By Nanna M. Andersen | Published: 24.02.22 | Edited: 02.07.23 A summary of: A Scientific Review of Age Determination for a Laboratory Rat: How Old is it in Comparison with Human Age? By Pallav Sengupta Biomedicine International 2011; 2: 81-89. Department of Physiology, Vidyasagar College for Women, University of Calcutta, Kolkata, West Bengal, India In this study, Sprague Dawley and Long-Evans rats were used in an attempt to show rat-human age correlations. These correlations widely differ depending on the stages of life and development. The study aims to illustrate the age period correlations, from weaning (12-21 days) to the aged phase of life (275 and up). Rats have a life expectancy of 2-3.5 years (Pass D, Freeth G.), with the average life expectancy being 3 years, which is the basis for the calculations made in the study. In Table 1, directly copied from the paper, we see the lifespan, body weight, developmental and reproductive events of the laboratory rat. While puberty and social maturity are mentioned in Table 1, rats are actually considered adults when they are between 2 and 5 months old (S. Sudakov, 2021). When contemplating the direct comparison between rat age and human age, it is suggested that 13.8 days are equal to 1 human year. That would equal approximately 1 human year every 2 weeks for the rat. That would mean each month would age the rats 2 more years in human years. Puberty starts in females at around the 5th week, while for males it is closer to the 6th week. Although these results will vary depending on the strain used in the study, in this study, Sprague-Dawley and Long-Evans rats were used. In this study puberty for females was measured to occur on day 32, and for males on day 42, which corresponds with the later onset of puberty in human males as well. However in other studies, the average time it took to reach puberty was 76.5 days, with a range from 53 to 142 days (Long JA and Evans AM). A Wistar strain reached puberty at 46.9 days, with a range of 36 to 66 days (Freudenberger CB), and in another lab study, it was reached at 52.7 days, with a range of 39 to 101 days. Another lab had an average of 80.6 days to reach puberty (Slonaker JR). A separate strain reached puberty between day 58.6 and 61.1 (Bogart R, Sperling G, Barnes LL, and Asdell SA). All of these results were narrowed down in the 1900s, where it was fixed to 40 to 60 days (Kohn DF, Clifford CB). While rats reached sexual maturity around 6 weeks, they don't reach social maturity until 5 to 6 months (Adams N and Boice R). Social maturity means that the rat has fully developed its social skills to function effectively in all social settings. These behaviors include social traits such as group compatibility, fair play, emotional adjustability, politeness, self-control, self-confidence, cooperation, and leadership. When trying to judge adulthood in rats based on musculoskeletal maturity, it becomes difficult to pinpoint. While skeletal growth tapers off in males and females around 7 to 8 months (Harlan), meaning they are done growing at this age. Fertility in female rats starts to decline around 6 months of age, especially when they have never mated and are allowed to continue the estrous cycle. This decline is also seen in human females at 35 years, and at 40 years, approximately 50% of females are infertile. Biological aging in females starts between 15 and 24 months of age (Meites J, Steger RW, Huang HH). This is when the gradual deterioration of the cell starts, and the cell ages and permanently stops dividing but does not die off. Menopause may start in rats between 15 and 18 months (Durbin PW, Williams MH, Jeung N, Arnold JS). Pregnancy during this period is difficult to accomplish but is possible. Pregnancies during this period are considered high-risk pregnancies, as they include a higher risk of pregnancy loss, complications during childbirth, preeclampsia (high blood pressure and clotting), congenital abnormalities, and maternal health problems. To determine how many days of a rat's life are equivalent to 1 human year, the study concluded that we have to take the different stages of life into consideration. So, 1 human year is equivalent to the following in rat days:
This gives us an average of 16.4 rat days. References
A summary of: The Laboratory Rat: Relating Its Age With Human's By Sengupta P. Int J Prev Med. 2013 Jun;4(6):624-30. The study discusses the commonly used methods for determining the age of rats, highlighting that none of these methods can precisely define a rat's age. Typically, multiple methods are employed in research studies to narrow down the accurate age of rats. The methods used to determine age in rodents include:
The weaning period in rats refers to the phase when the offspring start to consume solid foods. Weaning or nursing marks the initial stage of a rat's life. Usually, around day 18, the offspring begin ingesting solid foods, and in laboratory settings, pups are weaned from their mothers on day 21. After the weaning period, rats enter puberty, which is when reproduction first becomes possible. It is important to note that sexual maturity itself does not indicate the beginning of adulthood in rats; rather, it signifies the onset of adolescence or the juvenile period, characterized by increased risk-taking and social play. These behaviors persist well into adulthood (Spear LP). Biological aging in female rats begins at 15 months, as mentioned in the previous study. Most aging females experience persistent estrous, leading to constant sexual receptivity (Sengupta P). In humans, the equivalent biological aging would be menopause, characterized by the cessation of the menstrual or fertility cycle. Humans typically reach menopause around 51 years of age, while rats reach this biological aging stage between 15 and 20 months. Post-senescence, which denotes the period after menopause, is considered the last phase of life. On average, rats live for approximately 15.9 months after the end of menopause, correlating with a life expectancy of around 30 months for rats. The paper also includes a well-known graph that has been circulated in the rat community. This graph serves as evidence that females should not be mated before 6 months of age, as 6 months in rats corresponds to 18 human years. An important aspect of the graph is that it depicts social maturity when compared to humans. However, it does not account for sexual maturity or the ability to successfully wean a litter, both of which have been thoroughly proven to occur as early as 3 months, as evidenced in my previous blog post titled "The Importance of Peers During Early Development in the Norway Rat." The paper concludes by emphasizing that rats are not miniature humans. When comparing humans and rats in terms of age, it is essential to consider the differences in anatomy, physiology, development, and biological phenomena. References
My conclusion and thoughts It is evident that rats are considered adults between 2 and 5 months of age and reach social maturity around 5 to 6 months. However, reaching social maturity does not necessarily indicate the ideal breeding age, as fertility decreases in female rats around 6 months. Directly comparing the life stages of rats and humans is challenging due to their significant differences. Rats have an accelerated and short childhood compared to humans, reaching sexual maturity at 5 weeks for females and 6 weeks for males. This early maturation is crucial for the species' survival in nature, where rats typically live for about a year. Early procreation ensures the continuation of the species. While early matings may not be necessary for the survival of pet rats, it is essential to consider the physiological evolution of rats. Breeding them into old age poses numerous and serious risks. The rapid development of rats indicates that they are biologically equipped to handle litters at an early age, and studies have shown that offspring from younger does are generally healthier than those from older does. The second study highlights the importance of not treating rats as miniature humans, particularly when determining the ideal breeding age. Relying solely on social maturity, as we do with humans due to social norms and expectations, overlooks critical factors such as the physiological development and biological health of the female, as well as the cognitive, emotional, and social development of the offspring. Many countries have established a minimum age requirement of 6 months for female rats to be used ethically in breeding programs, but this approach neglects essential aspects of rat development. With the availability of readily accessible information, cherry-picking supportive data to align with personal opinions or club norms is becoming increasingly challenging within the rat breeding community. In the past, access to scientific studies was limited, which in turn shaped the community based on the personal ethics prevalent at the time. However, as breeders become more connected and recognize the importance of scientifically supported breeding ethics, there is a growing belief in the need to improve and advance the community based on updated information. Despite the ease of accessing updated information and new studies, clubs and organizations may still lag behind in accepting changes in breeding ethics due to outdated databases and sources. However, there is a positive trend among breeders towards embracing scientific knowledge and ethical practices to drive progress in the community, which is a stance that I wholeheartedly agree with and support.
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