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Social Relationship
Historically, European rabbits colonize and are able to survive on deserted islands for extended periods of time due to their incredibly rapid breeding capabilities. Female rabbits are able to become pregnant within a 12 hour time period and can utilize food extremely efficiently. Still, female rabbits can 'resorb' an entire litter of unborn fetuses. European rabbits' re-ingest faecal material similar to a cow 'chewing cud'. Females usually have about 5-6 young per litter and usually lose 1-2 from reabsorption (Boyd and Myhill, 1987). The female might want to do such a thing if the environmental conditions were not sufficient for litter survival or predation pressure is high in the area.
Reproductive success and density of the population have an inverse relationship where greater density lowers fecundity. Rabbits restrict their breeding groups to include only members of the same group. There is a difference among rabbits in their breeding success depending on if they are the dominant or subordinate within their social group. Dominant rabbits have a higher breeding success than subordinates rabbits because they have greater access to resources within their environment such as food and mates. Hence, the dominant males contribute more to the gene pool than subordinates do. However, the study done by Daly in Urana, New South Wales, found that dominant males were only somewhat more successful in breeding than subordinates, which could be attributed to the set up of the warren groups. If a rabbit’s social group is spread out over many warrens, or fragmented, then the subordinates form pairs with the younger subordinate females within that group. This can lead to a lower reproductive success rate for the females. Still, younger female rabbits do not have as many offspring as older rabbits. The layout of the warren groups are supposed to foster greater breeding success for subordinate females. This is because breeding groups that occupy warrens result in a higher number of nesting sites (Daly, 1981).
Even though females can produce an enormous amount of offspring, many do not survive. Their offspring have a mortality rate of 40% due to starvation, not enough nursing or disease. Ironically enough, the social rank of a rabbit places an enormous strain on them resulting in a lack of reproductive success or subordinates ‘stunting’ their young. The stress felt by subordinate female rabbits is so intense that heart rates increase, adrenocortical activity, and their immunoglobin are reduced as a result. Social rank, however, does not indicate how many kittens are in the litter(Von Holst, 2002).
Not only does social rank affect reproductive success, but Rodel et al. and Armstrong note how the density of the population, either high or low, affects how many offspring a female will have. This might be due to the parents having to make adjustments after the offspring have arrived. If a female knows resources are scarce and there is not an ample supply of materials needed for her young to survive, her reproduction rate will lower as a result of the conditions (Rodel, 2004). Of course, increased competition as a result of the high density environment can also have the ability to suppress a female’s reproductive performance (Rodel, 2004). Nevertheless, in highly dense populations of rabbits, females have been known to only produce 10-11 offspring a year, while in low density populations, females have produced up to 20 offspring. The reason this might occur is due to a higher amount of resources available for the mother and the young within the population (Armstrong, 1982).
In addition to density and social rank, the age of the rabbit also affects breeding. Younger females have shown a lower reproductive rate than older rabbits. The cause of their lower reproduction rate may be due to lower body weight and lower social rank. Females that survive the first reproductive season have an average lifetime of 2.3 years. The ranking of a female rabbit has an effect on their reproductive success because those with a higher ranking suppress the lower ranking rabbit. As the rabbits mature, the greater fitness rates females have, the more likely they will have a greater reproductive success. Hence, first-season females do not have a high reproductive rate when compared to older females (Rodel, 2004).
Male/Female Relationships:
European rabbits are often polygynous in large groups, but seem to be monogamous if only smaller dispersed warrens are available. Thus, the size of the burrow depends on whether or not a rabbit will display polygyny or monogamy. Having a large social group is beneficial to the rabbit for predator protection, greater resource gathering and is overall beneficial for the rabbits. Polygyny seems to be the most advantageous for a rabbit because the male invests the least amount of energy into caring for the offspring, yet the male adult rabbit does not meet the polygyny criteria. In order to be polygynous, the adult male must show resource-defense, female-defense and male dominance. Male rabbits do not meet these requirements because feeding sites are not defended, which eliminates resource defense and burrows are mainly occupied by females, which eliminates resource-defense (Roberts, 1988).
