• Norway rat - roof rat hybrids
• More on interspecies hybridization
  • Avoiding the wrong species
  • Do different species ever mate?
  • What happens if an interspecies mating takes place?
    • Hybrid inviability
    • Hybrid sterility
    • Hybrid fertility

Norway rats (Rattus norvegicus) and roof rats (Rattus rattus, also called black rats, ship rats) are different species. A species, according to the biological species concept, is a group of related individuals or populations that can interbreed and produce fertile offspring. Members of different species cannot produce fertile offspring together.

In rare circumstances, when two closely related species are kept together in captivity, mating may occur. The resulting pregnancy may be unsuccessful and the embryos die. Norway rat and roof rat crosses are usually unsuccessful. Gray (1972) reports unsuccessful matings between Norway and roof rats, and Chiasson (1980) records that Norway and roof rats will not produce offspring, even if artificially inseminated. Castle (1947) reports that crosses between R. norvegicus and R. rattus are very difficult to obtain, such that the embryos never come to term alive.

However, there is at least one anecdotal case of a cross between a male R. norvegicus and three female R. rattus which produced offspring, all of which were born alive but died shortly after birth. The rats were owned by Jane Adamo of New York, and her description of the event follows (pers. comm.):

Account of Norway Rat (Rattus norvegicus) -- roof rat (Rattus rattus) hybrids On October 23, 1999, I adoped four "wild Norway baby boys" from the AARK Wildlife Rehabilitation Center in Pennsylvania. The animals had been abandoned by their mother, hand raised from pinkies, then deemed unreleasable wildlife. We estimated the ratlets had been born around September 18, 1999. I installed them in a cage with Ick!, my mature Norway male. Over the next few weeks, I noticed a number of irregularities. There was never any fighting between Ick! and the babies; he loved them from the moment they were put in the cage with him. In addition, the babies seemed developmentally "stuck": they did not seem to be maturing, "bulking up" as Norways do when they grow into adulthood. Toward the end of December, the boys started getting "peevish" and bitey. On December 28, 1999, to my astonishment, one of the baby "boys" had a litter of seven pups. I took Ick! away permanently. Then I isolated the mother and her babies by taking her siblings away and leaving her alone with her babies in the cage. I left them alone for the night. In the morning, I looked for the babies and found that each one had been methodically bitten once in the head and killed. I put all the rats except for Ick! back in the same cage. Two days later, on Dec 30, a second rat "boy" had a litter of five; I thought I would do it differently and left them all together, but the next morning each of these were also found bitten once in the head. On January 12, 2000, a third rat "boy" had a litter of three; these babies were ignored and soon died. Note that the babies were all born alive, and moved and squeaked loudly. I did not observe if they nursed at all. I was very confused. These animals had been identified by the rehab center and my vet at Animal Medical Center, NYC, as wild Norway boys. How could they make such a mistake? The mystery continued until a few months later when a friend sent me a photograph of his Rattus rattus girl. To my astonishment, his rat looked exactly like the four "boys". I sought out more information, identified the four "Norway boys" to actually be four R. rattus girls and learned these relevant facts about R. rattus (From Claire Jordan's Rattus Website) -- The female ship/roof rat has a genital mound that makes it difficult to sex them at an early age. In addition, R. rattus has the physical aspect of a mouse and are petite and better climbers (almost "arboreal") compared to R. norvegicus. This explained why the "boys" never seemed to mature. Jane Adamo July 28, 2003

Avoiding the wrong species: reproductive isolating mechanisms

The overwhelming majority of species cannot and do not mate with each other. Crocodiles and horses, butterflies and falcons, earthworms and fleas, jellyfish and tuna and hundreds of thousands of other combinations of species cannot and do not interbreed with each other.

In a few rare instances, however, very closely related species may have the potential to interbreed. Horses, donkeys and zebras may interbreed, for example, as may lions and tigers, or bison and cattle.

In these cases it is nevertheless maladaptive for one species to mate with another. Mating with the wrong species is a waste of time and energy. Such matings are usually unsuccessful: the hybrid offspring die or are sterile, so all the reproductive effort of the parents is ultimately wasted.

