In altricial, litter-bearing species, huddling together with siblings during early life is a vital strategy to maintain a sufficiently high and stable body temperature. In this context, individual differences in huddling behavior within litters have been emphasized, as pups regularly occupying more central positions have relatively higher body temperatures, have quicker access to the mother’s nipples during nursing, and consequently show greater growth. However, it is not known whether such positive effects of a central litter huddle position on within-litter differences in growth translate into an overall higher weaning mass, taking into account strong contributors to among-litter growth variation, such as litter size and maternal parity. We used path analysis to investigate causal relations among these variables, based on data from 150 domestic rabbit pups from 24 litters. Our results confirmed positive, indirect effects of pups’ central litter huddle position on within-litter differences in early growth. This positive effect of a central litter huddle position also contributed to explaining a significant part of the overall across-litter variance in weaning body mass, apparent even when controlling for the direct negative effect of litter size, the direct positive effects of birth mass, and the lower offspring growth in primiparous compared to multiparous mothers. Thus, the results underline the key role of individual differences in litter huddle position in shaping within-litter but also overall variation in early growth. This might constitute an important mechanism accounting for how the positive association between body mass at birth and early growth is mediated in altricial, polytocous mammals. Huddling together with siblings during early life saves energy and thus can contribute to early growth in small, altricial mammals. However, this strategy can also lead to individual differences within the litter, as heavier pups typically occupy energetically more favorable positions in the center of the huddle. In our study, we show and compare the different causal pathways underlying this effect. Most importantly, our analysis shows that advantages in early growth arising from a more central position in the litter huddle are also apparent when comparing pups across all litters—even though there is typically a notable variation in growth among different litters, for example due to litter size and maternal (parity) effects. In conclusion, the results underline the key role of sibling interactions within the litter in shaping differences in early growth, with potential fitness consequences during later life.

中文翻译：

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窝窝位置的兄弟姐妹差异导致了小型哺乳动物断奶质量的整体变化

在晚生、产仔的物种中，在生命早期与兄弟姐妹挤在一起是保持足够高和稳定体温的重要策略。在这种情况下，强调了窝内蜷缩行为的个体差异，因为幼崽经常占据更多的中心位置，体温相对较高，在哺乳期间可以更快地接触到母亲的乳头，因此表现出更大的成长。然而，考虑到窝间生长变化的重要因素，如窝产仔数和母体胎次，尚不清楚中央窝窝位置对窝内生长差异的这种积极影响是否会转化为整体更高的断奶质量. 我们使用路径分析来调查这些变量之间的因果关系，基于来自 24 窝 150 只家兔幼崽的数据。我们的结果证实了幼崽的中央窝窝位置对早期生长中窝内差异的积极间接影响。中央窝窝位置的这种积极影响也有助于解释断奶体重总体跨窝差异的很大一部分，即使在控制窝产仔数的直接负面影响、出生体重的直接积极影响和与经产的母亲相比，初产的后代生长速度较低。因此，结果强调了窝窝位置的个体差异在塑造窝内以及早期生长的整体变化中的关键作用。这可能构成了一个重要的机制，可以解释出生时体重与早期生长之间的正相关是如何在多产哺乳动物中介导的。在生命早期与兄弟姐妹挤在一起可以节省能量，因此有助于小型、高龄哺乳动物的早期生长。然而，这种策略也可能导致窝内的个体差异，因为较重的幼崽通常在拥挤的中心占据更有利的位置。在我们的研究中，我们展示并比较了这种影响背后的不同因果途径。最重要的是，我们的分析表明，在比较所有窝的幼崽时，由于窝窝中更中心的位置而产生的早期生长优势也很明显——尽管不同窝的生长通常存在显着差异，例如，由于窝产仔数和产妇（胎次）效应。总之，结果强调了垃圾中兄弟姐妹互动在塑造早期生长差异方面的关键作用，在以后的生活中具有潜在的健康后果。