A study done by Roberts investigated the social relationships behind the European rabbit and the mating system seen within the species. He looked at burrow distribution, how many rabbits were together at the same time, overlap or spatial and temporal home ranges, friendly interactions between the rabbits, consortships and how close the rabbits were to one another. Roberts findings showed monogamous pairs were most common in single-entranced burrows with two females and two males occupying the area. Larger burrows with five females and three males tended to show promiscuity. In addition, the dispersal of females decided the mating system with highly dispersed females eliciting monogamy. It is likely monogamy will be observed because the males will only be able to guard one female the more spread out they are. Also, subordinate males will approach dominant females for mating when the local dominant male is not around (Roberts, 1988).
Boyd and Myhill found that after breeding, males still had active spermatogenesis, which suggests females have a larger window of opportunity to breed with males. Finding males with active spermatogenesis suggests females will find potent males more plentiful during various times of the year. For a female, Boyd and Myhill found that they have a ovulation rate of four months, which is not as lengthy as their actual reproduction time of 9 months. Therefore, female rabbits do not have synchronized reproductive seasons. There are differences in breeding between females attributed to weather and nutrition (Boyd and Myhill, 1987).
Having said this, discrepencies in the literature are found claiming that after fertilization, females do not show any interest in the male even though males have been known to mount unresponsive does. This is not as frequent in the wild due to the excessive energy expended without gain. The scent emanating from the female lets the male know if she is ready for copulation (Gonzalez-Mariscal, 1997). This conflicting information suggests that the interest of a female after copulation differs based on the availability of a female to find mates. If the female is in a highly dense area with lots of mate to choose from she will probably be less likely to try and find males who have active spermatogenesis because it would not be energy efficient (Gonzalez-Mariscal, 1997).
Chinning:
In order for male and female European rabbits to mark their territory and establish their social hierarchy, as well as show their mate they are ready to copulate, they perform a behavior called chinning. Chinning is when the animals rub their chins against an inanimate object while giving off odoriferous substances simultaneously with their submandibular (chin) gland. The scent that is released lets other rabbits know the territory is occupied as well as letting other females or males know potential mates. The scent can reveal how willing another rabbit is to copulate (Gonzalez-Mariscal, 1997).
Males display chinning behavior to try and get in as many ejaculations as possible, but chinning can be stopped by the behavior of an ejaculation. This would go against the logic that chinning is used to attract mates. If the male rabbit is not attracting as many mates through chinning frequences due to a single ejaculation, it would lead one to think that chinning behavior was not a beneficial mating behavior (Gonzalez-Mariscal, 1997). Gonzalez-Mariscal suggest that male rabbits may regulate their chinning behavior in order to maximize their fertilizing ability. Chinning frequently declines in rabbits after copulation and sexual behavior. Sexual behavior included stimulation or non-stimulation such as thrusting, which was enough to cease chinning. Their study looked at the effect sexual behavior had when tested against the four conditions of mounts and ejaculations, the effect of ‘sexual satiety’ on stopping chinning, effects of local anasthetic on mounting-induced chinning inhibition and the effect of lordosis responses on chinning frequency and ambulation in intact oesterous females. This concludes that both chinning and copulation stimulate a neural mechanism that does not enhance chinning behavior (Gonzalez-Mariscal, 1997).
For female chinning, Hudson and Vodermayer used eleven mature does, four mature bucks, three ovariextomized does and two castrated bucks to conduct their study. They found that chinning is an extremely important behavior for a female, but that there are four differences which have an effect on the chinning frequency. The four differences are individual discrepencies between the markings of female rabbits, daylength change, odour from other unknown rabbits, and habituation to all of these. The individual differences of female rabbits in chinning was simply the size of their marking gland amongst females. Female rabbits were observed to mark depending on the amount of light available. The reason differences were seen with different odor is because the more a female rabbit is aware of a stranger in the area, the higher amount of marking or chinning that occurs. The female is trying to attract as much attention to herself during her period in oestrus so she can mate. Since marking does occur in females when they are not in oestrus as well, Hudson and Vodermayer make the claim that it could relate to group identity as well (Hudson and Vodermayer, 1992).
