Parental care is an energetically costly period of the life history of many fish species characterized by extended high intensity activity. To date, there have been no studies that have investigated the cardiovascular correlates of extended parental care in fish. Using Doppler flow probes, the cardiovascular performance of six syntopic centrarchid fish species (N=232) that provide sole, male parental care was examined across a range of water temperatures that encompass their reproductive periods (14–26°C). Experiments were restricted to males but included both nesting and non-nesting individuals to evaluate the cardiovascular performance of fish during parental care. Resting values for cardiac output () and heart rate (fH) tended to be higher for nesting fish when adjusted for variation in temperature. Both of these cardiac variables also increased with water temperature. Stroke volume (VS) was similar among nesting and non-nesting fish and was generally thermally insensitive. When exposed to exhaustive exercise, nesting fish took longer to exhaust than non-nesting individuals. The high resting levels found in nesting fish accompanied by only minor increases in maximal values typically resulted in reductions in cardiac scope. Cardiovascular variables recovered more quickly in nesting fish, which could facilitate the high activity and bursting associated with parental care. Interspecifically, several cardiovascular variables were correlated with parental care activity. Parental care investment became more energetically expensive as the degree of cardiac frequency modulation decreased. Additionally, as the duration of parental care increased, so did the time required for fish to become exhausted, although this relationship was probably influenced by the fact that the larger species (e.g. smallmouth bass Micropterus dolomieu; largemouth bass Micropterus salmoides) provided the lengthiest care. Collectively, these data indicate that fish that provide parental care possess adaptations, including sufficient phenotypic plasticity, such that they can enhance their ability to provide high intensity protracted care, and emphasize the nexus between behavior and physiology.
Experiments were performed at the Queen's University Biology Station (QUBS) on Lake Opinicon in eastern Ontario, Canada. The six species of centrarchids that we investigated were syntopic and generally spawn over several weeks in the spring. Physiological assessments were undertaken across the range of temperatures within which all of these fish reproduce (∼14–26°C) (Wismer and Christie, 1987). Data for each species were collected across the range of water temperatures. However, most efforts focused on the specific temperature range during which each species provided parental care.
All fish used in this study were angled using rod and reel. We focused our study on males, the only sex that provides parental care among these species. Nesting males were identified by snorkelers and then captured from their nest. We restricted sampling to those fish that had offspring that were not free swimming and were usually in the egg or wriggler (i.e. embryo in the nest) stage. Herein we refer to these fish as ‘parental-care-providing’ or ‘nesting’; however, it is important to note that fish that were used in the study were freshly removed from nests so parental care was technically terminated upon their removal. Non-nesting fish were targeted by angling in areas away from the littoral zone where there was an absence of suitable spawning habitat. We included only sexually mature fish and attempted to collect fish of equal sizes for the two groups (i.e. nesting and non-nesting). All fish were landed within 20 s to minimize stress and were then transported to the laboratory in large, aerated coolers (75 l). In the laboratory, all fish were held in large common tanks (300 l). Fish were marked with binary dorsal spine clips to identify nesting and non-nesting fish. Flow-through lake water was provided for the fish so that they remained acclimatized to field conditions such as slight diel temperature variation even though there were briefly held in the lab (i.e. we were trying to avoid acclimation). Natural light through windows provided seasonally appropriate photoperiods. While in captivity, food was withheld. All fish used in this experiment were handled under an identical schedule. Fish were angled on day one and surgery was conducted 24 h later. Fish were then given 24 h to recover from surgery and then chased until exhaustion. Post-mortem calibrations were conducted the following morning as the next batch of surgeries was being completed. All surgeries were conducted by one of three trained individuals and the same individual conducted all calibrations.