Sometimes mother rats combine their litters in a single, communal nest. The mothers raise and nurse their litters together. Why do rats do this?
Communal nesting occurs when two or more mother rats rear their litters in a single nest. Such mothers may also share milk with the offspring of another mother.
Not all rats are equally likely to nest together. Familiarity and relatedness play a big role. Pairs of sisters who have grown up together are more likely to pool their offspring and to share parenting than females who have only known each other for a few weeks. Unfamiliar females tend not to pool their offspring, and when they do, one female tends to monopolize the litter. Familiar pairs of sisters are more successful than unfamiliar pairs: overall they produce more weaned offspring and are less likely to commit infanticide than unfamiliar pairs.
Infanticide can be a problem between unfamiliar pairs of mothers. Litters tend not to be synchronized, so they arrive at different times. In 44% of unfamiliar pairs, the second female, still pregnant with her litter, kills all or part of the first mother's litter within a few hours of its birth. In contrast, only 11% of familiar sister pairs experience infanticide.
Familiar sister pairs have another kind of inequality. The second-born litter of sister pairs is at a disadvantage compared to the first-born litter, and tends to suffer higher mortality. This occurs because in pooled litters of different ages the younger, smaller litter is forced to compete with the older, larger litter for milk. Another problem is that the older litter suckles at their aunt's teats before she gives birth. Such prepartum nursing renders her teats unusable to her own babies when they are born. Therefore, her own litter can only nurse from the first mother. So, not only are the second-born infants smaller and less competitive than their bigger cousins, they can only nurse from one of the two mothers.
If litters are not synchronized, with an age gap of more than two weeks between the two litters, the mortality of the second litter tends to be very high. The older litter invades the younger litter's nest, tramples and injures the young, monopolizes the mother's milk and damages her teats. Under these conditions, over two thirds of second-born pups die within a few days. Those few second-born pups that survive tend to have higher weaning weights, however.
However, if litters are synchronized (age gap of less than two weeks) and mothers of newborns don't nurse the older pups for several days, communal nesting may not increase pup mortality. In this case, birth synchrony may benefit the older litter without harming the younger litter: the first-born pups in this situation weigh more when they're weaned. The second-born pups experience no weight advantage, and survival of the litters isn't significantly increased.
In captive rats, raising a litter alone appears to be the best strategy. In general, captive females who raise their litters alone rear almost all of their young to weaning age (89%). Communal nesting does not enhance litter survival in captivity, and in some cases communal nesting leads to higher litter mortality (infanticide of the first litter, high mortality of the second litter). In the wild, however, communal rearing may be more beneficial because communal nests may protect young from the cold, infanticidal males, and may dilute the risk of predation.
Communal nesting, or nest sharing, occurs when multiple females share the rearing of one or more litters in the same nest. In rats, communal nesting involves two or more mothers rearing their litters together. Some mammals that share a nest, including rats, may also exhibit communal nursing, or allonursing, the sharing of milk with nonoffspring. Communal nesting and communal nursing have different sets of pros and cons and so must be considered separately (Hayes 2000).
The likelihood of communal rearing may increase with the degree of relatedness or familiarity. In other words, mothers, adult daughters, sisters, and aunts, and female rats who grew up together are more likely to share a nest than unfamiliar females.
Schultz and Lore (1993) studied reproductive success in familiar and unfamiliar pairs of pregnant female rats. Half the pairs were sisters who had been raised together, the other half were unrelated females who cohabited for the duration of their pregnancy. Here are the results:
Pairs of sisters rear more offspring than unrelated pairs
Sister pairs gave birth to more pups than unfamiliar pairs (437 vs. 368 pups). After birth, both sets of rats raised similar percentages of pups to weaning age (about 60%). Sister pairs were also more likely to pool their litters in a single nest. Unfamiliar females tended to maintain separate nests (Schultz and Lore 1993).
Sisters share parenting more than unrelated pairs
Familiar pairs tended to share pup care, with both females in contact with the litter at the same time. In contrast, in unfamiliar pairs with pooled litters one mother was more likely to care for all the pups from both litters. However, this didn't appear to affect survival rates: similar numbers of pups survived whether they were cared for by two mothers sharing duties or one mother doing it all (Schultz and Lore 1993).
Sisters are less likely to commit infanticide
Sisters are less likely to exhibit infanticide than unfamiliar females. Eleven percent of females from familiar pairs killed all or part of the other's litter, compared to 44% in unfamiliar pairs. In almost all the unfamiliar pairs, infanticide consisted of a pregnant female killing the first-born litter of the other female shortly after its birth. It usually started, not with an attack on the young, but with the consumption of the placentas which progressed to cannibalism (Schultz and Lore 1993).