There are a number of mechanisms which prevent individuals from mating with the wrong species. These are called reproductive isolating mechanisms:

• Physical separation: species who live in different geographic locations or occupy different ecological niches in the same location never have the chance to meet each other. In the wild, lions live in Africa and tigers in India, so there is no chance for them to mate. Two species may occupy different ecological niches and thus not mate with each other, for example, one species may lives in the jungle canopy and another may live on the jungle floor.
• Temporal isolation: Species that mate during different seasons or time of day cannot breed together. For example, ants mate on one day of the year: each colony sends up swarms of reproductive males and females. They mate and the females start colonies of their own. In the American Southwest, different ant species generally perform their mating flights on different days, thus preventing accidental matings between species.
• Behavioral isolation: members of different species may meet each other, but do not mate because neither performs the correct mating ritual. Elaborate mating rituals play an important role in species recognition. For example, tungara frog females prefer the calls of their own males (Kirkpatrick and Ryan 1991). Male zebra finches imprint on their mothers and seek out and court only females who look like their mom (Bischof 1994).
• Mechanical isolation: Copulation may be impossible because of incompatible size and shape of their reproductive organs.
• Morphological isolation: Copulation may be impossible because of the difference in body size or shape.
• Gametic isolation: Even if copulation occurs, the sperm and egg do not fuse -- fertilization cannot occur (e.g. rats and mice)

The reproductive isolating mechanisms described above are not always foolproof. In captivity, a human can intervene and create conditions that lead to an animal mating with another of a different species. For example, raise a baby male zebra finch with a Bengalese mother finch, and the zebra finch will grow up to ignore female zebra finches but devotedly court Bengalese finches (Bischof 1994).

Rarely, some crosses may happen naturally in the wild between very closely related species.

Interspecies mating usually fails, but it can fail at many different points after fertilization. These failures are called postzygotic reproductive isolating mechanisms, because they isolate one species from another even after fertilization has occured (postzygotic means "after fertilization").

The more related the species are to each other, the later in the reproduction process the mating tends to fail. Hybrids may die before or after implantation, during the pregnancy, or around the time of birth, For example, Norway rats and roof rats may produce non-viable fetuses or infants.

Species that are even more closely related to each other may produce viable but sterile offspring. For example, horses and donkeys produce sterile mules. Extremely closely related species may produce offspring that are partially fertile. For example, domestic cats and their wild relatives, and cattle and bison, tend to produce fertile female hybrids but sterile male hybrids.

Lastly, two species may be so closely related that they produce fertile offspring. For example, dogs and wolves produce fertile hybrids. Interspecies matings that produce fertile offspring challenge our notion of the biological species concept.

There is individual variation in interspecies matings, too. Not all matings between one species and another will fail at exactly the same point every time. For example, reproduction between goats and sheep almost always fails, but there is one recorded case of a viable, sterile goat-sheep hybrid. Similarly, Norway rats and roof rats tend not to produce offspring (fertilization does not occur or the embryos die in utero) but in a few cases hybrid offspring have been produced that died shortly after birth. As another example, horses and donkeys usually produce sterile mules, but in a few instances a female mule may be fertile. Because of this individual variation, it makes more sense to talk about success or failure rates: what percentage of matings (if any) between species A and B produce offspring? If offspring are produced, what percentage (if any) are fertile?

Hybrid inviability: Embryo death or stillbirth

If the species are related, fertilization may occur but the embryo dies. The miscarriage may happen very early, such that the fertilized egg fails to implant in the uterus, or it may happen at some point during the pregnancy. Lastly, the offspring may be brought to term but the offspring may be stillborn, dying before, during or shortly after birth.

Matings between these species do not produce viable offspring:

• Water buffalo and cattle: Water buffalo (Bubalus bubalis) and cattle (Bos taurus) embryos fail around the 8-cell stage (Patil and Totey 2003).
• Rats: Norway rat (Rattus norvegicus) and roof rat (Rattus rattus) matings are usually unsuccessful, but on a few occasions live offspring have been produced which died shortly after birth.
• Tiger-leopardess crosses (tigards) miscarry or are stillborn (Gray 1972).

Hybrid sterility: viable, sterile offspring

Two species may produce viable offspring which may survive to adulthood. Such hybrids are usually sterile. In some cases, however, some of the hybrid offspring may be fertile as adults. Generally, female hybrids are more likely to be fertile than males.

Matings between these species usually fail, but a few viable, sterile offspring are on record:

• Goats and sheep: Goats and sheep tend not to produce offspring, but in a single case a sterile goat-sheep hybrid was produced in Botswana.

