Reduced metabolic cost of locomotion in Svalbard rock ptarmigan (Lagopus muta hyperborea) during winter

The Svalbard rock ptarmigan, Lagopus muta hyperborea experiences extreme photoperiodic and climatic conditions on the Arctic archipelago of Svalbard. This species, however, is highly adapted to live in this harsh environment. One of the most striking adaptations found in these birds is the deposition, prior to onset of winter, of fat stores which may comprise up to 32% of body mass and are located primarily around the sternum and abdominal region. This fat, while crucial to the birds' survival, also presents a challenge in that the bird must maintain normal physiological function with this additional mass. In particular these stores are likely to constrain the respiratory system, as the sternum and pelvic region must be moved during ventilation and carrying this extra load may also impact upon the energetic cost of locomotion. Here we demonstrate that winter birds have a reduced cost of locomotion when compared to summer birds. A remarkable finding given that during winter these birds have almost twice the body mass of those in summer. These results suggest that Svalbard ptarmigan are able to carry the additional winter fat without incurring any energetic cost. As energy conservation is paramount to these birds, minimising the costs of moving around when resources are limited would appear to be a key adaptation crucial for their survival in the barren Arctic environment.     

The effects of season and sex upon the morphology and material properties of keratin in the Svalbard rock ptarmigan (Lagopus muta hyperborea)

The material properties and morphologies of the modified integumentary organs of birds (the keratinous bills, claws and feathers) have evolved to withstand the variety of mechanical stresses imposed by their interaction with the environment. These stresses are likely to vary temporally in seasonal environments and may also differ between the sexes as a result of behavioural dimorphism. Here we investigate the morphology and material properties of the claws of male and female Svalbard ptarmigan (Lagopus muta hyperborea) during the summer and winter using nanoindentation. Despite differences in locomotor demands between the sexes and pronounced seasonal differences in environmental conditions, like ground substrate, ambient temperature and day length, there was no significant difference in Young׳s modulus or hardness between the seasons for each sex. However, when comparing males and females, female claws were significantly harder than those of males and both sexes had significantly wider claws during winter. We propose that wider claws may follow winter claw moulting as the claws are regrown and form an important part of the ptarmigan׳s snowshoe-like foot that is an adaptation to locomotion on snow. Future work focusing on growth rates and more broad measures of material properties in both captive and wild birds is required to determine the extent of seasonal and sex differences in the material properties of their keratinous structures.     

Influence of energy stores on activation of reproductive function in male golden-mantled ground squirrels

Summary Male golden-mantled ground squirrels were captured in the field in late summer and placed on either unlimited or restricted (80% normal) food rations in the laboratory until each animal began to hibernate. At entrance into hibernation mean body mass was 274.5 g for the unlimited group and 224.5 g for the restricted group. Only six of 21 males subsequently underwent reproductive maturation during winter. Each of these six received the unlimited ration prior to hibernation, and these six included the five heaviest animals in the experiment. In addition to activating their reproductive systems, these heavier squirrels began hibernation later and ended hibernation earlier than the lighter squirrels. Squirrels that remained reproductively quiescent averaged 25.5% body mass as fat in early spring and thus were not severely limited in energy stores during winter.In another experiment a group of 13 male and female squirrels were housed together in a common outdoor enclosure in order to examine the possible relationship between reproductive condition and social standing of males. Although there appeared to be a social hierarchy among males, each male fattened substantially in fall and became reproductively active during winter.These results suggest that the level of energy stores accumulated as fat prior to hibernation in fall affects the potential of male ground squirrels to breed in spring. This dependence of reproductive development on energy stores may reflect the high energetic costs associated with breeding in males when they emerge from hibernation in spring when food availability is low.     

