Probably the most detailed data available for the energy demands of reproduction are derived from domesticated rodents. This is primarily because these animals are easily kept, and even quite invasive measurements can be made on them without the risk that they will desert their offspring. The time course of food intake, at 21°C, throughout reproduction in a strain of laboratory mouse (MF1) that we have been studying is shown in a
(Johnson et al. 2001a
). Food intake increased during pregnancy to approximately 8
g, compared with 5.5
g in the non-breeding females prior to reproduction. Although the pattern of foetal growth is essentially exponential between conception and birth, the pattern of food intake does not mirror this, but rather rises to a peak approximately 2–3 days earlier and then declines slightly before the day of parturition. The reason for this pattern is uncertain, but one hypothesis is that the expanding foetal mass competes for space with the alimentary tract in the abdomen, limiting the food intake. This may suggest that resources to support the gestation become limited in late pregnancy. Under such limitation, competing demands may have detrimental effects on the success of the pregnancy. One such competing demand may be the level of basal energy requirements (basal metabolic rate, BMR). We have recently shown in another strain of mouse (the C57BL/6 black mouse) that mice with higher BMR have a greater likelihood of mass anomalies occurring during pregnancy. These mass anomalies probably reflect foetal resorption events. Because BMR is correlated with body mass, this relationship might only be an artefact of both resorption rate and BMR being affected by mass, but the effect of BMR is also evident if the effect of mass is removed statistically (Johnston et al. 2007
). This effect supports the hypothesis that resource intake in late pregnancy may be limited, perhaps by space competition in the abdomen between the alimentary tract and the developing foetuses.
Figure 1 (a) Food intake each day throughout pre-breeding, pregnancy and lactation phases for MF1 mice. Data averaged across 71 litters. (b) Daily food intake averaged over the last days of lactation (10–18) plotted against litter size in MF1 mice. (c (more ...)
The most dramatic increase in food intake occurred during lactation. During the initial 10 days, this increase was linear, but then it reached a plateau at approximately 23
g of food per day. Food intake at the plateau (between days 10 and 18) was related to litter size. Small litters reached plateau intakes of less than 23
), but as the litter size increased food intake also increased to a plateau at approximately 23
g of food per day. The mice seemed to reach a limit in their food intake at this level. Although litter sizes increased, food intake did not increase in parallel.
We have shown that this limit in food intake is not mediated via the aspects of the cage the animals live in (Speakman & Krol 2005
), and therefore appears to be a physiological feature intrinsic to the animals themselves. This apparent physiological limit in the capacity of the mouse to ingest food at peak lactation may underpin an important life-history trait (maximum litter size) and an important life-history trade-off. Since the maximum asymptotic food intake at peak lactation is fixed, the energy that can be devoted to milk is also fixed. As the litter size increases, this milk must be divided between more and more offspring and, consequently, the pups wean at progressively smaller body masses (c
; Johnson et al. 2001a
). A physiological limit in the capacity to ingest food therefore appears to underpin the life-history trade-off between the number and size of offspring. Presumably, there is a minimum size of offspring at weaning, which would stand any chance of survival. Where the declining relationship between pup size and litter size intersects, this theoretical minimum viable pup size may define the maximum litter size. A distribution of natural litter sizes in this mouse strain is shown in d
. It is noticeable that the asymptotic intakes for the mice that raised 14 and 15 pups were actually higher than the average 23
g limit (d
). Perhaps these females were only able to raise such large litters because they were individuals that were capable of above-average intakes, and therefore still able to wean their pups at a pup mass of 7.5
g. Natural litter sizes of 16 and above in this strain may not be feasible owing to the interaction between the limit on maximal intake rate and the minimum viable pup size (this minimum viable pup size at weaning may also be physiologically mediated).
Understanding the physiological basis of this maximal intake limit is therefore of critical importance. Two hypotheses concerning the nature of this limit were proposed in the early 1990s (Peterson et al. 1990
; Weiner 1992
; Hammond & Diamond 1997
). The first hypothesis was that the limit was imposed by the capacity of the alimentary tract to absorb food (energy) and process it into a form for mobilization. This was called the ‘central limit hypothesis’. The second hypothesis was that the limit is imposed at the peripheral site where the centrally supplied energy is used: the mammary glands. This was called the peripheral limit hypothesis.
