Norway rat - roof rat cross
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.):
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.
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:
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:
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:
Matings between these species tend to produce viable, sterile offspring:
Matings between these species produce viable offspring that are usually sterile, but a few fertile female hybrids are on record:
Matings between these species produce sterile male and fertile female hybrids:
Matings between these species produce hybrids of unknown fertility:
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:
To find more hybrids, search the HybriDatabase.