I have been looking at weight values for rodents in the family muridae, specifically subfamilies: gerbillinae and deomyinae. I found some considerable discrepancies in the values for the same species from different references. Generally, I get similar values from sources concerned with African mammals (Mammals of Africa, Kingdon et al, 2013; Mammals of Sub-Saharan Africa, Monadjem et al, 2015; The Complete Book of the Southern African Mammals, Mills and Hes, 1997; The Contemporary Land Mammals of Egypt, Osborn and Helmy, 1982). The values I get from other sources, namely PanTheria, AnAge and Alhajeri et al (2015) are mostly similar amongst themselves but can be very different from those reported in the first (“African”) set of references.
The similarity within set cannot be solely explained as repeated citations from the same old reference; so I was wondering if it can be explained by biogeographic trends within widely distributed species. In other words, the set of references concerned with Africa is reporting species values from African populations only; while the other references report values from the world-wide distribution of the species. The observation that species with African and extra-African populations have wider ranges of values reported in PanTheria, AnAge and al-Hajeri compared to those in “African” sources for the species is consistent with this hypothesis. Furthermore, whenever a species is endemic to Africa, the two sets of references seem to largely agree.
Could somebody please corroborate/debunk this idea of mine, or suggest other explanations for these puzzling discrepancies?
Hi Haya Abdullah,
I work with small mammals in Brazil and we find references considering the mass of all animals captured or only of adult mass. I suggest to check that the authors are considering only adult weight mass and excluding reproductive females. These differences can bring a lot of variation. If not, you have an interesting task to follow. Best regards
Hi Haya, the variation could be considered as normal if the data are into the ranges. The variation could be seasonal, day time, sex, age, or any other sources like the way to take the measures. Some time the female is pregnant and it is not detected, or the animal was weighted close to feeding hours or not. many reason could be considered for body mass variation. The ranges variation is important and if your variation is into the normal ranges is ok. When the animal are rare or vary rare like some Caribbean mammals with only one or few specimens in collection and without morpholofgical data any effort to calculate the estimate body mass is a good contribution. Perhaps it are some comments that could be help your thinking.
I found the same for the weight of several mammal species where I compared averages between European sources (Handbuch der Saugetiere Europas) and PanTheria. So I guess it is indeed the difference between a global average and a 'local' average. Seems like a nice thing to write a paper about, as using global averages for local populations might introduce substantial error.
Hi Haya, quite often the grey literature may combine body mass data from both females and males, depending on the data sources they use. They often do not report the sample sizes and SD for these values. In PanTheria the values may be more similar as the authors have followed a constant procedure with the data handling procedures. Over all, grey literature values may not be suitable for population level comparisons but they are a good source for general studies. I prefer values from original studies, but it is hard work to find those. And as you can see from the previous
answers, there still are many sources for the value differences.
Dear Helena, Rafael, Tim and Kari, thank you all for your insightful answers. After taking another look on literature, I think it’s not likely that the discrepancies I am finding are related to differences in age since most references indicate that the measurements they provide are of adults. Also, except in the few cases where sexual dimorphism is notable, these values are indicated as combined averages for males and females. However, as mentioned by Kari, in almost all cases no mention is given in the grey literature to the inclusion or exclusion of pregnant females, or if seasonal changes are taken into account. All in all, I think the hypothesis that the discrepancies are a reflection of global vs local averages remains the most plausible.
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