Hammond & Diamond (1992)
manipulated litters of Swiss Webster mice and found that females did not elevate their food intake when given up to 23 pups. Similarly, we have also shown that MF1 mice given up to 19 pups also could not breach the 23
g limit that they reached during unmanipulated lactations with litters of greater than 10 offspring (Johnson et al. 2001a
). Faced with this problem of a litter size that is ‘too large’, females will often cull their offspring rather than eat more food (Johnson et al. 2001a
; Gandelman & Simon 1978
). In a separate experiment, Hammond & Diamond (1994)
prevented pups from weaning at their normal weaning age and hence their increasing growth demands needed to be supplied by the mother until the pups were 24 days old. In these conditions, the mothers also could not upregulate their food intake. The absence of an increase in the intake when pup numbers are increased or lactation extended confirms there is a physiologically imposed limit, but does not separate between the peripheral and central limits hypotheses.
Giving female mice additional energy-demanding tasks during late lactation can better separate the alternative hypotheses. The basis of these experiments is that, if the system is centrally limited, the total energy available will be fixed. Using some of this energy to perform another task will reduce the amount available to support milk production and the consequence will be a diminution of reproductive output. Alternatively, if the system is peripherally limited, the animals will increase their intakes to meet the combined demands, and reproductive output will remain unaffected. Three manipulations have been performed to test these ideas: lactating mice have been forced to exercise to obtain their food; they have been made simultaneously pregnant, combining the demands detailed in a, and they have been exposed to cold.
compared the reproductive strategies of house mice Mus domesticus
and deer mice Peromyscus maniculatus
by forcing females to run a preset number of revolutions (between 75 and 275) on a wheel to obtain each pellet of food. Despite the combined demands of lactation and locomotor activity, neither house nor deer mice exceeded the upper limit of food intake compared with unmanipulated mothers, given free access to food. As a result of the decreased amount of energy available for reproduction, the wheel-running house mice routinely killed some of the offspring throughout the first 12 days of lactation, whereas deer mice extended lactation well beyond normal weaning age.
Johnson et al. (2001b)
followed the intakes of mice that had been mated immediately post-partum and found that the mice concurrently lactating and pregnant did not respond to the increased energy burden by elevating their food intake. Instead, they delayed implantation at the start of the second pregnancy and the length of this delay was directly related to the numbers of pups. The animals therefore ‘avoided’ overlapping their energy demands, perhaps because they could not elevate their total intake at peak lactation to accommodate both. Similar observations were made in Rockland-Swiss mice (Biggerstaff & Mann 1992
), and in rats (Rattus norvegicus
: Koiter et al. 1999
), when food intake in late lactation was actually reduced in those rats that carried a simultaneous pregnancy, relative to rats just lactating. Together, these activity and pregnancy studies indicate that the limits are centrally, rather than peripherally, mediated; although, in case of concurrent pregnancy, the evidence is less strong as the animals avoided the problem.
However, when Hammond et al. (1994)
exposed lactating Swiss Webster mice to 8°C (approx. 22°C below the lower critical temperature), they found that food intake increased dramatically beyond the supposed centrally imposed limit. Similar observations have since been made in deer mice (Hammond & Kristan 2000
), MF1 mice (Johnson & Speakman 2001
) and cotton rats (Sigmodon hispidus
; Rogowitz 1998
). The capacity of the mice to elevate their food intake in the cold was completely at odds with the central limitation hypothesis. To test whether the limit was imposed peripherally, Hammond et al. (1996)
experimentally manipulated female mice by surgically removing some of their mammary glands during late lactation. The rationale behind this experiment was that if the capacity of the mammary glands was limited, then, if the mammary tissue was halved in size, the remaining tissue would be unable to compensate by elevating its milk production. However, if the capacity of the tissue was flexible and limited only by the centrally controlled supply of energy, then it would respond to the absence of half the tissue by expanding its capacity. They found that productivity in the halved glands did not increase, suggesting that the mammary gland was indeed the point at which the system was peripherally limited.
