Mammals are predicted to vary in body size following Bergmann’s rule, with individuals found at higher latitudes in colder temperatures being larger in size compared to conspecifics occurring at lower latitudes in warmer temperatures. Body size is similarly expected to vary temporally, with a decrease in size through time due to recent climate warming. While Bergmann’s rule is well-supported in mammals, there is increasing evidence of exceptions to the rule. Here, we present patterns of size variation in 17 North American mammal species using five morphological traits (condylobasal skull length, skull width, maxillary toothrow length, body weight, and head-and-body length) to determine if size varies predictably for each species in space and time. We found little support for a widespread Bergmannian pattern for these species at a broad spatial scale (across North America) and a contemporary temporal scale (the past 120 years). The effects of latitude or year on each trait were highly variable with three types of responses: an increase, a decrease, or no change in size across space or through time. Spatial size trends were detected more often than temporal size trends, as the temperature range was significantly larger in space than through time. Body weight (the most variable trait) and head-and-body length were more likely to conform to Bergmann’s rule than craniodental measurements. We did not detect any changes in size variability with latitude, and our study species either increased or decreased in size variability over time. Our findings demonstrate that size variation in mammals is highly context-dependent. As such, caution is needed when using rules of body size variation to predict the future response of species to climate warning while valid in theory, it is likely too simplistic of an approach.

中文翻译：

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洛基法则：哺乳动物空间和时间大小变化的特质

据预测，哺乳动物的体型大小会根据伯格曼法则而变化，与生活在较低纬度、较高温度下的同种动物相比，生活在较高纬度、较冷温度下的个体体型更大。同样，预计身体尺寸也会随时间变化，由于最近的气候变暖，身体尺寸会随着时间的推移而减小。虽然伯格曼的规则在哺乳动物中得到了充分的支持，但越来越多的证据表明该规则有例外。在这里，我们使用五种形态特征（髁突颅骨长度、颅骨宽度、上颌牙距长度、体重以及头身长度）展示了 17 种北美哺乳动物物种的体型变化模式，以确定每个物种的体型变化是否可预测在空间和时间上。我们发现，在广阔的空间尺度（整个北美）和当代的时间尺度（过去 120 年）内，这些物种存在广泛的伯格曼模式，但几乎没有证据支持这一点。纬度或年份对每个性状的影响差异很大，有三种类型的反应：随空间或随时间的大小增加、减少或没有变化。空间尺寸趋势比时间尺寸趋势更容易被检测到，因为空间上的温度范围明显大于时间上的温度范围。体重（变化最大的特征）和头身长度比颅牙测量更可能符合伯格曼规则。我们没有检测到尺寸变异性随纬度的变化，并且我们的研究物种的尺寸变异性随着时间的推移而增加或减少。我们的研究结果表明，哺乳动物的体型变化高度依赖于环境。因此，在使用体型变化规则来预测物种对气候警报的未来反应时需要谨慎，虽然理论上是有效的，但这种方法可能过于简单化。