Legged insects select the optimal locomotor pattern based on the energetic cost

 The gait transition in legged animals has attracted many researchers, and its relation to metabolic cost and mechanical work has been discussed in recent decades. We assumed that the energetic cost during locomotion is given by the sum of positive mechanical work and the heat energy loss that is proportional to the square of joint torque and examined the optimal locomotor pattern based on the energetic cost in a simple dynamical model of a hexapod by computer simulations. The obtained results well agree with characteristics in the locomotor patterns in legged animals; for example, the leg protraction time, step length, and the metabolic cost of transport are almost constant for many velocities, the leg cycling period decreases with velocity, and the energetic cost of locomotion induced by carrying loads linearly increases with mass loaded. This correspondence of the results of calculation to experimental results suggest that the heat energy loss for torque generation is proportional to the square of the torque during locomotion, and that the locomotor pattern in legged animals is highly optimized based on the energetic cost. Received: 22 December 1998 / Accepted: 14 April 2000     

Posture, gait and the ecological relevance of locomotor costs and energy-saving mechanisms in tetrapods

A reanalysis of locomotor data from functional, energetic, mechanical and ecological perspectives reveals that limb posture has major effects on limb biomechanics, energy-saving mechanisms and the costs of locomotion. Regressions of data coded by posture (crouched vs. erect) reveal nonlinear patterns in metabolic cost, limb muscle mass, effective mechanical advantage, and stride characteristics. In small crouched animals energy savings from spring and pendular mechanisms are inconsequential and thus the metabolic cost of locomotion is driven by muscle activation costs. Stride frequency appears to be the principal functional parameter related to the decreasing cost of locomotion in crouched animals. By contrast, the shift to erect limb postures invoked a series of correlated effects on the metabolic cost of locomotion: effective mechanical advantage increases, relative muscle masses decrease, metapodial limb segments elongate dramatically (as limbs shift from digitigrade to unguligrade designs) and biological springs increase in size and effectiveness. Each of these factors leads to decreases in the metabolic cost of locomotion in erect forms resulting from real and increasing contributions of pendular savings and spring savings. Comparisons of the relative costs and ecological relevance of different gaits reveal that running is cheaper than walking in smaller animals up to the size of dogs but running is more expensive than walking in horses. Animals do not necessarily use their cheapest gaits for their predominant locomotor activity. Therefore, locomotor costs are driven more by ecological relevance than by the need to optimize locomotor economy.     

Bi-parental care in willow ptarmigan: a luxury?

Willow ptarmigan, Lagopus lagopus, usually pair monogamously and both parents invest considerable time and energy in the care of offspring. However, the species is occasionally polygynous (5–9%), and in these cases only the females care for offspring. To examine the consequences of uni-parental care for the production of offspring and the body condition of mates, either the male or the female parent was removed at the onset of incubation and at hatch. Natural removals were also observed. Lone hen willow ptarmigan (widows) defended eggs and incubated clutches successfully, whereas single parent males (widowers) did not. After hatch, widows (mates removed either at the onset of incubation or at hatch) and pairs raised equivalent numbers of chicks to fleging, but widowers (mate removed at hatch) were less successful. There were no differences between parental types in numbers of offspring returning in a subsequent season or in the mating status of offspring in the first year of return. Widows and paired hens had similar rates of survival during the breeding season, and, in subsequent years, similar return rates and mate fidelity. Willow ptarmigan, after the onset of incubation, provided more parental care than is required to produce offspring and maintain survival and body condition of their mates. Thus, these hypotheses can be eliminated as probable factors maintaining monogamy for willow ptarmigan at this site during 1981–1984. Bi-parental care in willow ptarmigan may have developed as a salvage strategy for males as a consequence of their remaining throughout incubation to guard their mates and capitalize on the potential to re-nest.     

The social structure and dominance hierarchy of the Mashona mole-rat, Cryptomys darlingi (Rodentia: Bathyergidae) from Zimbabwe

Cryptomys darlingi is a social subterranean rodent mole which inhabits the mesic regions of south-eastern and central Africa. Mashona mole-rats live in small colonies (5-9 animals) in which reproduction is normally restricted to the largest male and female in the colony. The non-reproductive members in a mature colony cannot be placed into clearly defined work-related groups based on body mass.
The dominance hierarchy of a young colony was found to be linear, with a value of 1.00 calculated from Landau's linearity index, while that of a mature colony of nine mole-rats was almost linear (0.77). Dominance was found to be related to gender in the mature colony, with males more dominant than females, and to age in the young colony. The reproductive mole-rats are the dominant animals within their respective genders. Dominance appears to correlate positively with body mass (rs = 0.77 in the mature colony and rs = 0.93 in the young colony).
Popularity studies show that smaller animals and females tend to be more popular than the larger massed individuals or males. In the mature colony which contained predominantly adult animals, the reproductive pair was among the least popular. While in the young colony, composed predominantly of sub-adult and juvenile animals, the reproductive pair was the most popular.
Social organization within Mashona mole-rat colonies is compared with other southern African Cryptomys species.     