Matings between these species tend to produce viable, sterile offspring:

• Lovebirds: Fischer's lovebirds (Agapornis personata fischeri) and Peach-faced lovebirds (Agapornis rosecollis) can interbreed and produce sterile offspring. Interestingly, the lovebird hybrid displays intermediate nesting behavior. Fisher's lovebirds carry single strips of nest material in their beaks. Peach-faced lovebirds tuck many pieces of nest material between their rump feathers. The hybrids show a poorly organized mixture of the two strategies: they tuck nest material between their feathers but fail to let go, pull it out again, and start over. After several months, they can become partly successful, managing to transport some material back to the next site, but not in a manner that resembles either parent species. Sometimes they just turn their heads toward their rumps without tucking, then fly off with the material (Dilger, 1962).
• Lion-leopard crosses produce viable but sterile offspring called leopons. The most famous leopons were five cubs born in Hanshin Park in Japan (two in 1959, three in 1962). The last one died in 1985 (Doi and Reynolds 1967).

Matings between these species produce viable offspring that are usually sterile, but a few fertile female hybrids are on record:

• Whales: Several naturally occurring crosses between blue whales (Balaenoptera musculus) and fin whales (Balaenoptera physalus) have been identified. One male hybrid was sterile, and of the two female hybrids, one was sterile while the other was pregnant, though it is unknown whether her fetus would have been viable (Arnason et al. 1991, Spillaert et al. 1991, Bérubé and Aguilar 1998).
• Horse species: Crosses of horse species within the genus Equus tend to produce viable but sterile offspring. Zebra-horse, zebra-donkey and zebra-pony crosses produce sterile zorses, zedonks and zonies. Horse-donkey crosses produce sterile mules. Very rarely, a female mule may be fertile.
• Lion-tigress crosses produce sterile offspring called ligers. Tiger-lioness crosses, tigons, are more rare. In some cases, female ligers and tigons have proved to be fertile.

Matings between these species produce sterile male and fertile female hybrids:

• Cattle and bison: Domestic cattle (Bos taurus) and American bison (Bison bison) can be crossed to produce beefalo. Female hybrids are usually fertile, while males are sterile (Steklenev 1995, 1997).
• Domestic cats (Felis catus) can breed with their close wild relatives. Typically, the female hybrids are fertile while the males are sterile. For example, domestic cats crossed with servals (Felis serval) produce hybrids called Savannah cats; with Asian leopard cats (Felis bengalensis) produce hybrids called Bengal cats, and with jungle cats (Felis chaus) produce hybrids called chausies.
• Dolphin and false killer whale: There has been one case of a female bottlenose dolphin (Tursiops truncatus) and a male false killer whale (Pseudorca crassidens) producing a fertile female hybrid, called a wolphin. She went on to breed with a dolphin and produced a daughter.

 Matings between these species produce hybrids of unknown fertility:

• Bobcat and lynx: Bobcats (Lynx rufus) and lynxes (Lynx canadensis) may cross; several such crosses have happened naturally in the wild (also Q&A on Minnesota bobcat-lynx crosses (pdf)).
• Porpoises: Dall's porpoises (Phocoenoides dalli) and harbour porpoises (Phocoena phocoena) can conceive offspring. Many individuals with intermediate pigmentation have been observed, indicating that such offspring may be viable (Baird et al 1998).

Viable, fertile offspring

If the parent species are extremely closely related, they may produce fertile offspring. These are the edge cases in which the biological species concept can become too rigid. The biological species concept states that animals belong to a separate species if they cannot interbreed. So, if animals supposedly from different species interbreed and produce fertile offspring, then according to the biological species concept they should be one species. However, we usually don't consider them the same species because they differ in other features, such as geographic location, appearance, behavior, and genetics.

Matings between these species produce viable, fertile offspring:

• Canis species: Domestic dogs (Canis familaris), wolves (Canis lupus, Canis rufus) and coyotes (Canis latrans) can interbreed and produce fertile offspring:
  • Evidence for domestic dog DNA in wild Siberian wolves (Vila et al. 2003)
  • Evidence for domestic dog DNA in wild coyotes (Adams et al. 2003b).
  • Evidence for red wolf-coyote hybridization (Adams et al. 2003a); Northeastern coyotes may be the product of hybridization between Canadian wolves and Western coyotes.
• Cichlid species in Lake Victoria are extremely variable, displaying 500 color morphs. The species are isolated through mate choice, which is determined by coloration: mates choose each other by color pattern. However, in recent years, human activity has caused the water of Lake Victoria to become cloudy. In these areas, the cichlids can't differentiate between species. In these cloudy areas, bright color morphs have disappeared and the fish have become similar and dull in appearance through hybridization (Seehausen et al. 1997).

To find more hybrids, search the HybriDatabase.