Mammae numbers and litter sizes in an arboreal rodent fit the one-half rule

Abstract

Lactation is a defining mammal trait. Mammae numbers vary across orders, from 2 in humans to 29 in tenrecs (Hayssen et al., 1992). Why is there such a great variation of mammae counts across different taxa? This question attracted the attention of many scholars since Aristotle, who stated, “The animals that have large litters have their mammae upon the abdomen. Why is this? They have numerous young to feed, and so they need numerous mammae” (Aristotle, 1902).

Comparative studies have shown that mammae numbers coevolve with litter size. In general, litter size positively covaries with mammae count, that is, species with large litters nourish their sucklings from more nipples (Pearl, 1913a). Gilbert (1986) coined this relationship as a “one-half rule”: the average litter size in a given species is usually one-half of the available mammae. Thus, there are typically two nipples or teats per one pup. Across taxa, the mammae number explains more of the variation in litter size than any other species-level traits like body mass, diet, gestation length, or geography (Stewart et al., 2020). This indicates that mammae numbers might represent a constraint on fecundity across mammals.

Surprisingly, almost no attempts have been made to assess whether the one-half rule also applies within a species. Earlier researchers even directly excluded such possibility, claiming, “There seems no reason whatever to suppose that natural selection would tend to produce a correlation between the number of mammae in the mother and the size of her litters within a race” (Harris, 1916). More recently, Diamond (1987) considered it a “constant species characteristic,” and Gilbert (1986) claimed that “with few exceptions, mammary number in rodents is a species-typical invariant trait.” Even if some species were found with variable mammae counts, Gilbert excluded those species from his seminal study. Traditionally, only studies on domesticated animals paid attention to the issue of intraspecific variability in mammae numbers and litter size, with often inconclusive results (Bell, 1912; Kim et al., 2005; Korhonen, 1992; Parker & Bullard, 1913; Pearl, 1913b). Only one study on a wild mammal, the Naked-mole Rat (Heterocephalus glaber), assessed whether the one-half rule holds within a species but did not find support for it (Sherman et al., 1999).

The Edible Dormouse (Glis glis, Gliridae) is a small, nocturnal, arboreal rodent that inhabits deciduous woodlands across Europe. Earlier studies found variable numbers (8 to 14) of mammae within and between populations (Kryštufek, 2004, 2010; Naderi et al., 2014; Figure 1a). It is a typical tree cavity-dwelling species that readily accepts nest boxes as surrogates of natural tree hollows (Figure 1b). This acceptance of nest boxes enabled us to conduct a detailed longitudinal study of individually marked females over their lifespans. Between 2013 and 2015, we followed in detail every gestating female at our long-term field site near Dlouhá Loučka (49°49′ N, 17°12′ E) in the NE Czech Republic (Adamík & Král, 2023). Each dormouse was individually marked with a passive integrated transponder (PIT) tag, and we measured body mass and tibia length and counted the number of active mammae and litter size (Kukalová et al., 2013). Reproduction in the Edible Dormouse is tightly linked with the masting pattern in trees like European beech (Lebl et al., 2011; Pilastro et al., 2003). For this reason, we assessed the proportion of available masting beech trees around each breeding nest box. We considered only those litters when we checked the females right after the parturition (n = 145 litters, Holcová Gazárková & Adamík, 2025 data on Zenodo). We excluded 4 litters with birth dates after September 1 as they likely indicate second or replacement litters, 5 litters when the pups were older than 14 days, and the females brought them into the nest box from another cavity, and 18 litters with communal nests (for details on daily routines of fieldwork see Holcová Gazárková & Adamík, 2016). In the following years (2017–2022), we performed a less intensive field survey where we recorded nipple counts only in those lactating females that had been recorded before, and we could track their breeding performance over their lifespan (n = 112 litters). This second dataset assessed whether nipple count is a fixed character over the female's lifespan (Holcová Gazárková & Adamík, 2025 data on Zenodo). By mammae number in this study we mean mammae with active milk production. Anatomical mammae counts would require sacrificing the study animals.

