Nuchal humps are morphological traits that occur among vertebrate species and have multiple functions. The emergence of nuchal humps in Xiphophorus multilineatus males maintained in the laboratory, a species that does not develop humps in the wild, provided a unique opportunity to explore the development and function of this trait. The current study investigated the tissue composition of the hump and analyzed the influence of rearing temperature and diet restriction on hump development. Through histological examination and gene expression profiling, our findings show that the hump is composed primarily of adipose tissue, which suggests a role in energy storage via fat deposition. Nuchal humps grew to a larger size in the cold environment (20 °C) than in the warm (25 °C), and the differential gene expression pattern between temperature treatments suggests increased fat deposition in the cold versus the warm environment. For example, the G0S2 gene, which inhibits lipid catabolism, is upregulated in the cold environment, and the WARS-1 gene, which leads to increased fat stores when under-expressed, is upregulated in the warm environment. The hypothesis that temperature influences hump development by stimulating shifts in fat metabolism is further supported by the finding that males from the warm environment used fat stores in the nuchal hump at a faster rate when placed on a restricted diet than males from the cold. The influence of temperature, diet, and potentially activity levels on fat deposition suggests X. multilineatus as an evolutionary animal model for gaining insights into the mechanisms involved in human obesity.