06/02/2026
Great read, highly recommend for anyone interested in seriously breeding any rodent but specifically mice in general!
I was asked to share this publicly for educational purposes. This is a post from my group science behind mice (I can’t tag it for some reason lol)
Behavior in Culling
I was supposed to do a post on trauma bonding, and trust me, I will get to that. BUT, Interestingly, I recently bought a textbook called Behavioral Genetics of the mouse: genetics of behavioral phenotypes.
I wasn’t expecting to find a section on culling in it but I guess it does make sense it’s in here. Let me explain why section by section.
Page 89, section: Litter culling
“Pup culling is a delicate issue. Indeed, the necessity to obtain a minimal amount of tissue and/or biological samples (as required in a molecular biology study that also includes basic behavioral endpoints) is often in conflict with the requirements of a high-standard and exhaustive behavioral analysis. For many scientists performing breeding studies in a regulatory context, the culling of litters to a standard size shortly after birth (generally 3-5/gender/litter) is an acceptable practice, although it is often seen as a non-naturalistic strategy. While the origins of this habit seem to be lost in the mist of time, the perceived benefits (reduction in workload and costs, reduction in litter size variability, and in the growth and development of pups during the postnatal period) appear to justify the random discarding of some animals. Dams nursing larger litters were observed to be away from the nest more often than those nursing smaller litters (Grota and Ader, 1969; Priestnall, 1972) and the decrease of the total time that the rodent dams spend with their litters decreased rapidly in litters of 12 pups (Grota and Ader, 1969).”
I don’t think it’s any surprise that any living thing would want a break when the work load is too much. The less time a mom spends with her children, the less those pups get to eat which could be a contributing factor in why pups are smaller on top of the fact that they don’t have enough ni***es.
“The point that should not be missed by behavioral neuroscientists is that, when performing developmental work, data quality, reliability, and avoiding important experimental bias, all reside in keeping pup development within normal range. Maintaining litter size constant is a good prerequisite when studying pup development. Operationally, outbred mouse lines having been selected for being highly prolific easily overcome the natural number of “really wild” (feral) mice (5-6 pups, up to 12 for females at their 2nd, 3rd, and 4th delivery).”
Pups that get to spend more time with their mother end up being able to learn “how to mouse” better than those who don’t. A mother who isn’t as attenative of her pups either through selection, or culling, isn’t spending enough time with her pups to teach them to not be “feral”.
“In outbred Swiss-albino mice, which have been selected for several decades to be efficient breeders under laboratory conditions, the litter size may easily reach 10-12 pups without major reflections on the normal range of development of the neonatal pups.”
I’m sure a lot of us have heard about the Swiss-albino strain as it’s talked about often when people talk about selecting for milk production.
“For in**ed mice, this number is often halved. The same applies to transgenetic mouse lines in the absence of any relevant indication of these being particularly feeble.”
Let me highlight that:
*For in**ed mice, this number is often halved.*
This is significant. If the Swiss-albino which has been selected for decades to be prolific in their breeding can handle 10-12 pups without losing condition and in**ed lines often have that number halved, we should really be thinking about our “max number” we recommend to new breeders. Of course it’s line dependent. But newer breeders are going to be more hesitant of culling. If they hear they can keep 8 max, they hear they can keep 8 no matter what. And this won’t always be the case.
“In several cases, due to physical impairment to unexpected deficits in maternal care, or both, the culling of pups within a litter deserves particular attention. In fact, the number of pups should not exceed the capability of the dam to provide a sufficient amount of maternal care, both qualitatively and quantitatively (Alleva et al., 1989; Capone et al., 2005; Cirulli and Laviola, 2000; Cirulli et al., 2003b; D’Udine and Alleva, 1988; Hall and Rosenblatt, 1978; Laviola and Alleva, 1995).”
This means that health checks are more than just making sure pups have full bellies. Health checks should include making sure pups are meeting all milestones while mom isn’t losing condition as well as making sure mom is tending to pups as much as she should be. If milestones aren't being met, mom is losing condition, and mom isn't tending to pups as much as she should be, culling down further may be needed.
“A final relevant consideration is that, in the case of significant retardations in somatic and/or behavioral development, the first explanation (and the most parsimonious, sensu Occam’s razor) should be that maternal care by the dam is impaired. Any hypothesis attributing pups deficits to a developmental change should be at least accompanied in the discussion by this alternative explanation.”
Again, any behavioral issues the pups display as they develop and grow could be due to the dams maternal care isn’t up to par.
Alternatively the next section titled S*x ratio in the litter states:
“Mouse dams tend to take care of male pups more frequently and for longer periods in comparison with females(Laviola and Alleva, 1995; Moore and Morelli, 1979)…and, thus, pup survival may then also depend upon the s*x ratio, particularly in the case of very small litters. A high mortality level, or major bias in s*x survival, can be avoided by using a strategic combination of s*x dependent culling (Wainwright, 1999)”
I think this statement speaks for itself.
references:
effects of litter size and the behavior of lactating female mice (Mus Musculus):
https://drive.google.com/file/d/1KSOppEp-FsPYTeo9-ycCrR8XYvS5GDGk/view?usp=sharing
Effects of littersize on emotionality, adrenocortical reactivity, and susceptibility to gastric erosions in the rat:
https://drive.google.com/file/d/16EJYhAiwqQM5Y4HgcamoLJNRNqSSV6Fw/view?usp=sharing
Litter gender composition affects maternal behavior of the primiparous mouse dam (Mus musculus):
https://drive.google.com/file/d/1mLMg02vTY4gYMCV6wQf7pNL7aBCTcN1D/view?usp=sharing
Methods in the analysis of maternal behavior in the rodent:
https://drive.google.com/file/d/1QDQOLZ-Do4XMQZcNytfXPDwKJT-1YfoK/view?usp=sharing
Paradoxical effects of D-amphetamine in infant and adolescent mice: role of gender and environmental risk factors:
https://drive.google.com/file/d/1qpBOBat67ntxIZh8VXNSXi9O43FcPZpy/view?usp=sharing
Early disruption of the mother-infant relationship: effects on brain plasticity and implications for psychopathology:
https://drive.google.com/file/d/1e8bYUheYgOMqXAk4ojKp59qTE3yNp4HQ/view?usp=sharing
Development of nutritional control of food intake in suckling rat pups:
https://drive.google.com/file/d/1BA4T5Zh1gQXUUhVIe_7HLIv6m5uh2p0j/view?usp=sharing
Sibling effects on the behavior of infant mouse litters (Mus domesticus):
https://drive.google.com/file/d/1vOLng1CiYDMnSCroPUOFsfU3RUtsBe-l/view?usp=sharing
Methodological issues in the assessment of behavioral development in laboratory mice:
https://drive.google.com/file/d/1q2CXgMbimNNNQvTw4OWw1vjV8SdhG1YZ/view?usp=sharing