The above experiments suggested that there is a physiological limit in the capacity of the mammary tissue to secrete milk which underlies the asymptotic food intake in late lactation, and thus the trade-off between litter size and pup size and potentially defines the maximal litter size. When animals were manipulated at room temperature by giving them more pups to raise, food intake did not increase because milk production was limited by the capacity of the mammary glands. When exposed to cold conditions, food intake did increase (demonstrating a lack of central limitation) owing to the combined demands for milk production (at maximal capacity) and increased thermogenesis.
Several other studies have been performed, which also indicate that the limits to intake at peak lactation are not centrally mediated. These studies involve manipulation of the energy density of the food. If the energy density of food is decreased, but animals have a central processing capacity limit, they should be unable to upregulate their intake to compensate. Speakman et al. (2001)
fed MF1 mice a diet that provided 25% less digestible energy than their normal food and then mated them. Food intake in the mice fed on the low energy density food increased at peak lactation on average of 3.8
g (from 23.1 to 26.9
g per day). Similar data are available for brown hares (Lepus europaeus
; Hacklander et al. 2002
). When fed on a diet with lower energy content, asymptotic food intake in late lactation increased from 230–250 to 280–300
g per day. Consequently, milk production was stable across the dietary treatments at approximately 35
g per day for females raising single offspring and 70
g per day for females raising twins. Conversely, when energy density was manipulated in the opposite direction, there was no indication that energy intake was elevated. In another example, rats fed a high energy density diet during lactation decreased their food intake to sustain energy intake constant (Denis et al. 2003
An important aspect of the peripheral limitation idea is that milk production levels remain constant across the different manipulations—reflecting the fact that mammary glands are working at capacity. Several studies have measured milk production and support this prediction. Drummond et al. (2000)
studied milk production in rabbits (Oryctolagus cuniculus
) and observed that, following natural deaths of some offspring, the flow of milk was unaltered. Fink et al. (2001)
studied lactation in captive mink (Mustela vision
) and showed that in mothers raising litters of three, six and nine offspring, milk production did not increase when litters increased from six to nine offspring. Rogowitz (1998)
demonstrated in cotton rats that levels of milk production in rats at 21 and 8°C were similar—consistent with the mammary glands working at maximal capacity. However, other studies have failed to find this consistency (Johnson & Speakman 2001
; Król & Speakman 2003a
; Król et al. 2003a
). We have now studied food intake and milk production in mice at peak lactation at three different temperatures: 30, 21 and 8°C. As predicted by the peripheral limitation hypothesis, food intake increased across these measurements as temperature declined. However, unexpectedly, milk production and, consequently, pup growth were not constant across the different temperatures, but followed the pattern of food intake. Mice exported 88
kJ energy in milk per day at 30°C, 167
kJ at 21°C and 288
kJ at 8°C (Król & Speakman 2003b
). Moreover, the weaning masses of pups at 30, 21 and 8°C averaged 6.1, 7.0 and 7.3
g, respectively (Król & Speakman 2003a
). Consequently, the colder it got the more food the mice ate, the more milk they produced and the heavier the pups were at weaning.
These data are fundamentally inconsistent with both the suggestion that the limits are imposed by the capacity of the alimentary tract to process ingested energy and the suggestion that the limits reside in the milk production capacity of the mammary gland. There are a number of new ideas about the nature of the limit in food intake during lactation, which have been recently summarized (Speakman & Krol 2005
). For example, one of these is that the limits are imposed by aspects of the neuroendocrinological system that regulates food intake rates—the ‘neural saturation hypothesis’. An alternative, suggested by (Król & Speakman 2003a
), is that the capacity to expend energy during lactation at 21°C might be limited by the ability of the female mouse to dissipate heat. Hence, manipulations at 21°C—which aim to stimulate both food intake and milk production—notably increasing litter size (Hammond & Diamond 1992
), extending lactation (Hammond & Diamond 1994
), making them simultaneously pregnant (Biggerstaff & Mann 1992
; Koiter et al. 1999
; Johnson et al. 2001b
) and making them exercise (Perrigo 1987
)—all failed to increase either food intake or milk production, because the animals could not increase their heat production without risking fatal hyperthermia (see below). In effect, this hypothesis is a central limitation idea, but is focused around the ability to dissipate heat rather than assimilation from the gut. Under the heat dissipation limit hypothesis, when mice are exposed to the cold, this is not an additional demand, but a relaxation of the heat dissipation limit, allowing the animals to elevate not only their food intake but also their milk production and thus the size of their offspring. Similarly, when mice were placed in the hot, this reduced their capacity to dissipate heat, restricted their food intake and milk production, and led to smaller pups being weaned.
