Animals can definitively have identical twins, though the frequency and mechanism vary dramatically across the animal kingdom. Identical twins, known scientifically as monozygotic (MZ) twins, originate from a single fertilized egg that splits into two separate embryos early in development. This results in two individuals who share the same genetic material. This process is distinct from fraternal, or dizygotic (DZ) twinning, which occurs when two separate eggs are fertilized by two separate sperm. While fraternal twinning is common in many litter-bearing species, identical twinning is typically considered a random biological accident in most mammals.
The Biological Process of Monozygotic Twinning
Monozygotic twinning begins with a single zygote, the cell formed when an egg and sperm unite. The process relies on the developmental flexibility of the early embryo, where cells are initially able to form a complete organism. The precise timing of the split determines the membranes and placenta that the twins will share, which has significant implications for their development. If the split happens very early, before the morula stage, the resulting twins will have separate placentas and amniotic sacs, a configuration known as dichorionic-diamniotic.
Later splitting, often at the blastocyst stage, results in the twins sharing a single placenta, making them monochorionic. The split occurs in the inner cell mass, which is the part that will become the embryo itself. This later split is the most common form of identical twinning and carries higher developmental risks because the twins share a blood supply. Should the split occur very late, after the eighth day of development, the separation may be incomplete, leading to the formation of conjoined twins.
Obligate Polyembryony: The Case of the Armadillo
The nine-banded armadillo (Dasypus novemcinctus) represents the most striking example of identical twinning in the vertebrate class. For this species, the production of genetically identical young is an obligate reproductive strategy, termed obligate polyembryony. This means the splitting is a requirement for reproduction rather than an accidental occurrence.
The female armadillo typically fertilizes only one egg per reproductive cycle. After fertilization, the single zygote undergoes a period of delayed implantation. Once the blastocyst implants in the uterus, it systematically divides into four distinct, genetically identical embryos.
This consistent splitting is likely an adaptation to the female’s reproductive tract, which features a small uterine implantation site that can only accommodate a single blastocyst. By splitting into four separate embryos, the armadillo maximizes its reproductive output from that single implantation. The result is a litter of four identical quadruplets, all of the same sex, produced in nearly every pregnancy.
Spontaneous Identical Twinning in Domestic Mammals
Outside of the armadillo family, identical twinning in mammals is a rare and unpredictable event. In large domestic animals, such as cattle and horses, natural twinning is generally undesirable and carries risks to both the mother and the offspring. In cattle, only about 4 to 6 percent of naturally occurring twin births are monozygotic.
Confirming monozygosity in livestock without genetic testing can be difficult, as fraternal twins can often look very similar. Modern DNA genotyping confirms the identical genetic makeup of twin calves and foals, confirming the rare spontaneous event. Conversely, agricultural science has adopted embryo splitting to induce identical twins in livestock like cattle and sheep. This technique involves manually bisecting an early-stage embryo in a laboratory before implanting the two halves into surrogate mothers.
Twinning Phenomena Beyond Mammals
The biological mechanism of embryo splitting is not exclusive to mammals and is found across various parts of the animal kingdom. The most dramatic example is seen in certain parasitic wasps, such as the Copidosoma floridanum. These insects exhibit polyembryony where a single egg divides clonally to produce hundreds or even thousands of genetically identical offspring. This mass production allows the wasp to quickly consume its host from the inside.
In vertebrates like fish and reptiles, the occurrence of successful identical twinning is extremely uncommon, but incomplete splitting is observed. Monozygotic splitting in these groups frequently results in conjoined twins, often known as bicephalic or two-headed animals. The confined nature of the egg and developmental constraints restrict the separation process. Documented cases of monozygotic twinning have been confirmed through genetic analysis in species like the medaka fish.