Postnatal growth rate and adult body weight in mammals: a new approach

Labelling experiments in red deer provide a general model for early bone growth dynamics in ruminants

Growth rates importantly determine developmental time and are, therefore, a key variable of a species' life history. A widely used method to reconstruct growth rates and to estimate age at death in extant and particularly in fossil vertebrates is the analysis of bone tissue apposition rates. Lines of arrested growth (LAGs) are of special interest here, as they indicate a halt in bone growth. However, although of great importance, the time intervals between, and particularly the reason of growth arrests remains unknown. Therefore, experiments are increasingly called for to calibrate growth rates with tissue types and life history events, and to provide reliable measurements of the time involved in the formation of LAGs. Based on in vivo bone labelling, we calibrated periods of bone tissue apposition, growth arrest, drift and resorption over the period from birth to post-weaning in a large mammal, the red deer. We found that bone growth rates tightly matched the daily weight gain curve, i.e. decreased with age, with two discrete periods of growth rate disruption that coincided with the life history events birth and weaning, that were visually recognisable in bone tissue as either partial LAGs or annuli. Our study identified for the first time in a large mammal a general pattern for juvenile bone growth rates, including periods of growth arrest. The tight correlation between daily weight gain and bone tissue apposition suggests that the red deer bone growth model is valid for ruminants in general where the daily weight gain curve is comparable.

Allometries of maximum growth rate versus body mass at maximum growth indicate that non-avian dinosaurs had growth rates typical of fast growing ectothermic sauropsids

We tested if growth rates of recent taxa are unequivocally separated between endotherms and ectotherms, and compared these to dinosaurian growth rates. We therefore performed linear regression analyses on the log-transformed maximum growth rate against log-transformed body mass at maximum growth for extant altricial birds, precocial birds, eutherians, marsupials, reptiles, fishes and dinosaurs. Regression models of precocial birds (and fishes) strongly differed from Case's study (1978), which is often used to compare dinosaurian growth rates to those of extant vertebrates. For all taxonomic groups, the slope of 0.75 expected from the Metabolic Theory of Ecology was statistically supported. To compare growth rates between taxonomic groups we therefore used regressions with this fixed slope and group-specific intercepts. On average, maximum growth rates of ectotherms were about 10 (reptiles) to 20 (fishes) times (in comparison to mammals) or even 45 (reptiles) to 100 (fishes) times (in comparison to birds) lower than in endotherms. While on average all taxa were clearly separated from each other, individual growth rates overlapped between several taxa and even between endotherms and ectotherms. Dinosaurs had growth rates intermediate between similar sized/scaled-up reptiles and mammals, but a much lower rate than scaled-up birds. All dinosaurian growth rates were within the range of extant reptiles and mammals, and were lower than those of birds. Under the assumption that growth rate and metabolic rate are indeed linked, our results suggest two alternative interpretations. Compared to other sauropsids, the growth rates of studied dinosaurs clearly indicate that they had an ectothermic rather than an endothermic metabolic rate. Compared to other vertebrate growth rates, the overall high variability in growth rates of extant groups and the high overlap between individual growth rates of endothermic and ectothermic extant species make it impossible to rule out either of the two thermoregulation strategies for studied dinosaurs.

Manipulating reproductive effort leads to changes in female reproductive scheduling but not oxidative stress

The trade-off between reproductive investment and lifespan is the single most important concept in life-history theory. A variety of sources of evidence support the existence of this trade-off, but the physiological costs of reproduction that underlie this relationship remain poorly understood. The Free Radical Theory of Ageing suggests that oxidative stress, which occurs when there is an imbalance between the production of damaging Reactive Oxygen Species (ROS) and protective antioxidants, may be an important mediator of this trade-off. We sought to test this theory by manipulating the reproductive investment of female mice (Mus musculus domesticus) and measuring the effects on a number of life history and oxidative stress variables. Females with a greater reproductive load showed no consistent increase in oxidative damage above females who had a smaller reproductive load. The groups differed, however, in their food consumption, reproductive scheduling and mean offspring mass. Of particular note, females with a very high reproductive load delayed blastocyst implantation of their second litter, potentially mitigating the costs of energetically costly reproductive periods. Our results highlight that females use strategies to offset particularly costly periods of reproduction and illustrate the absence of a simple relationship between oxidative stress and reproduction.

A system for predicting energy and protein requirements of wild ruminants

Wild ruminants require energy and protein for the normal function. I developed a system for predicting these energy and protein requirements across ruminant species and life stages. This system defines requirements on the basis of net energy (NE), net protein (NP), and ruminally degraded protein (RDP). Total NE and NP requirements are calculated as the sum of NE and NP required for several functions (maintenance, activity, thermoregulation, gain, lactation, and gestation). To estimate the requirements for each function, I collected data predominantly for wild species and then formulated allometric and other equations that predict requirements across species. I estimated RDP requirements using an equation for cattle. I then related NE, NP, and RDP to quantities more practical for diet formulation (e.g. dry matter intake). I tabulated requirements over a range of body mass and life stages (neonate, juvenile, nonproductive adult, lactating adult, and gestating adult). Tabulated requirements suggest that adults at peak lactation require greatest quantities of energy and neonates generally require greatest quantities of protein, agreeing with suggestions that lactation is energetically expensive and protein is most limiting during growth. Equations used in this system were precise (allometric equations had R(2) generally ≥0.89 and coefficient of variation <31.1%) and expected to reliably predict requirements across species. Results showed that a system for beef cattle would overestimate NE and either over- or underestimate NP for gain when applied to wild ruminants, showing that systems for wild ruminants should not extrapolate from requirements for domestic ruminants. One prominent system for wild ruminants predicted at times vastly different protein requirements from those predicted by the proposed system. The proposed system should be further evaluated and expanded to include other nutrients.

Energy expenditure in European roe deer fawns during the suckling period and its relationship with maternal reproductive cost

We measured energy expenditure by means of indirect calorimetry in European roe deer (Capreolus capreolus) fawns at frequent intervals from birth to 45 days of age. Mean resting metabolic rate (RMR) and body mass (BM) did not differ significantly between males (N = 9) and females (N = 14). RMR significantly increased with the fawn age following the equation RMR (kJ·d-1) = 956 + 31.4 age (days). Growth in BM was linear: BM (kg) = 1.565 + 0.135 age (days). Mass-specific RMR decreased from a peak value of ca. 500 kJ·kg-1·d-1 in the first week to 290.4 ± 10.0 kJ·kg-1·d-1 in the sixth week. Concomitant metabolic trials performed with 14 lactating females and their 19 fawns indicated that metabolic weaning began in the third week, when the fawn's energy expenditure became markedly higher than could be compensated for by the mother's milk. Comparison of the energy expenditure of singletons and twins showed that female roe deer are energetically well adapted to nurse two fawns.

Growth and survival of juvenile yellow-bellied marmots (Marmota flaviventris)

We compared patterns of growth in juvenile yellow-bellied marmots (Marmota flaviventris) between 2 years in which precipitation differed, and we determined if mass at entry into hibernation affects over-winter survival. Juveniles exhibited an asymptotic growth pattern with mass gain for a mean of 60.8 days, followed by stable mass until entry into hibernation. Growth ceased in early September, shortly after the end of the plant growing season. Juveniles born in 1991, a year of late snowmelt and low summer precipitation, entered into hibernation at significantly lower mass than juveniles born in 1992. Juveniles born in 1992 survived better despite experiencing a long winter during which they spent more days in hibernation and lost more mass. Overall, heavier juveniles were more likely to survive hibernation. Further, mass at entry into hibernation apparently had a greater effect on over-winter survival than did winter severity.