Flexibility in the social behavior of captive female capybaras (Mammalia, Rodentia)

Highlights

• Captive female capybaras exhibit a linear social dominance hierarchy.

• As space per individual decrease there is more predictability in the dominance success.

• Capybara shows flexibility in social behavior.

Abstract

Capybara (Hydrochoerus hydrochaeris) lives in stable groups composed of adult males and females with their young. The species shows flexibility in social organization in response to short-term environmental changes, but apparently does not show flexibility in social behavior. To gain insights into mechanisms underlying changes in social relationships, we analyzed the social dominance hierarchy of five captive capybara groups, composed of four to 13 adult females kept in outdoor paddocks ranging from 400 to 4500 m². In addition, we evaluated the effects of group size and space allowance on two complementary properties of social structure: linearity and steepness. Captive female capybaras exhibit a linear social dominance hierarchy. There was also more predictability in the dominance success– hierarchical steepness − in the dominance hierarchy with a decrease in the space per individual. This variability in response to changing circumstances shows flexibility in capybara’s social behavior.

Introduction

The social organization (the composition of groups), the mating system, and the social structure (describing who interacts with whom) concurrently characterize the social system of a species (Kappeler and van Schaik, 2002). The basic unit of capybara (Hydrochoerus hydrochaeris) social organization is the family unit, which consists of a dominant male, several possibly related adult females with their young, and one or more subordinate males (Herrera, 2013). The most common mating system of capybaras is polygynous in which one male is responsible for most of the copulation after pre-copulation competitions (Herrera and Macdonald, 1993). A strictly linear dominance hierarchy characterizes the social structure of capybara males (Herrera and Macdonald, 1993). However, the free-range capybara females do not appear to have a particular social structure among them (Herrera, 2013). In captivity, Ferraz et al. (2013) observed a monopolization of feeding resources by only one female that won conflicts against all other females − characteristic of a despotic dominance hierarchy.

Capybara groups appear to be relatively stable and range from four to 16 adults in the seasonally flooded savannahs of Venezuela (Herrera and Macdonald, 1987). Moreover, the species shows flexibility in social organization −reversible changes in the social organization at a population level in response to short-term changes of the environment (Schradin, 2013). During dry seasons, several groups come together to the dwindling pools, which results in associations of more than 100 capybaras (Ojasti, 1973). In the following rainy season, however, they divide up into their original groups (Herrera and Macdonald, 1987). However, apparently capybara does not show flexibility in social behavior. Flexible social behavior refers to individual variability in response to changing circumstances (Barrett et al., 2013). The increase in capybara group size did not result in an increase in cooperative behavior; on the contrary. (1985) studied the effect of the size of breeding groups (5, 10 or 15 adults) and the space allowance (enclosures with 32 m² and 120 m²) upon the social dominance hierarchy, agonistic interactions rate, and reproduction. The authors recorded increases in rates of aggression and mortality, as well as a decrease in reproduction rate with the group size. However, there was no relationship between these parameter and space allowance (Ojasti and Sosa Burgos, 1985). Furthermore, Ojasti and Sosa Burgos (1985) verified that low-ranking individuals were the ones that lose weight and died during the study.

The lack of social dominance among free-ranging capybara females (Herrera, 2013) is probably explained because there is no need to compete for grass, the capybara’s main food resource (Ojasti, 1973, Macdonald, 1981), which is dispersed in its range. In contrast, farmed capybaras usually compete for food due to the spatial concentration of this resource. In captivity it is possible to manipulate ecological factors in ways to gain insights into mechanisms underlying changes in social relationships (Calhoun, 1950). Therefore, we aimed to describe the social dominance hierarchy of captive female capybaras and to evaluate the relationship between rank and body mass. We also intended to evaluate the effects of group size and space allowance on two complementary properties of social structure: linearity and steepness. The linearity is the main structural characteristic of a dominance hierarchy that ranges from zero (non-linear hierarchy) to 1.0 (perfect linear hierarchy) (Whitehead, 2009). The hierarchical steepness, a core aspect of social structure, is a continuous measure of dominance gradients measuring predictability of the dominance success (deVries et al., 2006). Adopting a different arrangement of the feeders, to avoid monopolization of feeding resources as occurred in Ferraz et al. (2013), we hypothesized that capybara females organize themselves in linear hierarchy dominance. Following previous results showed by Ojasti and Sosa Burgos (1985), we expected a correlation between rank and body mass independently of the group size and space allowance. We also expected an increase in agonistic rate with the increase of group size independently of the space allowance, as described by Ojasti and Sosa Burgos (1985). Moreover, we predicted decreases in both the hierarchical linearity and steepness along with the increase in group size and decreases in space allowance due to the increase in competition.