The whiptail lizards of the genus Aspidoscelis are a group of slender, fast-moving reptiles native to the southwestern United States and Mexico. The question of whether male whiptail lizards exist stems from the fact that several well-known species appear to be composed entirely of females. The Aspidoscelis genus contains both species that reproduce in a typical fashion and those that have evolved a unique reproductive strategy.
The Dual Nature of Whiptail Reproduction
The answer to whether male whiptail lizards exist depends on the specific species being examined. The genus Aspidoscelis is divided into two major groups based on their reproductive modes.
One group consists of bisexual species that reproduce like most other vertebrates, involving both males and females. These species are known as “bisexual” or “gonochoristic” whiptails and include common examples like the Western Whiptail (A. tigris) and the Little Striped Whiptail (A. inornatus).
The second group is composed entirely of females, with no males present. These all-female lineages, such as the New Mexico Whiptail (A. neomexicanus), reproduce without needing a male partner. They arose through ancient hybridization events between two different bisexual whiptail species. Because they are genetic hybrids, healthy male offspring are often not viable, which led to the establishment of stable, all-female populations.
The Mechanism of Asexual Reproduction
The ability of these all-female whiptail species to reproduce without fertilization is due to a process known as cloning, where the egg develops into a full embryo on its own. This strategy bypasses the need for sperm to form a viable egg. The process requires a unique modification of the cell division that forms eggs, which typically halves the number of chromosomes.
In all-female whiptails, the cells destined to become eggs duplicate their entire set of chromosomes, effectively doubling the genetic material. This doubling creates a pseudo-tetraploid cell, meaning it carries four copies of each chromosome instead of the usual two.
When the cell undergoes the necessary divisions to form an egg, the result is a full-fledged egg that retains the complete, diploid number of chromosomes. This means the egg contains all the genetic information needed to develop into a new organism without male contribution. This mechanism preserves the high level of genetic variety inherited from their two parent species. The resulting offspring is a genetic clone of the mother, ensuring the successful hybrid genome is passed down intact.
The Behavioral Paradox of Pseudocopulation
Despite the lack of males and the ability to reproduce alone, all-female whiptails engage in pseudocopulation. This behavior involves two females taking on alternating male-like and female-like roles. One female will mount the other, bite her on the neck or side, and entwine tails, mimicking the courtship and copulatory actions of their bisexual ancestors.
This behavior serves an important physiological function. It is driven by the female’s ovarian cycle and the corresponding fluctuation of hormones. When a female is preparing to ovulate, high levels of estrogen cause her to exhibit receptive, female-like behavior.
After she has ovulated, progesterone surges, which then switches her behavior to the male-like mounting role. The physical stimulation from being mounted by a partner is necessary to induce the mounted female to ovulate a healthy egg. Without this behavioral interaction, reproductive success declines, demonstrating that this complex social behavior is integrated into their unique reproductive cycle.
Evolutionary Advantages and Disadvantages
The all-female reproductive strategy offers significant evolutionary benefits. The most significant advantage is the potential for rapid population growth. Since every individual in the population is female and capable of producing offspring, the population can theoretically double in size much faster than a sexual species where approximately half the individuals are non-reproducing males. Furthermore, a single female can colonize a new area and rapidly establish a stable population, as she does not need to locate a mate.
However, this reliance on cloning introduces a major long-term disadvantage: a reduced ability to adapt to changing environments. Sexual reproduction constantly shuffles genes, creating the variation needed for a species to respond to new diseases or shifts in climate. While the hybrid origin provides a high degree of genetic diversity at the outset, the lack of gene mixing over time makes the lineage less flexible. A single environmental pressure that affects one individual could potentially wipe out the entire, genetically uniform population.