Individual variation in growth rates often generates variation in fitness. However, the ability to draw meaningful inferences from growth data depends on the use of growth models that allow for direct comparisons of growth between the sexes, between populations, and between species. Unlike traditional sigmoid functions, a recently parameterized family of unified growth models provides a reliable basis for comparisons since each parameter affects a single curve characteristic and parameters are directly comparable across the unified family. Here, we use the unified-models approach to examine the development of sexual size dimorphism in Damaraland mole-rats (Fukomys damarensis), where breeding males are larger than breeding females. Using skeletal measurements, we show here that the larger size of male Damaraland mole-rats arises from an increased growth rate across the entire period of development, rather than through sex differences in the duration or timing of growth. Male-biased skeletal size dimorphism is not unusual among rodents, and our measures of sex differences in size in captive mole-rats are close to sexual size differences in the wild, where size dimorphism = 1.04 (male:female). We hope our study will encourage the wide use of unified growth models by mammalogists.
Supplementary Data SD1.— Methods for the literature search of the use of different sigmoid growth formulations in the mammal literature.
Supplementary Data SD2.— Instantaneous growth rate equations for unified models of sigmoid growth.
Supplementary Data SD3.— Instantaneous growth rates (growth velocity) for the unified growth models applied to female incisor width (a), male incisor width (b), female body length (c), and male body length (d). Instantaneous growth rates represent the first derivative of the best-fitting non-linear mixed effects model in each case. Points on each curve highlight the estimated point of maximum growth in either model. All models were fitted to data on known-age, captive individuals.
Supplementary Data SD4.— The best-fitting random effects structure for each form of unified growth model fitted to skeletal traits.
Supplementary Data SD5.— Estimates for random effects, temporal autocorrelation, and power of variance covariate in models of skeletal growth on captive Damaraland mole-rats.
Supplementary Data SD6.— Parameter estimates and model comparison for ‘interval equation’ models of growth in wild Damaraland mole-rats.
Supplementary Data SD7.— Example R code to fit unified models to mole-rat skeletal data.