1. Body size is a critical component of organismal biology. Body size is known to be influenced by a plethora of environmental conditions, among which exposure to large-scale variations of salinity has been comparatively overlooked. Yet, exposure to salinity is known to affect energetic allocation toward growth and reproduction.
2. In this study, we investigated the morphological differences between inland and coastal individuals of spined toads (Bufo spinosus) in Western France. 
3. We measured adult morphology both outside and during the reproductive season on 190 individuals, and assessed reproduction in pairs originating from inland (N=20) and coastal (N=30) environments.
4. Overall, we found that adult coastal toads were smaller and lighter than inland individuals. Reproductive correlates of these differences included lower fecundity and smaller egg size (but higher egg density) in coastal females. Interestingly, these differences were not allometric correlates of body size, as coastal females invested proportionally less in all components of reproduction (fecundity, egg size and egg protection).
5. These results suggest altered resource allocation to growth and reproduction in coastal amphibians, which may be related to the marked spatial gradient of salinity (measured in reproductive ponds) and the associated costs of osmoregulation (higher osmolality in coastal individuals), for which local adaptation and higher tolerance to salinity remains to be tested.

The dataset is related to the article “The costs of living on the coast: reduction in body size and size-specific reproductive output in coastal populations of a widespread amphibian”. This article was meant to compare coastal and inland individuals of the species Bufo spinosus. These individuals were captured either during reproduction (50 amplectant pairs [amplexus], each constituted of one male [M] and one female [F], captured in ponds, from 16/02/2022 to 02/03/2022), or outside reproduction (90 individuals captured on roads, from 08/09/2022 to 15/10/2022). We provided two datasets, each one corresponding to one period of capture. These datasets allow us to determine differences in investment during reproduction, but also in morphology, between coastal and inland populations. Precise locations can be given on demand but were not included in the dataset as some captures were performed in natural reserves. Site (coastal or inland) is indicated, as well as date of capture. Individuals or amplectant pairs are identified by a letter corresponding to the site of capture (C for coastal and M for inland) as well as a number corresponding to their order of capture. 

During reproduction, for each amplexus (constituted of one male and one female), we assessed time to laying (time between capture and laying), egg strings’ length (mm), eggs’ diameter (mm), eggs’ density (number of eggs/mm) and number of eggs (all determined using the ImageJ software), size of both males and females (SVL: Snout-Vent-Length, given in mm), and calculated the size difference between a male and female in the same amplexus (both absolute and given in percentage), the initial (at capture) and final (after egg laying) of males and females, and the difference in mass between capture and egg laying (both absolute and given in percentage). 

Outside reproduction, we captured 45 males (M) and 45 females (F) independently (these individuals were opportunistically captured on roads in either coastal or inland locations), and computed their size (SVL: Snout-Vent-Length, given in mm) and mass (given in g).