RESUMEN
OBJECTIVES: Ecological similarity between species can lead to interspecific trophic competition. However, when ecologically similar species coexist, they may differ in foraging strategies and habitat use, which can lead to niche partitioning. As the body tissues of consumers contain a stable isotope signature that reflects the isotopic composition of their diet, stable isotope analysis is a useful tool to study feeding behavior. We measured the isotopic niche width, which is a proxy for trophic niche width, of mantled (Alouatta palliata) and black (A. pigra) howler monkeys. Specifically, studied populations in allopatry and sympatry to assess whether these species showed niche partitioning. MATERIALS AND METHODS: Between 2008 and 2012, we collected hair samples from 200 subjects (113 black and 87 mantled howler monkeys) and used continuous flow isotope ratio mass spectrometry to estimate δ13 C and δ15 N. We described the isotopic niche width of each species in allopatry and sympatry with the Bayesian estimation of the standard ellipse areas. RESULTS: In allopatry, isotopic niche width and isotopic variation were similar in both species. In sympatry, black howler monkeys had a significantly broader isotopic niche, which was mainly determined by high δ15 N values, and included the majority of mantled howler monkeys' isotopic niche. The isotopic niche of mantled howler monkeys did not differ between sympatry and allopatry. CONCLUSIONS: The coexistence of these ecologically similar species may be linked to trophic niche adjustments by one species, although the particular features of such adjustments (e.g., dietary, spatial, or sensory partitioning) remain to be addressed.
Asunto(s)
Alouatta/fisiología , Ecosistema , Conducta Alimentaria/fisiología , Simpatría/fisiología , Animales , Isótopos de Carbono/análisis , Dieta , Femenino , Guatemala , Cabello/química , Masculino , México , Isótopos de Nitrógeno/análisisRESUMEN
The presentation of food may affect feeding competition and the well-being of captive social species. We hypothesized that feeding competition in a captive group of 5 black-handed spider monkeys (Ateles geoffroyi) should increase in response to certain food presentations in terms of size, distribution and quality of food, and that higher feeding competition should lead to an increase in agonism and physiological stress (measured by faecal glucocorticoid metabolites, FGCM) as well as to a decrease in affiliation, proximity among individuals and feeding activity. We used 5 experimental treatments representing different combinations of size, distribution and quality of food. We observed social interactions for 100 h, collected 6,500 proximity and feeding activity records, and gathered 226 faecal samples. When food was clumped, individuals spent less time feeding, and there was also significant individual variation in feeding activity within treatments. FGCM levels were higher when food was clumped. These results are probably linked to an increase in feeding competition when food is concentrated. At least in small groups of spider monkeys, dispersing food in two feeding stations may be sufficient to decrease differences among individuals in priority of access to food resources, hence reducing physiological stress and interindividual differences in feeding activity.
Asunto(s)
Atelinae/fisiología , Conducta Alimentaria , Glucocorticoides/metabolismo , Conducta Social , Alimentación Animal , Animales , Atelinae/psicología , Heces/química , Femenino , Masculino , Estrés FisiológicoRESUMEN
The measurement of hormones in fecal samples allows for the noninvasive assessment of the endocrine status of free-ranging primates. However, procedures and techniques for hormone analysis in feces must be validated, both analytically and physiologically. Few studies have addressed the endocrinology of black howler monkeys (Alouatta pigra). Due to its conservation status, direct handling of individuals from this species and invasive sample collection are highly regulated, and therefore traditional methods for the validation of hormone assays, such as pharmacological challenges, are not allowed. As a consequence, sometimes studies of the fecal hormones of free-ranging black howler monkeys do not report physiological validations and therefore the biological reliability of such measurements cannot be assessed. In order to stimulate future research with this species, the present study aimed at providing methodological bases for fecal endocrine monitoring. Specifically, we compared the validity of two immunoassays (radioimmunoassays, RIA; solid-phase chemiluminescent enzyme immunoassay, SPCEI) performed with commercial kits to measure cortisol, testosterone, estradiol, and progesterone; and demonstrate how the physiological functions of these steroid hormones can be determined through non-pharmacological validations. We found no differences between the analytical validity of RIA and SPCEI assays to measure cortisol and testosterone, whereas for estradiol and progesterone RIA showed better results. Concerning the physiological validation of our assays, we demonstrated that: (1) comparisons between pre- and post-stress situations may be used to assess cortisol response, (2) comparisons between females and males may be used to assess variation in testosterone levels, and (3) comparisons between pregnant and non-pregnant females may be used to determine variation in estradiol and progesterone activity. The analytical and physiological validations that we performed demonstrate that there are currently commercial kits that allow for correct endocrine monitoring of this species, and that there are non-pharmacological alternatives to assess the biological validity of hormone measurements.