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Bipes biporusFive-toed Worm Lizard

By Morgan Casanova

Geographic Range

Bipes biporus (Mexican Mole Lizards) are found solely on the Baja California Peninsula of Mexico. Their range extends throughout the southern half of the Baja California peninsula, west of the Peninsula Ranges. They can be found as far south as Cabo San Lucas and north to the northwest edge of the Vizcaíno Desert. (Grismer, 1955; Papenfuss, 1982)

Habitat

The distribution of Bipes biporus reflects the type of environment it inhabits. The range is widest in the Vizcaíno Desert and the Magdalena Plain because the soil is loose and dry. The climate is seasonably cool in these areas. Mexican Mole Lizards build intricate burrowing systems that run horizontally right below the surface of the soil. The systems usually emerge to the surface under rocks or logs. Because Bipes biporus is a burrowing species, data are difficult to collect regarding the depths that they are found, and the average and maximum depths in the soil that they inhabit is unknown. However, Papenfuss (1982), found B. biporus from 2.5 cm to 15 cm deep, with most being found at 4 cm. (Grismer, 1955; Papenfuss, 1982)

  • Range elevation
    0.025 to 0.15 m
    0.08 to 0.49 ft
  • Average elevation
    0.04 m
    0.13 ft

Physical Description

Bipes biporus can be identified by its small, ossified head, a cylindrical body covered with scales in a pattern of vertical rings (annuli), and two preanal pores. Juveniles are mainly pink in color, turning white as they mature. Mexican Mole Lizards do not exhibit sexual dimorphism, so sexual identity can only be determined by looking at the gonads. The species within the family Bipedidae differ from other Amphisbaenians in that they have forelimbs. All other members of this group are completely legless. The three species of Bipedidae have small, powerful forelimbs that are specialized for burrowing. Bipes biporus is the only member of the family with five clawed digits on each forelimb. When compared to the other two species in the family, B. canaliculutus and B. triductylus, B. biporus has the shortest average tail length. All species in the family Bipedidae exhibit tail autotomy, but they are unable to regenerate their tails. In B. biporus, tail autotomy occurs between caudal annuli 6 and 10. There is a positive correlation between tail autotomy and body size. Since the larger specimens are usually older, it can be inferred that older specimens are more likely to be tailless than younger specimens. This is due to the higher likelihood of older animals coming into contact with a predator at some point in their lives. (Grismer, 1955; Papenfuss, 1982)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    240 (high) mm
    9.45 (high) in
  • Average length
    190-210 mm
    in

Development

In the close relative Bipes canuliculutus, hatchlings (less than 1 year of age), juveniles (greater than one year of age but not yet sexual mature), and adults can easily be distinguished by a size range: those 90 to 130 mm in length are considered hatchlings, 130 to 185 mm are considered juveniles, and greater than 185 mm are considered adults and sexually mature. In B. biporus, however, hatchlings and juveniles cannot be reliably distinguished from one another by size, so size can only be used for identifying adults (greater than 185 mm long). Due to the burrowing lifestyle of B. biporus and difficulty in obtaining specimens, information on sex determination is unavailable. (Grismer, 1955; Papenfuss, 1982)

Reproduction

No studies have been done to determine the type of mating system that Bipes biporus may use. Of the 2719 specimens collected in the extensive study done by Papenfuss (1982), the ratio of adults to hatchlings stayed relatively constant throughout the study. This suggests that reproductive success remains somewhat stable from year to year and does not depend on the annual rainfall as previously thought, because the study took place during a time of drought. Papenfuss (1982) also found that, on average, larger females tended to lay more eggs than smaller females. Almost twice the number of female adult and juvenile specimens were collected compared to the number of males, although the difference was not statistically significant. Whether or not this difference was due to sampling bias or if the primary sex ratio was actually skewed could not be determined. (Hollingsworth and Frost, 2007; Papenfuss, 1982)

Bipes biporus is an oviparous species. Females lay from 1 to 4 eggs per clutch, with an average of 2 eggs per clutch. The eggs take roughly 2 months to hatch, but there is no mention in the literature on whether the females guard the eggs or display any type of parental care once the eggs are hatched. The eggs are generally laid at the end of June to the beginning of July and begin hatching in late September. Females reach sexual maturity at about 45 months of age. At 45 months, most females are 185 mm or longer, and this length is considered the start of the adult stage in B. biporus. Once sexually mature, females only reproduce, at most, once every other year. Late sexual maturity, small clutch size, and a biennial breeding interval makes the reproductive potential of this species much lower than in most lizards. (Grismer, 1955; Papenfuss, 1982)

  • Breeding interval
    Female Mexican mole lizards lay clutches biennially.
  • Breeding season
    Mid-summer
  • Range number of offspring
    1 to 4
  • Average number of offspring
    2
  • Average gestation period
    2 months
  • Average age at sexual or reproductive maturity (female)
    45 months

No data has been collected regarding the extent, if any, of parental care performed by Bipes biporus.

