Dawson Terence J, Norton Melinda A, Rodoreda Suzette, Abbott Sarah K, McLeod Steven R
School of Biological, Earth and Environmental Sciences University of New South Wales Sydney NSW Australia.
Fowlers Gap Arid Zone Research Station Fowlers Gap NSW Australia.
Ecol Evol. 2021 Jun 14;11(13):9062-9078. doi: 10.1002/ece3.7750. eCollection 2021 Jul.
Juvenile mammals in their postweaning developmental stages face many challenges in transitioning to adulthood. Among large grazing species such as ruminant bovids and cervids, an overarching challenge is acquiring and processing sufficient nutrients to survive and grow, with a gut that may not yet be fully developed. Marsupial kangaroos of Australia face similar challenges; they also digest vegetation by fermentation in a large foregut. In red kangaroos, (=), the dominant species of Australia's arid interior, females may breed continuously; however, juvenile recruitment to the adult population is irregular and coincident with sporadic rainfall.As compared with adult females, the nutritional requirements of juvenile are high in relation to their body mass (BM), largely due to the cost of their rapid growth. We examined processes that juveniles have in their morphology, physiology, and behaviors to meet their elevated nutritional needs, by comparing recently weaned juveniles of both sexes and adult female in their desert habitat. Features studied include relative body sizes, relative dimensions, and capacities of principal gut regions, the foregut, small intestine, caecum, and large intestine with rectum. Also examined were digesta attributes and rates of digesta excretion. Additionally, the rates of change in skull parameters and dental characteristics to maturity were assessed. Field determinations of diet choice were made for both age classes.In juveniles, the content masses of major gut structures were related to body mass (BM), as were those of adult females, that is, ~BM. In both age classes, the digesta mass of the foreguts exceeded 75% of the total digesta mass. Diets of both juvenile and adult largely focused on grasses. Juveniles had higher rates of digesta excretion while foraging than adults. In addition, the foregut contents in juveniles occupy proportionally less of the total gut than in adult females. Together, the higher excretion rate and smaller relative foregut of juveniles suggest that they necessarily focus on forage that can be rapidly digested, such as young, green grasses, or herbage.Comparison of the skulls of juveniles and adults revealed how this harvest can occur. Relative to BM, juveniles had skulls of larger volume than adults. Additionally, during growth the skull lengthens proportionally faster than increasing BM. By weaning, the dimensions of the incisor bite of juveniles neared those of adult females. The area of wear on premolars/molars increased only slowly relative to the development of incisors, further pointing to juveniles selecting more digestible forage than adults. The intermittent availability of such forage, principally young grasses, appears key to the significant recruitment into the population in their arid habitat.
处于断奶后发育阶段的幼年哺乳动物在向成年过渡过程中面临诸多挑战。在反刍类牛科动物和鹿科动物等大型食草物种中，一个首要挑战是获取并处理足够的营养以生存和生长，因为它们的肠道可能尚未完全发育。澳大利亚的有袋类袋鼠也面临类似挑战；它们同样通过在前肠进行发酵来消化植物。在红袋鼠（澳大利亚干旱内陆的优势物种）中，雌性可能会持续繁殖；然而，幼崽进入成年群体的过程并不规律，且与零星降雨同时发生。与成年雌性相比，幼年红袋鼠的营养需求相对于其体重而言较高，这主要是由于其快速生长所需的成本。我们通过比较沙漠栖息地中刚断奶的幼年红袋鼠和成年雌性红袋鼠，研究了幼年红袋鼠在形态、生理和行为方面为满足其更高营养需求所具备的过程。研究的特征包括相对身体大小、主要肠道区域（前肠、小肠、盲肠以及带有直肠的大肠）的相对尺寸和容量。还检查了消化物属性和消化物排泄速率。此外，评估了头骨参数和牙齿特征到成熟阶段的变化速率。对两个年龄段的饮食选择进行了实地测定。在幼年红袋鼠中，主要肠道结构的内容物质量与体重相关，成年雌性也是如此，即约为体重的比例关系。在两个年龄段中，前肠的消化物质量均超过总消化物质量的75%。幼年和成年红袋鼠的饮食主要以草类为主。幼年红袋鼠在觅食时的消化物排泄速率高于成年红袋鼠。此外，幼年红袋鼠前肠内容物在总肠道中所占比例比成年雌性小。总体而言，幼年红袋鼠较高的排泄速率和相对较小的前肠表明它们必然专注于可快速消化的草料，例如嫩绿草或草本植物。幼年和成年红袋鼠头骨的比较揭示了这种获取方式是如何实现的。相对于体重，幼年红袋鼠的头骨体积比成年红袋鼠大。此外，在生长过程中，头骨长度的增长比例比体重增加更快。到断奶时，幼年红袋鼠门齿咬合的尺寸接近成年雌性。相对于门齿的发育，前臼齿/臼齿的磨损面积增加缓慢，这进一步表明幼年红袋鼠比成年红袋鼠选择了更易消化的草料。这种草料（主要是嫩草）的间歇性可得性似乎是干旱栖息地中大量红袋鼠幼崽成功加入种群的关键。