Second-born litters in trouble in sister pairs
First and second-born litters had roughly equal survival rates (49.4 vs. 58%) in unfamiliar pairs. However, when the mothers are related, more of the first-born litter survived (76.1%) than the second-born litter (45.1%). The poor survival of the second-born litter happened because the second mother shared care for the first litter: the younger litter was therefore at a competitive disadvantage with the older pups when competing for teats (Schultz and Lore 1993).
If older pups suckle at the teats of mothers about to give birth, they make the nipples unusable to newborns. Sachs and Rosenblatt (1974) found that pups born to a mother who sucked older offspring before giving birth loose weight rapidly and suffer a very high mortality rate. This happens because the nipples of a mother rat about to give birth are suited to the suckling of newborns. As the babies age, her nipples gradually adapt to their stronger suck. Nipple development and pup suckling are therefore synchronized and remain mutually adapted as the pups grow. If older pups nurse from the nipples of a mother rat who is about to give birth, their strong suckling damages her nipples. Such nipples become very red and may become caked with dried blood. Clotted blood may also block the nipple ducts (Sachs and Rosenblatt 1974).
Therefore, in a communal nest with litters of unequal ages, the second litter has access only to the first female's milk because their own mother's nipples are too damaged for them to use. In addition, the second litter has to compete with an older, stronger litter for the milk of the first mother. This puts the second litter at a great competitive disadvantage.
This competitive disadvantage is particularly pronounced when the age gap is between 15-28 days. In this situation, the older litter invades the nest shortlyand damage the teats. Young in this situation typically die within 3 days (Mennella et al. 1990).
Communal nesting may not harm pups if age gap is small
If litters are born within two weeks of each other, survivorship may be similar to that of litters born alone. The two mothers begin nursing both litters at the same time: when the new litter is about 4 days old. Mothers share the burden of lactation about equally (Mennella et al. 1990).
Therefore, when births are synchronized, both litters tend to survive, but if litters are not synchronized the second, much younger litter tends to die. Interestingly, if the mother of the second, asynchronous litter loses her litter she soon comes back into heat and becomes pregnant again. In this case, the next time around the two mothers' litters are more synchronized. Therefore, inter-litter competition may result in synchronized birth cycles (Mennella et al. 1990).
In some cases, communal nursing may increase pup weight
Only about 30% of newborns survive in the presence of much older pups. Those that do survive, however, have a higher average weaning weight than those born in the presence of younger pups (86.7 g vs. 73.2 g) (Mennella et al. 1990).
From the perspective of the first-born litter, birth synchrony benefits the older litter: first-born litters in synchronized situations (age gap 0 - 2 weeks) weighed more at weaning than first-born litters in asynchronized situations (age gap 2 - 4 weeks) (90 vs. 80 g for males, 84 vs. 73 for females). Birth synchrony did not increase the weight of the second-born litter (Mennella et al. 1990).
Dominant mothers raised more offspring than subordinates
Schultz and Lore (1993) found that dominant females gave birth to the same number of pups as subordinate females, but more of the dominant females' young survived to weaning than the subordinate's young (228 pups vs. 188 pups). Note that dominance was unrelated to birth order: mothers of first-born litters were as likely to be dominant as mothers of second-born litters. In unfamiliar pairs, the dominant rat was also the one more likely to monopolize the entire litter.
Reports about communal rearing in wild rats are mixed: some excavated nests show mixed-aged litters which indicate communal rearing (Steiniger 1950, as reported in Schultz and Lore 1993), others found only small groups of same-age young indicating single litters (Telle 1966, as reported in Schultz and Lore 1993). This discrepancy may be due to the relatedness of the mothers: Steiniger's rats belonged to stable populations of wild rats in large enclosures, so the females were likely to have been sisters who were raised together and went on to pool their offspring. Telle's rats, however, were from recently formed groups of unrelated individuals who had been reared apart.
Calhoun (1963) reports that mothers and their older offspring in a semi-wild setting may sometimes share a common burrow, which is conducive to communal nesting, but he did not actually record nest sharing.
In captivity, female rats kept alone raise a higher percentage of their young to weaning (89%) than females who nest communally (60%). Therefore, in captivity females will raise more young to weaning if they rear their young alone (Schultz and Lore 1993).
This is different from mice in captivity, which tend to raise more young when they nest communally than when they nest alone (König 1994).