Lifespan/Longevity

Due to its secretive lifestyle, there has been little research done on the average lifespan of B. biporus. Because females do not reach sexual maturity until roughly 45 months of age and only reproduce every other year, average life expectancy must exceed this point. A captive specimen, captured as an adult, lived for 3 years and 3 months. (Papenfuss, 1982; Slavens and Slavens, 1999)

  • Range lifespan
    Status: captivity
    3.25 (high) years
  • Average lifespan
    Status: wild
    Over 3 years years

Behavior

Bipes biporus is unique among the other members of the Bipedidae in that it has an enhanced capacity for thermoregulation. Because Mexican Mole Lizards are ectotherms, they attain their body temperature from the soil of their environment. B. biporus can regulate its body temperature by moving deeper underground or closer to the surface through its underground tunnels. Typically, Mexican Mole Lizards spend the cooler morning hours near the surface of the ground, and as the temperature of the surroundings rise throughout the day, they retreat deeper into the soil. The capacity for thermoregulation and inhabiting a reliably warm climate enables year-round activity in Mexican Mole Lizards; they appear to lack a regular period of hibernation. (Papenfuss, 1982)

In terms of locomotion, B. biporus exhibits concertina locomotion. This motion, used by many elongate burrowing animals, occurs when one part of the body anchors in one spot while the anterior portion extends forward while pushing against the anchored spot. Then the anchored portion moves forward and the body anchors at a different spot. While this process has been found to be energetically expensive in snakes, Bipes may use it more economically, because they seem to move entirely using aerobic respiration. When constructing and extending the underground tunnels, B. biporus flexes and extends its forelimbs for clearing of the soil and for stabilizing and forward propulsion of its body. (Dial, et al., 1987)

Communication and Perception

In contrast to the middle ear structure of most amphisbaenians, the stapes and cartilaginous extracolumella of Bipes biporus are connected by fibrous tissue to a deep fold on the skin of the neck, apparently making B. biporus more efficient at receiving sounds. This structure is likely most effective at intercepting air-borne sounds. The importance of this highly specialized structure is not completely understood for this otherwise rather primitive species. However, this unique ear structure may be important for locating prey above the surface. (Wever and Gans, 1972)

Food Habits

Bipes biporus can be described as an opportunistic carnivore. Mexican Mole Lizards will target a variety of prey including ants, ant eggs and pupae, cockroaches, termites, beetle larvae and other insect larvae, and most other small invertebrates. They are considered generalist predators because they will seize anything of suitable size that they contact. If they encounter a large number of ants, they will eat enough to fill up their stomachs, while their next meal may be a single adult cockroach. The one characteristic that remains the same for all prey is the practice of B. biporus to capture their prey and then retreat underground to consume it. As is true of many squamates, their teeth are attached to the sides of the jaws in a pleurodont manner. (Kearney, 2003; Papenfuss, 1982)

  • Animal Foods
  • eggs
  • insects
  • terrestrial non-insect arthropods
  • terrestrial worms

Predation

Due to its burrowing lifestyle, B. biporus avoids many reptile-eating predators such as birds. Nocturnal mammals, such as badgers and skunks, that often prey on burrowing animals may be unable to find and dig up B. biporus because the latter usually burrows deeper at night. The most likely predators of Mexican Mole Lizards are snakes; snakes are easily able to enter the burrow systems constructed by Bipes. However, all species of Bipes are able to autotomize (drop) their tails as a defense mechanism; it is thought that the autotomized tail may distract the predator or even block the burrow while the Mole Lizard escapes. However, Mole Lizards cannot regenerate the lost tail for future encounters with predators. (Papenfuss, 1982)

  • Known Predators
    • Serpentes (Snakes)

Ecosystem Roles

Bipes biporus plays a role in the ecosystem as a predator on terrestrial or burrowing invertebrates. Mexican Mole Lizards may act as a control on certain pest population by consuming numbers of mites, insects, and their larvae. In turn, B. biporus represents a food source (prey) for slender, burrowing snakes, such as Phyllorhynchus decurtatus, on the Baja California Peninsula. (Kearney, 2003; Papenfuss, 1982)