THE PHYSIOLOGICAL TRANSITION FROM FASTING TO FEEDING IN WEANED ELEPHANT SEAL PUPS

We studied energetics and food utilization in young elephant seals as they were first introduced to solid food following their long post-weaning fast. Using radioactive tracer techniques, we monitored changes in body composition, protein metabolism, and metabolic rate during fasting and initial feeding. In fasting animals, fat stores supplied nearly all energetic requirements. In feeding animals, 49% of protein ingested was retained as body tissue, allowing protein mass to increase. Body fat was lost at rates comparable to rates in fasting animals and continued to fuel the bulk of metabolism. Weight loss was arrested when animals consumed 786 g/d, or 40 kcal/kg^0.75/d, which was far less than their metabolic rates (63–206 kcal/kg 0.75 /d). Surprisingly, the young seals were able to maintain weight and store protein while energy intake was below metabolic needs. This was possible because animals gained weight as water; they retained wellhydrated proteinaceous tissue while losing poorly-hydrated adipose tissue.     

An analytical estimation of the energy cost for legged locomotion

Legged locomotion requires the determination of a number of parameters such as stride period, stride length, order of leg movements, leg trajectory, etc. How are these parameters determined? It has been reported that the locomotor patterns of many legged animals exhibit common characteristics, which suggests that there exists a basic strategy for legged locomotion. In this study we derive an equation to estimate the cost of transport for legged locomotion and examine a criterion of the minimization of the transport cost as a candidate of the strategy. The obtained optimal locomotor pattern that minimizes the cost suitably represents many characteristics of the pattern observed in legged animals. This suggests that the locomotor pattern of legged animals is well optimized with regard to the energetic cost. The result also suggests that the existence of specific gait patterns and the phase transition between them could be the result due to optimization; they are induced by the change in the distribution of ground reaction forces for each leg during locomotion.     

Variable reproductive effort for two ptarmigan species in response to spring weather in a northern alpine ecosystem

Predicting how animal populations respond to climate change requires knowledge of how species traits influence the response of individuals to variation in anuual weather. Over a four‐year study with two warm and two cold years, we examined how sympatric rock ptarmigan Lagopus muta and white‐tailed ptarmigan L. leucura in the southern Yukon Territory respond to spring weather in terms of breeding phenology and the allocation of reproductive effort. The onset of breeding was approximately synchronous; for each one‐degree rise in spring temperature, mean breeding dates of rock and white‐tailed ptarmigan advanced by about 2.7 and 4 days respectively. Although onset of breeding was similar, the two species differed in their reproductive effort. As breeding was delayed, average first clutch sizes of rock ptarmigan declined from 9.4 to 5.8 eggs over the breeding period, while those of white‐tailed ptarmigan only declined from an average of 7.8 to 6.8. Rock ptarmigan were also less likely to re‐nest if their first clutch was lost to predators and as a consequence they had shorter breeding seasons. White‐tailed ptarmigan produced about 25% more offspring annually than rock ptarmigan and contributed more young through re‐nesting. While white‐tailed ptarmigan had higher annual reproductive output, adult rock ptarmigan had a 20–25% higher annual survival rate, which may indicate a reproduction–survival trade‐off for the two species. These results show that even within the same location, closely related species can differ in how they allocate effort as environmental conditions fluctuate.     

Photoperiodic and ambient temperature control of the annual body mass cycle in Svalbard ptarmigan

The present study describes the photoperiodic control of annual body mass changes in captive. Svalbard ptarmigan, in particular the onset of autumnal increase and midwinter decrease in fat content under natural and simulated daylight-conditions in Tromsø (69° 46 N). Autumnal fattening commences when the birds become photorefractory and presumably depends on this condition. At present it is not known if any causal relationship is involved. Under outdoor ambient temperature, body mass begins to decline in November. However, when Svalbard ptarmigan are caged indoors at higher ambient temperatures and exposed to natural or simulated annual changes in daylength, body mass remains high until February. In these birds the depletion of fat stores appears to be triggered by the increasing daylength, since body mass remained high under permanent exposure to short days throughout spring but decreased promptly following photostimulation in May. When ptarmigan caged indoors were starved in midwinter body mass fell but increased briefly upon refeeding and thereafter declined as in the control birds throughout spring. This indicates that the winter body mass profile in Svalbard ptarmigan is not merely the passive outcome of shifts in the energy expenditure associated with thermoregulation, and that a sliding set point for body mass exists and is temporally fixed at the seasonal maximum in mid winter in birds caged under indoor ambient temperatures. The possibility is discussed that the decline in body mass seen outdoors may be associated with the increased hypothalamus-pituitary-gonadal activity which follows the breaking of photorefractoriness, and that this activity is sufficiently suppressed in Svalbard ptarmigan caged indoors under exposure to short days, to delay the reduction until they are photostimulated.Abbreviations BM body mass - GH growth hormone - HPG hypothalamic-pituitary-gonadal system - LL continous light - NL natural light - SD short day (4 hours light per day) - SL simulated annual changes in daylength - T _a ambient temperature     