Parent/Offspring Relationship:
They noted that offspring receive very little care from their parents, but this is a mechanism so the female will be able to prepare for her next breeding season. Once an adult female gives birth, she needs to conserve her energy due to her rapid breeding cycle, and thus, cannot give time and energy to caring for her young (Boyd and Myhill, 1987).
Newborn rabbits are atricial and within the first week of life restrict their behavior to sucking and huddling together for warmth. Thermoregulation is important and most young huddle together to conserve heat. When rabbit young are born, they have sealed eyes and outer ears with little motor skills. They receive food and strength from the female in the nest during this time, while the male defends the mother and young. Since the young does not receive any maternal care from the female besides milk, they learn to become independent very rapidly. However, the time and age the young find a mother’s nipple are crucial in the nursing process for young (Distrel and Hudson, 1984).
Odor factors play a large role in the regulation of how females feed their young. Coureaud et al. and Distrel and Hudson looked at newborn rabbits to investigate how they locate their mother’s nipple to feed. Depending on how acute the young's sense of smell is determines how quickly they will locate the nipple. Mature females nurse their young once a day for 2-4 minutes by standing motionless over them (Distrel and Hudson, 1984). There are a certain behaviors the young must meet in order to successfully find the mother’s nipple. The young must be able to detect different abdominal odors the mother emits, detect the change in smell if the mother alters her milk and the odor of the lactating mother when other females are around. Other factors included in the young’s ability to find a mother’s nipple are their oral grasping ability (Coureaud, 2001). When the young detect the presence of the mother through smell, they will lift their head and search for the nipple as she stands motionless over them (Distrel and Hudson, 1984). Finding the nippe usually takes about 10 seconds for the young, but it can take longer during the first days of birth (Distrel and Hudson, 1984). The highest odor detection period was right after birth most likely due to the hormones the female released during this time. The odor was emited from the mother’s abdominal area. In addition, the study conducted found that the young of European rabbits are not able to tell the difference between lactating female milk and freshly obtained milk and do not respond differently (Coureaud, 2001). The implications for this are understanding the preferences of young European rabbits and milk from the female can carry be an odor cue.
Young offspring are born more confident than adolescents. Within the nest their confidence is high because they have not been exposed to the periles looming outside the burrow and have the protection within the nest. After about 30 days, the confidence declines coinciding with the female giving birth to her next litter. When this time approaches, some offspring are forced to leave the nest in order to make room for the new litter. After the third month, rabbits reach sexual maturity. Aggression starts to develop during this time between parents and members of the same social group. Adolescents cope with the aggression either by leaving the territory or putting up with the agonistic encounters. If the adolescent decides to leave, it is forced to try and become a member of another social group. After 91-120 days, adolescents become accostomed to their new territory and are accepted as members (Dudzinski, 1977).
In caring for their offspring, females take care of the young without aid from the males. The female rabbit builds the nest, prepares the nest and defends it. Due to the high amount of investment placed by the female on the nest, how successful reproduction is for a female mainly depends on the resources within their environment, instead of relying on the male. Dominant males have first choice to mate with the dominant female and younger subordinate males have to try and mate with subordinate females. For subordinate males, it is best to search outside of the natal unit if they desire a mate. This may be one reason why dispersal rates away from the natal unit are higher in males than females (Kunkele, 1996).
Even though females do not take care of their young, research has noted that those females who have a higher investment in their offspring had more reproductive success in future seasons than others. An interest quality the female rabbit possesses is her abdominal fur is a different color than the rest of her body when it is the reproductive season. When she goes to build her nest, she plucks out this hair which is left in the nest. When the young are born, the maternity of each litter can be found with the color of the fur seen in the nest. This is one evolutionary marker that helps the young distinguish which nest is theirs. Other ways that rabbits develop maturnity is through nursing, marking, defense of the nest and behavior of the young (Von Holst, 2002).