Predation is known to have a significant effect on life history diversification in a variety of species. However, physical constraints of body shape and size can sometimes limit life history divergence. We test this idea in the Costa Rican livebearing fish Alfaro cultratus. Individuals in this species have a narrow body and keeled ventral surface, and females do not develop a distended abdomen when pregnant like other livebearing fishes. Here, we describe the life history of A. cultratus from 20 different populations across both high-predation and low-predation environments. We found significantly lower reproductive allotment in females from high-predation environments than in females from low-predation environments, but no significant difference in female or male size at maturity, number of offspring produced by females, or size of offspring. We found that A. cultratus exhibit isometric patterns of allocation for clutch dry mass in relation to female dry mass in high-predation and low-predation environments. Our results suggest that body shape constraints in this species limit the life history divergence we typically see between populations from high-predation and low-predation environments in other species.

We collected these fish over 3 different years (2006, 2007, and 2019) from Costa Rica using a mesh seine. We euthanized the fish using MS-222 and then preserved them in 70% ethanol prior to dissection. We separated males and females and then we dissected the females on the left lateral side. We removed embryos and stomachs. We measured the length of the females (prior to dissection), staged the embryos according to the Haynes classification method (Haynes, J. L. (1995). Standardized Classification of Poeciliid Development for Life-History Studies. Copeia 1, 147–154.), and measured the dry mass of the females as well as the dry mass of the embryos for each female. We also calculated the average size for an embryo in each brood by dividing the dry mass of the embryos by the number of embryos in each brood. Information for an individual fish and the population it originates from is found in a single row. We looked at the length of males and females within the population to decide where the size at maturity was for each population. More details on methods can be found in our manuscript as well as in Johnson and Belk 2001 (Johnson, J. B., and Belk, M. C. (2001). Predation environment predicts divergent life-history phenotypes among populations of the livebearing fish Brachyrhaphis rhabdophora. Oecologia 126, 142–149. doi:10.1007/s004420000504.) We analyzed each life history trait in a linear model looking for the effects of predation. We incorporated covariates into the model. When analyzing number of offspring, we included female dry mass as a covariate. When analyzing offspring size and reproductive allotment, we used female dry mass and developmental stage of embryos as covariates. We did not include any covariates for male or female size at maturity. Brood dry mass was our measure of reproductive allotment. In addition, we log transformed reproductive allotment and number of offspring in the analysis to satisfy assumptions of the linear model.

Here is a key to the column names: 

FieldID: This is a number used that shows the year and the site collected at by our lab group for that year. 19-15 would mean that the location was the 15th collected from in 2019. 

Collection: This is a shorthand for fieldID that is used to differentiate the sites in the analysis. The numbers do not necessarily mean anything but they are grouped according to locality. 

BYUID: the ID given to each individual fish that it is tagged with in the Monte L Bean Museum. 

Length: Length in mm from the tip of the nose to the attachment of the caudal fin.

Maturity: 1 means mature or an individual with developing embryos. 0 is immature. 

DevStage: Development stage of embryos. This scale goes from 1-11 and is explained in the Haynes article cited above. 

NumOff: This is the number of offspring or count of developing embryos contained in a given female. 

FemaleDryMass: This is the dry mass of the females with embryos and stomachs removed and 24 hours in a dessication oven. This is measured in grams. 

broodDryMass: This is the dry mass of the brood of a female after 24 hours in a dessication oven. This is measured in grams. 

Year: The year that the collections were made. 

Locality: The name of the river or tributary where the collections were made. 

Latitude

Longitude

Predation: 1 means predators were found while seining and 0 means they were not. 

SizeMaturity: This was the size maturity of females in the population. This was decided as the mm length where over half of the females were mature in that size group. 

N.Females: This is the number of mature females out of the number of collected females.

N.Males: This is the number of collected males. 

M.Mean.Size: This is the average length of the collected males which we used as the male size at maturity. 

size: This is the brood dry mass divided by the number of embryos or the average size of embryos for each brood.