The number of active (i.e., lactating) mammae ranged from 2 to 12 (mean ± SD: 10.19 ± 1.32, n = 145, Figure 2), and the litter size from one to 11 pups (6.81 ± 1.58, n = 145, Figure 2). Mammae number positively and significantly predicted litter size; linear regression: intercept ± SE 1.63 ± 0.93, b = 0.51 ± 0.09, t₍₁₄₃₎ = 5.6, p < 0.001, R² = 0.18. This relationship was consistent even after controlling for the positive but nonsignificant effects of female body size and availability of masting trees within the territory (Appendix S1: Figures S1). In addition, we evaluated whether females had the same number of active mammae over their lifespans. We found moderate repeatability of this trait (intra-class correlation 0.29, 95% CI: 0.07–0.51). At the population level, we did not observe a change in mammae number between the first and subsequent breeding attempts (Appendix S1: Figure S3, mean difference in mammae number between first and second litters = −0.087, n = 46 females). Overall, in 27 cases, the females had the same mammae number between the first and second breedings; in six cases, we observed an increase by one; in two cases, an increase by two; in eight cases, it decreased by one; and three times, a decrease by two nipples (Figure 3).

Our data show an interesting fit to the one-half rule that was initially coined for interspecific comparisons (Gilbert, 1986; Stewart et al., 2020). Litter size in dormouse females increases by one pup per additional pair of nipples. Comparative studies showed that the mammae/litter size relationships are universally positive, but the steepness of the slopes varies across mammal clades. For example, marsupials and carnivores have steeper slopes than Eulipotyphla (Stewart et al., 2020). Even within rodent families, the steepness of the slopes differs, with the highest being among terrestrial squirrels (Gilbert, 1986). As we mentioned earlier, the numbers of mammae are frequently considered an invariant species trait. However, several studies reported mammae count variability between populations in a range of species (e.g., Cockburn et al., 1983; Kryštufek, 2004; Ward, 1998). At the intraspecific level, only one field study evaluated the one-half rule. Sherman et al. (1999) did not find support for the rule in both captive and wild Naked-mole Rats. However, this species is at its extreme, having unusually high mammae numbers relative to the litter size. In contrast, this issue attracted considerable attention in animal husbandry, but with mixed results (e.g., Bell, 1912; Kim et al., 2005; Korhonen, 1992; Parker & Bullard, 1913; Pearl, 1913b). For economic reasons, many domesticated lines of mammals have been selected for high offspring productivity, where mammae numbers equal litter size. Therefore, in captive breeds that underwent selection for productivity, we might not observe the relationship of mammae number to litter size. However, captive breeds have the potential for exploring the genetic mechanisms of this rule.

We also show a remarkable variability in the number of active nipples within females' lifespans. Almost one-half of the females had different numbers of active nipples in subsequent breeding events. Surprisingly, this is an unexplored area. For example, in the Naked-mole Rat, only 6% of females changed mammary numbers over an individual's lifetime (Sherman et al., 1999). We suggest that more attention be paid to the variability within species in mammae numbers. Possibly, in some species, this trait might have been overlooked, and instead of having a constant mammae count, the females flexibly activate mammae relative to their actual litter size. The meaning of the term mammae numbers needs to be clarified in each study. The number of mammae per species can be counted anatomically (usually on dead animals), ignoring the lactation status, or it can mean the number of nipples/teats used for active milk production (as in this study). We suggest the second is biologically meaningful because it shows the caring capacity of the female relative to her litter size. In addition, there is a solid piece of evidence that in some species the pups show high fidelity to particular mammae (Skok, 2018). Species with mammae clinging have more pups per mamma than the non-clinging ones (Gilbert, 1995).

The authors declare no conflicts of interest.

Permits to handle the dormice were issued by the Regional Authority of the Olomouc Region (KÚOK 31204/2011, KUOK 61548/2017).