DOI: 10.5061/dryad.k6djh9w8b
title: Late term fetus caudal luring behavior in a pitviper

description: 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.
description: In 2015 and 2018, a total of 18 different
 pregnant female copperheads (<em>Agkistrodon contortrix</em>)
 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 (35<sup>o </sup>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.