The inability to produce viable offspring affects various animal species due to reasons ranging from genetic incompatibility to social suppression. Animals that cannot reproduce are sterile or infertile, meaning they cannot produce functional gametes or viable young through sexual reproduction. This inability can be an inherent outcome of mixed parentage, a consequence of social structure, or the result of genetic defects and environmental pressures.
Sterile Offspring Resulting from Hybridization
One of the most common causes of sterility in the animal kingdom is hybridization, the mixing of two different species. When two closely related species interbreed, the resulting hybrid offspring are often sterile due to an incompatibility in the number or structure of their parents’ chromosomes. For an animal to produce reproductive cells, or gametes, its chromosomes must precisely pair up and separate during a cell division process called meiosis.
A well-known example is the mule, which is the offspring of a female horse (64 chromosomes) and a male donkey (62 chromosomes). The mule inherits 63 chromosomes, an odd number that makes it impossible for the chromosomes to form the necessary pairs during meiosis. This disruption prevents the formation of viable sperm or eggs. The same mechanism explains the sterility in other interspecies hybrids like a liger, the offspring of a male lion and a female tiger, or a zorse, the cross between a zebra and a horse.
The sterility is not always absolute and can sometimes follow a pattern known as Haldane’s Rule, where the sex that is affected is the one with two different sex chromosomes (XY in mammals). For example, male ligers tend to be sterile, while female ligers may occasionally be fertile. Beyond the chromosome count, the sterility can also be polygenic, meaning it is tied to numerous genes scattered across the chromosomes. This creates an imbalance in gene expression when the parental genomes are combined.
Functional Sterility in Socially Structured Species
In some animal societies, the inability to reproduce is not a genetic failure but a consequence of a strict social structure, resulting in what is termed functional sterility. This phenomenon is most pronounced in eusocial insects, like honeybees, ants, and termites, where a reproductive division of labor exists. Within a honeybee colony, the vast majority of female workers are functionally sterile.
The queen bee produces pheromones, chemical signals that circulate throughout the colony, which actively suppress the development of the workers’ ovaries. The workers are thus prevented from laying eggs and instead dedicate their lives to foraging, defense, and brood care, supporting the queen as the sole reproducer.
This functional sterility is also present in some cooperative breeding mammals, such as naked mole rats, where only one female, the queen, and a few males reproduce. The queen uses a combination of physical aggression and chemical signals to suppress the reproductive hormones and behavior of subordinate females. Similarly, in packs of wolves or African wild dogs, the dominant breeding pair will often suppress the breeding efforts of other adults through behavioral dominance, ensuring that only their genes are passed to the next generation in that social group.
Infertility Caused by Genetic Anomalies and Environmental Factors
Infertility can also arise in individuals and populations due to internal genetic anomalies or external environmental pressures. At the individual level, specific genetic mutations or chromosomal abnormalities can directly impair reproductive function. For instance, in various animal species, including horses and cats, sex chromosome abnormalities like XXY, which is similar to Klinefelter syndrome in humans, can lead to sterility.
The presence of an extra X chromosome interferes with the development of reproductive organs or the process of sperm production. Other single-gene defects, such as a mutation in the TMEM95 gene in certain cattle breeds, can cause bulls to be infertile. This defect prevents the necessary protein from forming on the sperm head, which is required for fertilization.
Beyond inherent genetic defects, environmental factors, particularly pollution, are increasingly causing reproductive failure at the population level. Endocrine-disrupting chemicals (EDCs), such as certain pesticides and industrial compounds, mimic or interfere with natural hormones in animals. For aquatic life, EDCs can alter the sex-specific development of reproductive tissues, leading to permanent abnormalities and reduced fertility in fish and amphibians. These chemicals can affect the processes of steroidogenesis, sperm motility, and embryo development, ultimately lowering reproductive rates.