The effects of winter severity and population density on body stores in the Iberian wild goat (<Emphasis Type="Italic">Capra pyrenaica</Emphasis>) in a highly seasonal mountain environment

We studied the factors that determined kidney fat stores (KFs) and kidney stores (Ks)—defined as the residuals from the linear regression of kidney mass and kidney fat, respectively, on body weight—in 463 Iberian wild goats (Capra pyrenaica) from the Sierra Nevada (southern Spain). Despite the fact that body stores in both sexes were highest during the warmest months of the year and lowest during the coldest months when food resources are limited, the observed pattern was sex- and age-dependent. The KFs of male goats fell more than those of females in winter, and the yearlings of both sexes needed one season more than young or adults to restore their KFs. Goats of all age classes showed the same seasonal patterns in their Ks, although Ks were lower in females than in males throughout the yearly cycle. In addition, we found strong delayed effects of both snowfalls and population density on body stores, and in years with a lot of snow, goats' KFs reached their lowest levels in the current winter–spring, but the highest in the following summers and autumns. This pattern was less noticeable in the Ks. Population density negatively affected the body stores of wild goats, especially in winter, and the amount of snow fallen in the year of birth (cohort effect) did not seem to influence the body stores in our data set. In addition, we assessed the accuracy of the residuals from the regression between body size and body mass for monitoring body condition of live wild goats and concluded that, although it poorly indicates fat stores, it could be used as a general proxy of body condition. Finally, we discuss the expected effects of climate warming on body stores in this Mediterranean Caprinae species.     

Physiological ecology of aquatic overwintering in ranid frogs

In cold-temperate climates, overwintering aquatic ranid frogs must survive prolonged periods of low temperature, often accompanied by low levels of dissolved oxygen. They must do so with the energy stores acquired prior to the onset of winter. Overwintering mortality is a significant factor in their life history, occasionally reaching 100% due to freezing and/or anoxia. Many species of northern ranid frogs overwinter in the tadpole stage, which increases survival during hypoxic episodes relative to adults, as well as allowing for larger sizes at metamorphosis. At temperatures below 5 degrees C, submerged ranid frogs are capable of acquiring adequate oxygen via cutaneous gas exchange over a wide range of ambient oxygen partial pressures (PO(2)), and possess numerous physiological and behavioural mechanisms that allow them to maintain normal rates of oxygen uptake across the skin at a relatively low PO(2). At levels of oxygen near and below the critical PO(2) that allows for aerobic metabolism, frogs must adopt biochemical mechanisms that act to minimise oxygen utilisation and assist in maintaining an aerobic state to survive overwintering. These mechanisms include alterations in mitochondrial metabolism and affinity, changes in membrane permeability, alterations in water balance, and reduction in cellular electrochemical gradients, all of which lead to an overall reduction in whole-animal metabolism. Winter energetic requirements are fueled by the energy stores in liver, muscle, and fat depots, which are likely to be sufficient when the water is cold and well oxygenated. However, under hypoxic conditions fat stores cannot be utilised efficiently and glycogen stores are used up rapidly due to recruitment of anaerobiosis. Since ranid frogs have minimal tolerance to anoxia, it is untenable to suggest that they spend a significant portion of the winter buried in anoxic mud, but instead utilise a suite of behavioural and physiological mechanisms geared to optimal survival in cold, hypoxic conditions.     