In the wild
In rats, communal rearing in captivity is less successful than rearing a litter alone. In the wild, however, communal rearing, especially by related females, may have other benefits such as defense against infanticide, dilution of predation risk, and warmth. See below for more on the benefits of communal rearing.
There are many hypotheses about why females may share a nest. Some of these hypotheses have supporting evidence, but many are yet untested for rodents in general or rats in particular. These hypotheses come primarily from a review of communal nesting in rodents by Hayes (2000).
In a crowded habitat, all the good places to live may be occupied, so older young may stick around and nest communally with their mothers, sisters, and aunts. An increase in communal nesting with greater population density occurs in white-footed mice and some voles, but it has not been tested in rats.
Clumped resources: maybe
If resources are clumped, rodents may congregate near these clumps and may nest communally. Conversely, if food is spread thinly and widely, rodents would not nest communally. This correlation been found in commensal house mice, who nest communally in barns (where food is all in one place) but not in fields (where food is spread out). No studies have examined this in rats yet.
Benefits of group living
Rodents could be nesting communally for the benefits brought by grouping, such as warmth. Infant rodents depend entirely on their mother for heat, so more mothers would bring a more constant supply of heat. However, the heat hypothesis is controversial because mother rodents in burrows appear quite capable of warming their litter alone.
Defense against infanticide: maybe
Communal nesting may help because it means more females are available to defend against infanticide. Communal defense against infanticide has been found in ground squirrels and mice, but is untested in rats.
Defense against predation: probably not
More mothers could mean the nest is more likely to be defended against predators. However, defense against predation may not be meaningful for rodents because even determined rodents are largely defenseless against large predators.
Dilution of predation risk: maybe
However, if a female nests communally it does mean that if a predator attacks there's a chance that the offspring eaten won't be her own, so she's diluting the risk to her own offspring. The dilution effect has been found in prairie dogs, but has not been examined in rats.
Adoption: probably not
If female mortality is high, females may nest communally so that if they die their young may be adopted by the other female. This would be particularly attractive if the mothers are already related, so the adoptive mother is actually raising her own siblings, grandchildren, or nieces and nephews. This appears to be the case in voles, but this hypothesis has not been supported for other rodent species despite their high mortality.
Does communal nursing help the survival of a mother's pups, or is it a maladaptive byproduct of communal nesting?
Proposed benefits of communal nursing
Mothers may benefit from communal nursing because their pups get fed when they're away from the nest (see Hayes 2000 for a review). In mice, females who nurse communally with close relatives raise an average of five more offspring in their lifetimes than females who nest alone, or females who share a nest but don't share nursing (König 1994). However, the study described above by Schultz and Lore (1993) indicates that familiar rats who nest and nurse communally do not raise as many offspring as singly housed rats.
Or is it a maladaptive byproduct?
Nursing is extremely costly, so feeding more than one litter may exhaust the mother. The more young a female has to nurse, the more milk she produces. However, this increase is not linear, so in large litters each infant gets less milk per head, and the quality of the milk decreases as well. König et al. (1988) found that mother mice with 12 offspring produced only 40% more milk than mothers with 6 offspring, and their milk had only 30% more energy. The mouse pups from the larger litters grew more slowly and had a lower weaning weight than those from small litters. Therefore, if a female is nursing multiple litters in a communal nest, the cost to her may be even higher than it is for one large litter of her own, and the offspring may end up getting less milk overall even though more mothers are providing milk.
Some mothers may cheat, leaving the enormous burden of nursing largely to another. Such unequal care occurs in pairs of rats: in unfamiliar pairs one female tends to monpolize the care. In related pairs care is shared equally, but the younger litter may only be able to nurse from one mother, so nursing is again distributed unevenly between related mothers (Schultz and Lore 1993). Thus the benefits of communal nesting may fall unevenly on the participating mothers.
Just as mothers may not benefit equally from communal nesting, the litters may not benefit equally either: in mixed-age nests the younger animals must compete with older, larger animals for milk, which puts them at a disadvantage. In rats in particular, the younger animals may be unable to nurse from their own mother because the older litter has rendered her teats unusable (Sachs and Rosenblatt 1974).
The costs of increased lactation may be offset by the benefits of communal nesting. Females selectively nurse just their own litter, so communal nursing may simply be an unavoidable byproduct of sharing a nest (e.g. in mice, Manning et al. 1995; see Hayes 2000 for a review). In the wild, however, the costs and benefits may differ: if the food supply is irregular the costs of increased lactation from communal nursing can be severe.