Research in a variety of vertebrate taxa has found that cardiac function is a major limiting factor in the ability of animals to cope with physiological challenges, and thus is suggested to play an important role in mediating fitness-related behaviors in the wild. Yet, there remains a paucity of empirical assessments of the relationships between physiological performance and biological fitness in wild animals, partially due to challenges in measuring these metrics remotely. Using male smallmouth bass (Micropterus dolomieu) as a model, we tested for relationships between cardiac performance (measured using heart rate biologgers) and fitness-related behaviors (assessed using videography and snorkeler observations) in the wild during the parental care period. Our results showed that heart rates were not significantly related to any measured parental care behaviors (e.g., nest tending) except for individual aggression level. After accounting for the effect of water temperature on heart rate, we found within-individual heart rate differed between days and also differed between nights. There was, however, evidence of diel variation in heart rate, where heart rate was higher during the day than at night. Although fitness is thought to be dependent on physiological capacity for exercise in wild animals, inter-individual variation in heart rate alone does not appear to relate to parental care behavior in smallmouth bass at the temporal scales examined here (i.e., hours to days). Further studies are required to confirm relationships between physiological performance and parental care behavior to elucidate the apparently complex relationships between physiology, behavior, and fitness in wild animals.
From May 23 to 28, 2016, snorkelling surveys identified 24 nesting male smallmouth bass (mean LT 422 mm ± 78 mm; mean ± SD) on nests in Big Rideau Lake (Ottawa, Ontario, Canada, 44.7706° N, 76.2152° W). Brood size was estimated by scoring the nests on a scale from 1 to 5, where 1 was a nest with few eggs and 5 was a nest with thousands of eggs (Philipp et al., 1997; Kubacki et al., 2002). All snorkelers were trained at the beginning of the season until there was consistency in brood size scoring among snorkelers and within individual snorkelers. The eggs are quite visible such that this simple method has been used reliably in a number of studies (e.g., Siepker et al., 2006; Algera et al., 2017a). Zuckerman et al. (2014) reported that fish with a brood score <3 and egg development >4 days (out of approximately 6 days of egg stage and 22 days of parental care; Cooke et al., 2002a) were more likely to abandon. Therefore, to reduce the risk of smallmouth bass abandoning their nest with the fH logger, only fish with a brood score of 3 or more, and an egg stage of 4 days or less were used in this study. Nests that met the criteria were labelled using weighted numbered tags.
Surgery and instrumentation
Smallmouth bass were collected off their nest by angling coupled with assistance from the snorkeler providing hand signals to ensure the correct bass was caught. Fish were landed within 20 s after hookset to reduce anaerobic exercise and stress associated with angling (Cooke et al., 2003a; Lawrence et al., 2018), and brought into the boat using a rubberized net to avoid injury. Once un-hooked, bass were placed onto a surgery table with water being continuously pumped over the gills, and were electro-sedated using fish handling gloves (Smith-Root, Inc., Washington, USA, http://www.smith-root.com; 10 mA). An approximately 5 cm longitudinal incision was made half-way between the pectoral and pelvic fins, posterior to the pericardial membrane. A fH logger (DST milli HRT, 8 g, 13 mm × 39.5 mm, Star-Oddi, Iceland; http://www.star-oddi.com/) was inserted immediately posterior of the pericardial membrane and sutured to the ventral musculature (PDS II polydioxanone suture; violet monofilament, 3–0). fH loggers were programmed to monitor ECG at 100 Hz, and to record fH every 2 min and ECG every 1.5 h to validate fH readings. The incision was closed using four to five square knot sutures, and the total length of the fish was measured, after which the fish was recovered and then released close to the nest. Throughout the entire process, a snorkeler was protecting the nest from predators and conspecifics until the adult bass returned to the nest and resumed its nesting behaviors. The loggers began collecting data at midnight on the same day of insertion (on average 9 h post-surgery).