Gen Comp Endocrinol. 2017 Jun 23. pii: S0016-6480(16)30388-4. doi: 10.1016/j.ygcen.2017.06.016. [Epub ahead of print]
- 1
- Centre for the Neurobiology of Stress, Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada. Electronic address: boonstra@utsc.utoronto.ca.
- 2
- Centre for the Neurobiology of Stress, Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada; Department of Ethology, Institute of Animal Science, Praha CZ-104 00, Czech Republic.
- 3
- Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada; Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada.
- 4
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada.
Abstract
Male reproductive success in most mammals is determined by their success in direct inter-male competition through aggression and conflict, resulting in female-defense mating systems being predominant. This is linked to male testosterone levels and its dynamics. However, in certain environments, a scramble-competition mating system has evolved, where female reproductive behavior takes precedence and male testosterone dynamics are unlikely to be related to inter-male competition. We studied the North American red squirrel (Tamiasciurus hudsonicus), a species with a well-established scramble-competition system. Using an ACTH hormonal challenge protocol as a proxy for competitive interactions, we compared the testosterone dynamics in breeding males in late winter with that in nonbreeding males in late spring in the Yukon. To gain an integrated picture of their physiological state, we also assessed changes in their stress response, body mass, energy mobilization, and indices of immune function. Testosterone levels at the base bleed were high in breeding males (2.72ng/mL) and virtually absent in non-breeding males (0.04ng/mL). Breeding males were in better condition (heavier body mass, higher hematocrit, and higher erythrocytes), had higher indices of immune function (neutrophil:lymphocyte ratio), but a similar ability to mobilize energy (glucose) compared with non-breeding males. Though total cortisol was higher in non-breeding males, free cortisol was twice as high in breeding males as their corticosteroid binding globulin levels were half as high. In response to the ACTH challenge, testosterone levels in breeding males declined 49% over the first hour and increased 36% over the next hour; in non-breeding males levels showed no change. Free cortisol increased only modestly (26% in breeding males; 23% in non-breeding males). Glucose levels changed similarly in breeding and nonbreeding males, declining for the first 30min and then increasing for the next 60min. Thus, testosterone and components of the stress axis function in a profoundly different manner in male red squirrels than in males of mammals with female-defense mating systems. There are four probable interrelated reasons for these adaptations in male red squirrels: the marginal benefits of each mating, the constraints of mate searching away from their own resource-based territories, energy mobilization in a harsh environment, and a long life span.
Copyright © 2017. Published by Elsevier Inc.
KEYWORDS:
ACTH challenge; Aggression; Corticosteroid binding globulin; Energy mobilization; Free cortisol; Seasonal reproductive changes; Stress response