Effects of salinity on the immune response of an ‘osmotic generalist’ bird

Salt stress can suppress the immune function of fish and other aquatic animals, but such an effect has not yet been examined in air-breathing vertebrates that frequently cope with waters (and prey) of contrasting salinities. We investigated the effects of seawater salinity on the strength and cost of mounting an immune response in the dunlin Calidris alpina, a long-distance migratory shorebird that shifts seasonally from freshwater environments during the breeding season to marine environments during migration and the winter period. Phytohaemagglutinin (PHA)-induced skin swelling, basal metabolic rate (BMR), body mass, fat stores, and plasma ions were measured in dunlins acclimated to either freshwater or seawater (salinity: 0.3 and 35.0 ‰, respectively). Seawater-acclimated dunlins mounted a PHA-induced swelling response that was up to 56 % weaker than those held under freshwater conditions, despite ad libitum access to food. Freshwater-acclimated dunlins significantly increased their relative BMR 48 h after PHA injection, whereas seawater-acclimated dunlins did not. However, this differential immune and metabolic response between freshwater- and seawater-acclimated dunlins was not associated with significant changes in body mass, fat stores or plasma ions. Our results indicate that the strength of the immune response of this small-sized migratory shorebird was negatively influenced by the salinity of marine habitats. Further, these findings suggest that the reduced immune response observed under saline conditions might not be caused by an energy or nutrient limitation, and raise questions about the role of osmoregulatory hormones in the modulation of the immune system.     

Metabolic implications of a 'run now,pay later' strategy in lizards: an analysis of post-exercise oxygen consumption

Lizards and many other animals often engage in locomotor behaviors that are of such short duration that physiological steady-state conditions are not attained. It is sometimes difficult to estimate the energetic costs of this type of locomotor activity. This difficulty is addressed by considering as reflective of the metabolic cost of activity (C(act)) not only the oxygen consumed during the activity itself, but also the excess post-exercise oxygen consumption (EPOC) and any excess metabolites persisting at the end of EPOC. Data from both lizards and mammals demonstrate that EPOC is the major energetic cost when activity is short and intense. This paper evaluates the major metabolic components of EPOC in lizards. We then examine how behavioral variables associated with locomotion (duration, intensity, frequency) can influence EPOC and C(act). Short and intense activity is much more expensive by this measure than is steady-state locomotion. Evidence is provided that intermittent activity of short duration can be more economical relative to single bouts of the same activity. Metabolic savings appear greatest when the pause period between behaviors is short. In contrast, endurance is enhanced by short activity periods and longer pause periods, suggesting a tradeoff between endurance and EPOC-related metabolic costs.     

Is hunting mortality additive or compensatory to natural mortality? Effects of experimental harvest on the survival and cause-specific mortality of willow ptarmigan

1. The effects of harvest on the annual and seasonal survival of willow ptarmigan Lagopus lagopus L. were tested in a large-scale harvest experiment. Management units were randomly assigned to one of three experimental treatments: 0%, 15% or 30% harvest. Seasonal quotas were based on the experimental treatment and estimates of bird density before the hunting season. Survival rates and hazard functions for radio-marked ptarmigan were then estimated under the competing risks of harvest and natural mortality. 2. The partially compensatory mortality hypothesis was supported: annual survival of ptarmigan was 0·54 ± 0·08 SE under 0% harvest, 0·47 ± 0·06 under 15% harvest, and was reduced to 0·30 ± 0·05 under 30% harvest. Harvest mortality increased linearly from 0·08 ± 0·05, 0·27 ± 0·05 and 0·42 ± 0·06 from 0% to 30% harvest, whereas natural mortality was 0·38 ± 0·08, 0·25 ± 0·05 and 0·28 ± 0·06 under the same treatments. 3. Realized risk of harvest mortality was 0·08-0·12 points higher than our set harvest treatments of 0-30% because birds were exposed to risk if they moved out of protected areas. The superadditive hypothesis was supported because birds in the 30% harvest treatment had higher natural mortality during winter after the hunting season. 4. Natural mortality was mainly because of raptor predation, with two seasonal peaks in fall and spring. Natural and harvest mortality coincided during early autumn with little potential for compensation during winter months. Peak risk of harvest mortality was 5× higher than natural mortality. Low natural mortality during winter suggests that most late season harvest would be additive mortality. 5. Environmental correlates of natural mortality of ptarmigan included seasonal changes in snow cover, onset of juvenile dispersal, and periods of territorial activity. Natural mortality of ptarmigan was highest during autumn movements and nesting by gyrfalcons Falco rusticolus L. Mortality was low when gyrfalcons had departed for coastal wintering sites, and during summer when ptarmigan were attending nests and broods. 6. Our experimental results have important implications for harvest management of upland gamebirds. Seasonal quotas based on proportional harvest were effective and should be set at ≤ 15% of August populations for regional management plans. Under threshold harvest of a reproductive surplus, 15% harvest would be sustainable at productivity rates ≥ 2·5 young per pair. Impacts of winter harvest could be minimized by closing the hunting season in early November or by reducing late season quotas.     

