The burden of size and growth for the juveniles of large mammalian herbivores: structural and functional constraints in the feeding biology of juveniles relative to adults in red kangaroos, Osphranter rufus

Juvenile mammals in their post weaning 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, Osphranter rufus (= Macropus rufus), 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 O. rufus 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 behaviours to meet their elevated nutritional needs, by
comparing recently weaned juveniles of both sexes and adult female O.
rufus 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, i. e. ~BM1.0. In both age classes the digesta mass of the
foreguts exceeded 75 % of the total digesta mass. Diets of both juvenile
and adult O. rufus largely focused on grasses. Juveniles had higher rates
of digesta excretion while foraging than adults. In addition, the foregut
contents in juveniles occupies proportionally less of the total gut than
in adult females. Together, the higher excretion rate and smaller relative
foregut of juveniles suggests 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. 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 O.
rufus population in their arid habitat.

处于断奶后发育阶段的幼年哺乳动物，在向成年过渡的过程中面临诸多挑战。在反刍牛科、鹿科等大型植食性物种中，核心挑战在于获取并处理足够的营养以维持生存与生长，而它们的肠道可能尚未完全发育成熟。澳大利亚的有袋类袋鼠面临类似困境：它们同样依靠大型前胃发酵来消化植物性食物。赤袋鼠（Osphranter rufus，曾用名Macropus rufus）是澳大利亚干旱内陆的优势物种，其雌性个体可持续繁殖，但幼崽成功存活并加入种群的比例并不稳定，且与偶发的降雨事件同步。相较于成年雌性赤袋鼠，幼年个体按体重（body mass, BM）计算的营养需求更高，这主要源于其快速生长带来的营养消耗成本。本研究通过在赤袋鼠的沙漠栖息地中，对比近期断奶的两性幼年个体与成年雌性个体，探究了幼年个体在形态、生理与行为层面适应更高营养需求的策略。本次研究观测的特征包括：相对体尺、主要肠道区域（前胃、小肠、盲肠以及带直肠的大肠）的相对尺寸与容积；同时测定了食糜特性以及食糜排泄速率。此外，还评估了头骨参数与牙齿特征向成年阶段发育的变化速率。研究还针对两个年龄组的赤袋鼠开展了野外食性选择测定。结果显示，幼年个体的主要肠道结构内容物质量与体重（BM）呈约BM^1.0的线性相关，成年雌性个体亦是如此。两个年龄组的前胃食糜质量均占总食糜质量的75%以上。幼年与成年赤袋鼠的食性均以草本植物为主。幼年个体在觅食期间的食糜排泄速率高于成年个体。此外，相较于成年雌性，幼年个体的前胃内容物占总肠道容积的比例更低。综合来看，更高的食糜排泄速率与相对更小的前胃，表明幼年个体必然会选择易消化的饲草，例如鲜嫩的青绿草本植物。对幼年与成年赤袋鼠头骨的对比分析，揭示了它们如何实现这种饲草获取策略：按体重归一化后，幼年个体的头骨容积大于成年个体；此外，在生长过程中，头骨长度的相对增长速率快于整体体型的增长速率。至断奶阶段时，幼年个体的门齿咬合尺寸已接近成年雌性。相较于门齿的发育，幼年个体前臼齿/臼齿的磨损面积增长缓慢，这进一步表明幼年个体相较于成年个体，会选择更易消化的饲草。这类饲草（主要是鲜嫩草本）的间歇性可获得性，似乎是干旱栖息地中赤袋鼠种群成功实现幼崽补充的关键因素。