three species examined, only the data for the Peruvian D. catesbyi and Ecuadorian/Peruvian I. cenchoa are sufficient to indicate continuous or aseasonal reproduction. At the other localities for these two species and all localities for C. fissidens, the data are insufficient to discriminate between continuous and seasonal reproduction; however, we intuit the Central American samples, particularly the Mexican ones, to have seasonal reproduction for Coniophanes and Imantodes.

Our bias is to assume cyclic or seasonal reproductive patterns in subtropical and tropical snakes unless data indicate otherwise. Fitch (1970) and other biologists have the opposite bias. No matter what the researcher's preference, we must all be aware of the potential diversity in reproductive cycles at a single site and the potential of a species for modifying its pattern at different locations. The first point is demonstrated by the recognition of six reproductive patterns in Cambodian snakes (Saint Girons and Pfeffer, 1971): (1) polyestrous with aseasonal reproduction, (2) polyestrous with midwet season reproduction, (3) monoestrous with spring reproduction, (4) monoestrous with early summer reproduction, (5) monoestrous with early fall reproduction, and (6) double period of reproduction (data can also be interpreted as monoestrus with winter or dry season reproduction). The snakes of the Iquitos region, Peru, show both continuous and seasonal reproduction (Dixon and Soini, 1977, table 1); data are insufficient for finer subdivisions. The modification of scheduling pattern by a tropical colubrid at different localities is indicated by our data and those of others, although as yet not convincingly so.

Neill (1962) postulated seasonal reproduction for Belize snakes by estimating the age of posthatchlings on the basis of yolk-sac scar condition. His evidence suggested an August to October period for hatching and birth and a May to July period for fertilization and egg laying. The synchronization mechanism was assumed to be the cooler temperatures of December through February and the resulting period of inactivity. Henderson and Hoevers (1977) agreed with the reproductive schedule proposed by Neill but disagreed with the mechanism and its periodicity. They suggested the February to May dry season as the mechanism. Unwittingly, Henderson had presented data earlier (1974, table 1) that demonstrates that the period of inactivity is December through March, but he did not analyze these data. A comparison of the growth rates of Belize Oxybelis aeneus shows that the rate is 0.12 ± 0.15 mm/day during the dry season (December to March) and 0.62 ± 0.56 mm/day during the wet season (April to November)—a strong indication of reduced activity during the dry season.

The annual growth rate for Oxybelis aeneus (calculated from Henderson, 1974, table 1) is 0.87 ± 0.47 and 1.18 ± 0.75 mm/day in immature (less than 700 mm snout-vent length) females and males, respectively. These rates are considerably faster than our estimates of 1 mm/week for Coniophanes fissidens, 1 mm/week Dipsas catesbyi, and 3.5 mm/week Imantodes cenchoa. With the possible exception of I. cenchoa, our estimates of growth may be underestimates, for aside from Carphophis vermis (Clark, 1970), other snakes have a growth rate greater than 0.5 mm/day, e.g., Tropidoclonion (Blanchard and Force, 1930), Australian elapids (Shine, 1978).

Underestimates of growth will result in overestimates of age at sexual maturity. Our estimates are two to three years for these tropical snakes. Estimates for temperate-zone colubrids are usually no longer than these and often shorter. Three Australian elapids, Unechis gouldii, Hemiaspis daemelii, and H. signata, reach sexual maturity in the surprisingly short time of one year (Shine, 1978). It seems doubtful that any tropical species will mature faster than these temperate-zone species.

The available evidence on reproduction in tropical snakes suggests that they are not very different from their temperate-zone conspecifics and congenerics. Only the possibility of continuous reproduction and two or more clutches per year is available to tropical colubrids, but not all tropical localities will permit even these reproductive adaptations.

Imantodes cenchoa (common names: blunthead tree snake, neotropical blunt-headed tree snake and fiddle-string snake) is a species of mildly venomous, rear-fanged snake in the family Colubridae. The species is native to in Mexico, Central America, and South America.