There are two putative mechanisms for how the capacity to dissipate heat may influence lactation performance. At high ambient temperatures, lactating mice may continuously face difficulties dissipating heat. This would lead to a perpetually elevated body temperature (see below) which might influence the regulation of milk production. It is well established that endogenous opioids in the preoptic anterior hypothalamus are of key importance in the regulation of body temperature, and are reduced during lactation (Kim et al. 1997
). Projections from this area terminate in the paraventricular nucleus (PVN), where the magnocellular cells synthesize oxytocin. Rayner et al. (1988)
showed that intracerebroventricular (ICV) administration of morphine inhibits oxytocin production in the PVN. Thus, elevated endogenous opioids under hyperthermia might directly reduce oxytocin secretion. Morphine administration also disrupts maternal suckling behaviour (Cox et al. 1976
; Bridges & Grimm 1982
). In addition, thyroid hormone, which may be regulated by differences in body temperature, is an important modulator of prolactin production. Continual maternal hyperthermia in relation to external ambient temperatures (or differential capacity to dissipate body heat) may therefore directly inhibit oxytocin and prolactin secretion, thereby reducing milk production. Finally, there may be a more direct link between hyperthermia and milk production. A response to hyperthermia might be to direct blood flow away from the mammary glands to other peripheral areas to dissipate heat by vasodilatation (Black et al. 1993
). Blood flow in the mammary glands has been shown to have a direct effect on milk production (Vernon & Flint 1983
A second mechanism is that suckling schedules of the mice may be influenced by the development of maternal hyperthermia in the nest. Although pups act as heat sinks early in their development (Scribner & Wynne-Edwards 1994a
), in late lactation offspring are capable of considerable heat production. The suckling unit of mother and pups, therefore, may generate heat that leads to maternal hyperthermia, ultimately forcing the female to discontinue suckling (Croskerry et al. 1978
). The progress to hyperthermia would be more rapid as the capacity to dissipate heat declines. The sucking stimulus is one of the primary factors stimulating oxytocin release and milk let down, and also feeds back onto prolactin release, thereby regulating milk production. Continual disruption of suckling, due to intermittent hyperthermia, would be a second mechanism linking heat dissipation capacity to lactation performance.
Many studies have examined the pattern of change in food intake across the reproductive cycle among other small non-domesticated mammals. Some examples of these measurements are in . This is not an exhaustive review. Using the data for rodents and converting estimated food intakes into energy for those species where only food intakes are reported in the original papers, there was a significant effect of body mass (F=42.6, p<.001) and a significant effect of reproductive status (F=30.6, p<.001), but no significant interaction between these two variables (F=1.55, p=0.225; a). Using pairwise Tukey comparisons, the non-breeding energy demands did not differ significantly from those in pregnancy, but the demands in lactation were significantly elevated above both the non-breeding and pregnancy demands. This analysis reveals that the energy costs of pregnancy are relatively trivial. This does not mean that demands in pregnancy do not impose limits on reproduction (see above arguments regarding foetal volume competing for abdominal space with the alimentary tract). Nevertheless, the pattern observed in laboratory mice and rats that the costs of reproduction are substantially greater during lactation than in pregnancy appears to be very broadly applicable.
Table 1 Some examples of peak energy intakes of non-domesticated small (less than 1kg) mammals at different phases of the reproduction cycle. LS is litter size. Under P this is litter size at birth; under L, this is litter size at weaning. Where original (more ...)
Figure 2 (a) Peak energy intake of female small rodents, when not breeding (black), pregnant (red) and lactating (green). There was no significant elevation in intake during pregnancy, but intake during lactation was significantly elevated compared with both non-breeders (more ...)
