Reptile Reproduction Notes

Sexual Reproduction in Reptiles

• Most reptiles reproduce sexually, involving two distinct sexes.
• Each sex contributes a gamete during reproduction.
• All reproductive activity occurs within the cloaca, a single exit/entrance at the base of the tail for both waste and reproductive products.
• Eggs leave the female's body through the cloaca.

Female Reproductive Anatomy

• Follicles are produced in the ovary and shed during ovulation.
• They mature in the oviduct through vitellogenesis.
• Diagrams illustrate non-breeding and breeding states of the female reproductive tract, specifically in the brown anole.

Hemipenes

• Lizards and snakes possess hemipenes, structures that deliver sperm into the female's cloaca during mating.
• Hemipenes are usually stored inside the body of the male.

Turtle Genitalia

• Turtles have a single genital tubercle.
• Turtle and mammal penis designs are anatomically convergent.
• The penis is a hydraulic structure reinforced by an axial orthogonal array of collagen fibers.

Tuatara Reproduction

• Male and female tuataras press their cloacas together as the male excretes sperm.
• Tuatara sperm has the fastest velocity of any reptile sperm yet analyzed.
• Male tuataras have no penis.
• Reference to Lamar et al 2021 study: Initial collection, characterization, and storage of tuatara (Sphenodon punctatus) sperm offers insight into their unique reproductive system.

Hemiclitores

• Female genitalia are historically under-studied compared to males.
• Hemiclitores in snakes are diverse across a range of species and likely functional.
• Little is known about the possible functional role and evolution of the hemiclitores in squamates.
• Reference to Folwell et al. 2022 study on hemiclitoris structure across 9 species of snake.

Sexual Behavior

• Elaborate coloration and displays are particularly well-studied in lizards.
• These are linked to male-male competition and female choice.

Male Mate Choice

• Males can also be the "choosy" sex.
• Both male and female juveniles have red tails; females retain red tails in early adulthood.
• Males preferred red females, suggesting that red coloration is a sexual signal involved in male mate selection.
• A red tail could signal pre-ovulatory receptiveness and reduced sperm competition (young unmated adult female).
• Reference to Belliure et al., 2018 study on male mate choice in spiny-footed lizards (Acanthodactylus erythrurus).

Mating Behavior in Snakes

• Male Competition: Ritualized combat to "win" access to the female.
• Females release pheromones to attract males, who follow scent trails.
• Males perform courtship behaviors (e.g., chin rubbing, body alignment) to encourage copulation.
• Females of many species can store sperm for months or even years, allowing them to fertilize eggs when conditions are ideal.

Mating Behavior in Turtles

• Highly variable given variation in terrestrial/aquatic habits.
• Display behavior – e.g., freshwater turtles "flutter" claws in the female's face.
• Softened plastra in males enables mounting.

Mating Behavior in Crocodilians

• Aggressive maintenance of territories by males.
• Tactile displays, e.g., snout rubbing.
• Vocalizations – auditory, infrasonic vibrations, head-slapping.

Internal Fertilization

• Unlike in amphibians, gametes fuse and develop inside the female.
• Allowed reptiles to become fully terrestrial – gametes don't dry out internally, so reliance on water was removed.

Variation in Reproductive Mode

• Oviparous: egg-laying
• Viviparous: live-bearing

The Ovo-Viviparity Continuum

• Egg-laying (oviparity)
• Live-bearing – offspring nourished by yolk (lecithotrophic viviparity)
• Live-bearing – offspring nourished by mother (matrotrophic viviparity)
• Examples: Zootoca vivipara, Chalcides chalcides, Mabuya brachypoda

Transitions to Viviparity

• Approximately 20% of squamates (lizards and snakes) are viviparous, whereas all crocodilians and turtles are oviparous.

Egg-layers

• Amniotic eggs first evolved in reptiles.
• Contain membranes that hold water and nutrients for the embryo and facilitate gas exchange.
• Have a solid shell that prevents water loss.
• Have a fatty food source (the yolk) for the embryo.

Adaptations to Oviparity

• Yolk-rich eggs.
• Variation in sex-determining mechanisms:
  • Chromosomal
  • Temperature-dependent
• Nesting behavior:
  • Nests can stabilize temperatures/ maintain warm conditions.
  • May buffer reptiles from some climate change effects…

Live-bearers

• Extraembryonic membranes serve as the site of gas exchange and possibly transfer small quantities of organic and inorganic nutrients.
• The chorion and allantois fuse to form a rudimentary placenta.
• Linked to colder climates – internal development allows optimal temperatures to be maintained.
• Examples: Lizards – e.g., skinks, chameleons, phrynosomatids, cordylids; Snakes – e.g., boids, colubrids, natricids, viperids, elapids.

Adaptations to Viviparity

• Internal developmental time is longer (but there is no incubation period).
• The shell membrane is extremely reduced; the egg loses its crystalline layer.
• Example: Lerista bouganvillii

Reptiles vs. Amphibians

• Reptiles hatch/are born in the body plan/form they will remain in for their whole life.

Asexual Reproduction in Reptiles

• Parthenogenesis: reproduction without a male gamete, resulting in all-female populations.
• Facultative: only occurs in rare circumstances, e.g., captive ball pythons.
• Obligate: only method of reproducing.
• May be an advantage in the short term – allows rapid increase in population - but generally leads to a reduction in genetic diversity.
• Examples: Brahminy blindsnake, Whiptail lizard
• Parthenos (Greek) = virgin, Genesis (Greek) = creation, Parthenogenesis

Parthenogenesis in Whiptail Lizards (Aspidoscelis)

• About a third of whiptail species are parthenogenetic.
• Thought to have arisen from hybridization of sexually reproducing species.
• Chromosome number doubles during meiosis - gametes are produced with a complete set of chromosomes, preserving genetic diversity.
• Reference to Lutes et al. Nature 2010 study showing oocytes from parthenogenetic A. tesselata contain twice the amount of chromosomal DNA compared to sexual A. gularis.

Social Behavior and Parental Care

• The majority of reptiles "lay and go away," so interactions within the family are limited.

Viviparity and Social Grouping

• Viviparity is linked with social grouping in squamates.
• Reference to Halliwell et al., (2017) study in Nature Communications.

Intergenerational Aggregation

• Starting point for social evolution?
• Examples: Crotalus, Egernia, Phrynocephalus, Xantusia
• Benefits:
  • Thermoregulation
  • Enhanced protection against predators
  • Rudimentary parental "care" (defense of common burrow, nest site)

Parental Care

• ~3% of 938 squamate reptile genera exhibit care.
• All 24 crocodilians care for young – biparental care in 8 species.
• Reference to Reynolds JD, Goodwin NB, Freckleton RP. 2002 study on evolutionary transitions in parental care and live bearing in vertebrates.

Egg Guarding/Thermoregulation

• Python curling around eggs – may warm them through "shivering thermogenesis."
• Five-lined skink Plestiodon fasciatus guards and tends eggs, removes moldy eggs, moves them to optimize humidity.

Parental Care in Crocodilians

• Build large nests on carefully chosen nest sites.
• Incubate and guard eggs.
• Post-hatching care, sensitive to hatchling vocalizations (e.g., distress calls) and even adjust responses based on call info (Nile crocodiles are more responsive to calls of smaller offspring).
• Reference to Chabert et al., 2015 study in Scientific Reports.

Learning Objectives (Revisited)

• Sexual reproduction in reptiles – morphology and behavioral adaptations
• Evolutionary importance of internal fertilization
• Reproductive modes across the reptiles
  • Oviparity
  • Viviparity
• Asexual reproduction
• Social behavior and parental care