Fat ablation and food restriction influence reproductive development and hibernation in ground squirrels

The role of white adipose tissue in development of the reproductive apparatus of male golden-mantled ground squirrels was assessed by surgical removal of fat (lipectomy) immediately prior to onset of hibernation or by manipulation of fat levels through food restriction for 5 wk preceding hibernation. Animals then were maintained without food at 6 degrees C, and they hibernated from November 1983 until April 1984. At that time, blood plasma was assayed for hormone levels, and body mass, body composition, and masses of the testes and seminal vesicle-prostate complex were determined. At autopsy, testes and sexual accessory organs were heavier in Control squirrels than in Food-Restricted or Lipectomized (LIPX) animals. Paired-testes mass was positively correlated with body mass. Testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) levels were uniformly low in animals sampled during torpor, but were elevated in 2 animals that had regained the euthermic state. The Food-Restricted animals spent more time in torpor than did the other two groups, and the percentage of decrease in body mass over the hibernation season was less for Food-Restricted than for LIPX or Control animals. Adequate lipid stores may be essential for the normal development of the reproductive apparatus of male ground squirrels. In the field, animals with insufficient fat stores may forego reproduction and thereby conserve energy for survival.     

Deferred costs of compensatory growth after autumnal food shortage in juvenile salmon

Growing animals are often able to offset the effects of periods of reduced food availability by subsequently undergoing a phase of elevated compensatory or 'catch-up' growth. This indicates that growth rates are not normally maximized even when food is not limiting, suggesting that fast growth may be costly. Here, we show experimental evidence of a long-term deferred cost of compensatory growth after a period of food shortage. Juvenile salmon subjected to a short-lived low-food regime in autumn subsequently entered a hyperphagic phase, leading to complete restoration of lipid reserves and partial recovery of lost skeletal growth relative to controls. However, several months later they entered a prolonged phase of poorer performance (despite food now being freely available), so that by the following spring they were substantially smaller than controls and had lower lipid reserves for their body size. The incidence of sexual maturation in males the following breeding season was also reduced. Salmon thus appear to trade off the benefits of short-term restoration of fat stores prior to winter against long-term performance.     

Understanding sex differences in the cost of terrestrial locomotion

Little is known regarding the physiological consequences of the behavioural and morphological differences that result from sexual selection in birds. Male and female Svalbard rock ptarmigans (Lagopus muta hyperborea) exhibit distinctive behavioural differences during the breeding season. In particular, males continuously compete for and defend territories in order to breed successfully, placing large demands on their locomotor system. Here, we demonstrate that male birds have improved locomotor performance compared with females, showing both a lower cost of locomotion (CoL) and a higher top speed. We propose that the observed sex differences in locomotor capability may be due to sexual selection for improved male performance. While the mechanisms underlying these energetic differences are unclear, future studies should be wary when pooling male and female data.     

Lability of fat stores in peripubertal wild house mice

Summary Traditionally, the adaptive value of mammalian white fat stores is considered in relation to longterm needs such as providing protection against the vagaries of winter or signalling the reproductive system when energy reserves are sufficient to risk pregnancy. As shown here, the fat stores of young house mice could not serve such needs. Despite prolonged acclimation and excess nesting material, food deprivation at 10°C significantly lowered the fat stores of peripubertal female house mice in only 12 h, and would exhaust them in 30 h. Even close to thermoneutrality (24°C) the calculated time to exhaustion was only 70 h. The fat stores of a young house mouse are obviously too meager to offer any meaningful protection over a winter of several months duration, or even over a 5–6-week cycle of pregnancy and lactation. Furthermore, in a wild habitat where food availability and ambient temperature can vary rapidly and greatly, such fat stores would be too labile to effectively coordinate puberty with somatic development.