There was a wide variation in the energy demands at peak lactation (; a
). The residual variation, once the effects of body mass had been removed, was significantly associated with litter size (b
), which explained 49.3% of the residual variance (p
=0.0103). Although litter size and body size explain much of the variation in energy intake at peak lactation, there is still considerable variation in the energy intakes across species. One factor that may be of importance in this variation is the developmental strategy. Studies of the patterns of development in eutherian mammals have identified two separate developmental strategies (reviewed in Martin & MacLarnon 1985
; Martin 1989
), with some species having prolonged periods of gestation followed by birth of relatively well-developed offspring that rapidly become independent of the mother and are capable of feeding themselves relatively quickly after birth (precocial development). In contrast, other species have a relatively short gestation that is followed by a more protracted period of lactation during which they are completely dependent on nutrient supply from the mother (altricial development). The use of these strategies is not independent of body mass with mammals weighing more than 100
kg using only the precocial strategy, while species weighing less than 100
g use the altricial strategy almost exclusively, although some species follow what has been called an ‘intermediate’ strategy (defined in Martin & MacLarnon 1985
), e.g. Acomys caharinus
(Degen et al. 2004
). As anticipated from their size, mice follow the altricial strategy. Between 100
g and 100
kg species are found that follow either precocial or altricial strategies with a few ‘intermediates’ (Martin & MacLarnon 1985
; Martin 1989
Clearly, this dichotomy in developmental strategy may impact on the levels of energy investment by the mother during lactation (Oftedal 1984
; Hill 1992
; Kam et al. 2006
). Since it is the rarer strategy, relatively few studies have addressed the levels of food intake of small rodents following the clearly precocial strategy (but see, for example, the studies of the guinea pig Cavia porcellus
; Kunkele & Trillmich 1997
) with many more studies following the patterns of investment during lactation of species raising clearly altricial or intermediate offspring. Gestation period (days) reflecting position on the altricial–precocial continuum was not an additional significant factor influencing peak energy demands (p
=0.13). The absence of an effect of this dichotomy might be expected given the selection of species that were either altricial or intermediate strategists. The only rodent with clear precocial development in the data reviewed in was the guinea pig (Krebs 1950
), but the manner in which the data were presented in that paper relative to unstated levels in non-breeding animals precluded its inclusion in the data analysis.
The residual variation once the effects of litter size and body mass were removed to an extent follow different strategies for coping with the direct costs of reproduction. Hence, the low values in the cotton rat (Sigmodon hispidus) and the hamster (Phodopus sungorus) reflect the fact that these animals deposit fat stores during the early phase of reproduction, which are withdrawn in lactation and reduce the need to supply all the energy from food intake. Low demands in other species may reflect the use of compensatory mechanisms that reduce expenditure on other components. These strategies will be discussed in detail below.
Expressing the peak lactation energy intake as a multiple of the intake of non-breeding animals, the average across all the rodents in was 2.1 (s.d. =0.72, n=21). This is considerably lower than the ratio in MF1 mice at 21°C, which equalled 4.2. However, the intake of the MF1 mice at this temperature is within the 95% confidence limits of the prediction from the fitted regression equation from the non-domesticated rodents, given the body size and litter size for this animal.
The importance of the differences in magnitude between the peak levels of intake in wild rodents and laboratory mice and rats remain uncertain. In most of the species studied in , and in other species, peak intake of food during lactation is dependent on litter size. Generally, however, these patterns do not reach an asymptotic level as observed in the MF1 mice (b). Two examples are shown in . However, despite this absence of an asymptote in both species (and others), there is still an inverse relationship between the litter size and the mean offspring size, indicating that increases in investment were insufficient to match elevated demand. These data suggest that the trade-off between litter size and pup mass is more complex than the simple model of the mother reaching an asymptotic intake at which point a fixed investment is divided between increasing numbers of offspring. Part of this equation, however, may be capacity limits in the offspring themselves and how these depend on litter size. Pup–pup competition may be a key element of the impact of litter size on their growth efficiency.
Figure 3 (a) Food intake at peak lactation (grams over 3 days) in relation to litter size in Peromyscus leucopus (drawn from tabulated data in Millar 1978). (b) Energy intake during lactation (kJd−1) in relation to litter size in Sigmodon hispidus (more ...)
