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Late term fetus caudal luring behavior in a pitviper


Smith, Charles (2022), Late term fetus caudal luring behavior in a pitviper, Dryad, Dataset,


With the advent of powerful imaging tools, the prenatal behavior of vertebrates is far more complex than previously believed, especially concerning humans, other mammals, and birds. Surprisingly, the fetal behavior of squamate reptiles (lizards, snakes, and amphisbaenians), a group of over 11,000 extant species, are largely understudied. Using ultrasonography, 18 late-term pregnant copperhead snakes (Agkistrodon contortrix) from a single population were inspected for fecundity (number of fetuses). Unexpectedly, during the ultrasound procedure that involved 97 fetuses, we observed sinusoidal tail movements in 11 individuals from eight different copperhead mothers. These movements were indistinguishable from caudal luring, a mimetic ambush predatory strategy which is exhibited by newborn copperheads and other snakes. Caudal luring is initiated shortly after birth and is employed to attract susceptible vertebrate prey. Using the same ultrasound equipment and methods, we tested for this behavior in two species of rattlesnakes (genus Crotalus) not known to caudal lure and none of the late-term fetuses showed any type of tail movements. Prenatal movements in humans and other vertebrates are known to be important for musculoskeletal and sensorimotor development. The fetal behaviors we describe for copperheads, and possibly other snakes, may be similarly important and influence early survival and subsequent fitness.


In 2015 and 2018, a total of 18 different pregnant female copperheads (Agkistrodon contortrix) were collected for ultrasonography (Table 1). Seven females were collected from 7 July to 16 August in 2015, and 11 females were collected on 14 and 16 August 2018. The collection site was a 485 ha parcel of basalt trap rock ridge ecosystem situated 4.75 km north-west of Meriden, Connecticut [43]. In 2015, females were brought to the laboratory and provided with private enclosures, which consisted of plastic cages (61 cm L x 40 cm W x 12 cm H) supplied with paper as a floor covering and substrate heating by heat tape (8 cm wide) situated beneath and across the front end of the cage (35o C). Artificial lighting (eight 40 W fluorescent tubes) positioned 3m above the cage was timer-controlled to simulate natural (Connecticut time) photoperiod. Water was available in glass bowls ad libitum. Because copperheads rarely eat in the latter stages of pregnancy (CF Smith, pers. observ.), food (thawed mice) was not offered until after parturition. After birthing, all females (n = 7) brought to the laboratory were safely returned to their exact capture sites (e.g., GPS coordinates and field notes). In 2018, females were tested in the field (14 and 16 August) and released after ultrasonography testing.


To visually access the fetuses, we scanned the pregnant subjects using a SIUI-CTS-8800+ portable ultrasound (Shantou Institute of Ultrasonic Instruments Co., Ltd., Guangdong, China) equipped with a L7L50K-G Linear 5-12 MHZ probe set at a frequency of 7.5MHz, 60dB in gain, and resolution depth of 3.2 cm. In 2015, parturition dates were recorded for females (n = 7) housed in the laboratory. Because females captured in 2018 were released at their capture sites following the ultrasound procedure, parturition dates for these females are not known. However, all females tested in 2018 were deemed to be late term based on the ultrasound results.