Research Article: When more is not merrier: Using wild population dynamics to understand the effect of density on ex situ seahorse mating behaviors

Date Published: July 2, 2019

Publisher: Public Library of Science

Author(s): Heather D. Masonjones, Emily Rose, Ludek Bartos.


Seahorses are considered one of the most iconic examples of a monogamous species in the animal kingdom. This study investigates the relationship between stocking density and mating and competitive behavior from the context of the field biology of the dwarf seahorse, Hippocampus zosterae (Jordan & Gilbert). Animals were housed in 38 liter tanks at a range of densities and sex ratios (from 2–8 animals per tank), and their reproductive and other social behaviors were monitored from tank introduction through copulation. At low tank densities and even sex ratios but comparatively high field densities, frequency of both mating and competitive behaviors was low in trials. A higher level of males in tanks across all densities increased competition, activity levels, and aggression leading to egg transfer errors and brood expulsion, resulting in lower reproductive success. Across seahorse species, mean and maximum wild densities were consistently lower than those used in ex situ breeding, with adult sex ratios that were significantly female biased. However, significant variation exists in wild seahorse densities across species, with higher densities detected in focal/mark recapture studies and on artificial habitat structures than reported with belt transect sampling techniques. Interchange of knowledge gained in both aquarium and wild contexts will allow us to better understand the biology of this genus, and improve reproduction in captivity. Interpreting ex situ reproductive behaviors of seahorses within various densities reported from natural populations will help us predict the impact of conservation efforts and increase the likelihood of long-term persistence of populations for this threatened genus.

Partial Text

Maladaptive behaviors have been observed in captivity in animals brought in from the wild, often associated with holding and/or capture stress [1–3]. However, much of the research on behavior in captivity has focused on animals bred in aquaria, with the expectation that either genetic or epigenetic factors were involved in the development of behaviors that lead to lower survival or lower reproductive value in captivity [4,5]. In addition, research exists to suggest that there are behaviors observed in the wild and when placed in that context are adaptive, but become maladaptive in the captive context, especially given the constraints of environments like aquaria [6]. Partnering with researchers studying both the population biology and behavior of animals in the wild is critical to identifying the root of these behaviors and how to manage them ex situ.

To better understand the impact of population densities on mating behaviors and determine their effects on the reproductive output of seahorses, more natural populations will need to be surveyed. There is also need for studies to determine the causes behind the variation in densities across species worldwide and whether they are a result of variation in survey methods, interspecies variations, methodology for calculating seahorse density, the particular type of habitats that are being surveyed and when for each study, or from differences in the impacts of anthropogenic factors [70]. From a conservation perspective, it is important that when using seahorse densities for determining whether or not all species of seahorses should be included in the IUCN Red List, the sampling techniques used for each study must be considered. Ultimately, in this study we have demonstrated the importance of understanding how the behavioral changes across densities and sex ratios might be important for the success of these species and outline some of the challenges associated with placing ex situ studies in the context of natural population densities of seahorses.




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