(ii) Protein and calcium
Previous discussion of limits on lactational performance has focused almost entirely on energy. Yet growing offspring also require large amounts of protein, calcium and other micronutrients. At weaning, an MF1 mouse has produced on average 12 pups each weighing 7.5
g: a total of 96
g of wet tissue. Of this tissue, approximately 13.5
g is protein and 1.8
g is calcium, both of which must be supplied by the mother. The diet we feed our mice on contains 22.45% crude protein (RM3 breeder diet SDS diet services, Witham Essex). Thus, over the last 8 days of lactation when they are taking in 23.2
g each day, their total protein intake amounts to 41.6
g of protein—substantially more than ultimately appears in the pups. This rough balance suggests that in the lactating MF1
mouse system, protein supply is unlikely to exert limits on the animals. Several studies have been performed, however, where the level of protein in the diet is much lower.
examined the effects of feeding nursing rats and mice on four diets containing 16.3, 13.6, 11.4 and 9.4% protein. Food intake in lactation was broadly independent of the protein content of the diets, except that it was reduced in mice feeding on the lowest protein diet and ‘highly variable’ in rats on this diet. However, the growth of offspring was directly affected in both rats and mice, with offspring size at weaning being positively related to the protein content of the diet (a
). Maternal mean protein intake in these mice throughout lactation averaged only 0.5
g per day in the lowest protein group which even accounting for the mean litter size difference between the strains (12 in our work and 7.1–7.5 in Goettsch 1960
) is still substantially lower than that for our mice (5.2
g/day at peak lactation). There are several additional studies which show that low maternal dietary protein levels affect pup growth. The mothers appear constrained, however, owing to the maximal amounts of energy that they can process—perhaps defined by the heat dissipation limit. Hence, this energetic limit prevents them from simply eating more food to meet their protein demands. These data indicate that energy limitation imposes a primary constraint on the animals under low protein supply.
Figure 4 Mean mass at weaning of male (grey) and female (white) pups of (a) rats and (b) mice when fed on diets of varying protein content (from 16.3% to 9.4% protein). Drawn from data in Goettsch (1960).
If low protein in the maternal diet imposes a restriction on the growth of offspring then, perhaps, protein content of the diet is the key factor that links food intake to pup growth. When we exposed lactating females to hot and cold conditions, we may have manipulated the constraint on their total energy intake that was imposed by their capacity to dissipate heat (Krol & Speakman 2003a
). However, the impact on pup growth may have been mediated via the consequences of these different levels of energy intake for levels of protein intake—indeed the mice at 8°C were ingesting approximately 7.2
g of protein each day, those at 21°C only 5.2
g per day and those at 30°C only 2.91
g per day. Perhaps the trade-off between litter size and pup size (c
) may move depending on dietary protein contents—to the right when protein in the diet is elevated and to the left when it is reduced. Supporting these ideas, Hitchcock (1927)
fed nursing rats a base diet that contained 29% protein, and additionally supplemented some of the animals with raw meat. Litter size in both groups was the same yet the offspring from the mothers fed meat daily weighed 61.8
g at weaning, compared with only 47.3
g in the rats not given the meat ration.
A similar situation probably pertains to calcium supply. The diet we use in our studies of mouse lactation contains 1.24% Ca. Thus, in the last 8 days of lactation, the mice ingest 2.30
g of calcium, compared with an estimated calcium content of the pups of 1.8
g. Hence, as long as calcium and protein levels in diets are above a certain limit, the constraint on maximal energy intake is unlikely to severely impact offspring in terms of their protein and calcium status.
For wild animals, the situation where calcium and protein contents of the diet are below the critical point, where the energy limit becomes restrictive, may be more routinely breached. One situation that has attracted considerable attention is in microchiropteran bats. Because microchiropteran bats are almost exclusively nocturnal, a constraint probably imposed by ecological factors such as predation risk and competition (Speakman 1990
; Rydell & Speakman 1995
; Speakman et al. 2000
), they do not routinely get any exposure to sunlight. This may influence their production of vitamin D (Cavaleros et al. 2003
), which is an essential component of calcium physiology. Moreover, most microchiropteran species are predominantly insectivores and these prey are very low in calcium content. Their inability to manoeuvre effectively on the ground restricts them from finding additional mineral sources of calcium that may be available to other animals. Insectivorous bats have very low litter sizes for their body sizes (Kurta & Kunz 1987
) and one interpretation of these low rates of productivity is that they are primarily constrained by their capacity to obtain calcium (Barclay 1994