Economic Importance for Humans: Positive

Due to the large number of insects and other small invertebrates that Bipes biporus feeds on, Mexican Mole Lizards undoubtedly play a role in controlling the populations of their prey. Whether they are ever abundant enough to have a significant economic impact is currently unknown. (Kearney, 2003)

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

Bipes biporus does no harm to humans or human economic interests; however, people sometimes kill them due to their intimidating appearance. Many people believe them to be snakes, so if they are dug up during agricultural activities, they are often unnecessarily killed out of fear. (Hollingsworth and Frost, 2007)

Conservation Status

Despite their low reproductive potential, Mexican Mole Lizards are considered a relatively stable species, with no immediate concern of endangerment or extinction. These lizards would seem to have at least some ability to adapt to changing environments. If the habitat of B. biporus is disturbed by excavation, they appear to respond largely by digging deeper into the soil or burrowing in a new area. The Mexican Mole Lizard is a secretive species that stays hidden underground most of the time, thus limiting its exposure to predation and negative human interactions. They occur in some protected areas and are considered protected wildlife under Mexican law (catagory Pr). (Hollingsworth and Frost, 2007)

Contributors

Morgan Casanova (author), Michigan State University, James Harding (editor), Michigan State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

tactile

uses touch to communicate

terrestrial

Living on the ground.

vibrations

movements of a hard surface that are produced by animals as signals to others

References

Dial, B., R. Gatten, S. Kamel. 1987. Energetics of Concertina Locomotion in Bipes biporus (Reptilia: Amphisbaenia). Copeia, 1987 (2): 470-477. Accessed December 18, 2016 at http://www.jstor.org.proxy1.cl.msu.edu/stable/1445785?Search=yes&resultItemClick=true&searchText=bipes&searchText=biporus&searchUri=%2Faction%2FdoBasicSearch%3Famp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26wc%3Don%26prq%3Dbipes%2Bbiporus%2Breproduction%26hp%3D25%26fc%3Doff%26Query%3Dbipes%2Bbiporus%2B%26so%3Drel%26acc%3Don&seq=5#page_scan_tab_contents.

Grismer, L. 1955. Amphibians and Reptiles of Baja California, Including Its Pacific Islands and the Islands in the Sea of Cortés. Berkeley, California: University of California Press. Accessed December 19, 2016 at https://web-b-ebscohost-com.proxy2.cl.msu.edu/ehost/ebookviewer/ebook/ZTAwMHhuYV9fOTA0NjVfX0FO0?sid=57d8e672-136f-4dba-9fc5-6b9e504d7851@sessionmgr105&vid=0&format=EB&lpid=lp_254&rid=0.

Hollingsworth, B., D. Frost. 2007. "Bipes biporus" (On-line). International Union for Conservation of Nature and Natural Resources. Accessed December 16, 2016 at http://www.iucnredlist.org/details/63723/0.

Kearney, M. 2003. Diet in the Amphisbaenian Bipes biporus. Journal of Herpetology, 37: (2): 404-408. Accessed December 16, 2016 at http://www.jstor.org.proxy2.cl.msu.edu/stable/pdf/1566161.pdf.

Papenfuss, T. 1982. The Ecology and Systematics of the Amphisbaenian Genus Bipes. Occasional Papers of the California Academy of Sciences, 136: 1-42. Accessed December 18, 2016 at http://s3.amazonaws.com/academia.edu.documents/34858036/Bipes.pdf?AWSAccessKeyId=AKIAJ56TQJRTWSMTNPEA&Expires=1482205672&Signature=%2Bf8f%2BBZbZX5qfZrKmbpCaANDj6k%3D&response-content-disposition=inline%3B%20filename%3DOCCASIONAL_PAPERS_The_Ecology_and_System.pdf.

Slavens, F., K. Slavens. 1999. Reptiles and Amphibians in Captivity: Breeding— Longevity and Inventory Current January 1, 1998. Seattle, Washington: Slaveware.

Wever, E., C. Gans. 1972. The Ear and Hearing in Bipes biporus. Proceedings of the National Academy of Sciences of the United States of America, 69 (9): 2714-2716. Accessed December 17, 2016 at http://www.jstor.org.proxy1.cl.msu.edu/stable/61851?Search=yes&resultItemClick=true&searchText=bipes&searchText=biporus&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3Dbipes%2Bbiporus%2B%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26amp%3D%26acc%3Don%26fc%3Doff%26hp%3D25%26so%3Drel%26wc%3Don%26resultsServiceName%3DdoBackToBasicResults%26prq%3Dbipes%2Bbiporus%2Breproduction&seq=1#page_scan_tab_contents.

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