Experimentally Altered Navigational Demands Induce Changes in the Cortical Forebrain of Free-Ranging Northern Pacific Rattlesnakes <i>(Crotalus o. oreganus)</i>

Abstract

The hippocampus of birds and mammals plays a crucial role in spatial memory and navigation. The hippocampus exhibits plasticity in adulthood in response to diverse environmental factors associated with spatial demands placed on an animal. The medial and dorsal cortices of the telencephalon of squamate reptiles have been implicated as functional homologues to the hippocampus. This study sought to experimentally manipulate the navigational demands placed on free-ranging northern Pacific rattlesnakes (Crotalus o. oreganus) to provide direct evidence of the relationship between spatial demands and neuroplasticity in the cortical telencephalon of the squamate brain. Adult male rattlesnakes were radio-tracked for 2 months, during which time 1 of 3 treatments was imposed weekly, namely 225-meter translocation in a random direction, 225-meter walk and release at that day’s capture site (handling control) or undisturbed (control). Snakes were then sacrificed and the brains were removed and processed for histological analysis of cortical features. The activity range was larger in the translocated (Tr) group compared to the handled (Hd) and undisturbed control (Cn) groups when measured via 95% minimum convex polygon (MCP). At the 100% MCP level, Tr snakes had larger activity ranges than the Cn snakes only. The volume of the medial cortex (MC) was larger in the Tr group compared to the Cn group. The MC of Hd snakes was not significantly different from that of either of the other groups. No differences in dorsal cortex (DC) or lateral cortex volumes were detected among the groups. Numbers of 5-bromo-2′-deoxyuridine (BrdU)-labeled cells in the MC and DC 3 weeks after BrdU injection were not affected by treatment. This study establishes a causal relationship between navigational demands and greater MC volume in a free-ranging reptile.

From chemotaxis to the cognitive map: The function of olfaction

“…There is a correlation, however, between relative medial cortex (medial pallium homologue) size and active predation, whereby medial cortex size is larger in active than in sit-and-wait lizards (63). In snakes, rattlesnakes forced to navigate after experimental displacement have an increased volume of medial, but not dorsal or lateral, cortex (64).…”

Hippocampal Pallium and Map-Like Memories through Vertebrate Evolution

“…Free-ranging northern Pacific rattlesnakes (Crotalus o. oreganus) that return to their home area after experimental displacements have larger MC relative to the dorsal cortex, compared with hand manipulated or undisturbed snakes [40]. Interspecific comparisons in closely related species of lizards have revealed that species with higher demands on spatial abilities have increased hippocampal volume.…”

Effects of testosterone on the spatial ecology, coloration, and brain regions in western fence lizards, Sceloporus occidentalis

“…In reptiles, the medial and dorsal cortices (MC and DC, respectively) are considered hippocampal homologues and similarly display plasticity with regards to spatial ecology. In male Northern Pacific rattlesnakes (Crotalus oreganus), short distance translocations caused increased home range size and an increase in MC volume (Holding et al, 2012). In male side-blotched lizards (Uta stainsburiana), non-territorial morphs had smaller DC volumes than morphs holding larger territories (LaDage et al, 2009).…”

“…This increase in volume of these brain regions, or the hippocampus in mammals and birds, is mediated, at least in part, through increases in neurogenesis (Rats: Drapeau et al, 2003;Roof and Havens, 1992;Birds: LaDage et al, 2010). In reptiles, the medial and dorsal cortices (MC and DC, respectively) are considered hippocampal homologues and similarly display plasticity with regards to spatial ecology and territory or home range size (Holding et al, 2012;LaDage et al, 2009;Roth et al, 2006. Male cottonmouth snakes (Agkistrodon piscivorus) have larger MC compared to female conspecifics (Roth et al, 2006), presumably due to a sexual dimorphism in spatial movements.…”

“…Male cottonmouth snakes (Agkistrodon piscivorus) have larger MC compared to female conspecifics (Roth et al, 2006), presumably due to a sexual dimorphism in spatial movements. In male Northern Pacific rattlesnakes (Crotalus oreganus), short distance translocations caused increased home range size and an increase in MC volume (Holding et al, 2012 (Brown et al, 2003).…”