Socio-sexual and probable mating behavior of Cook Inlet beluga whales, Delphinapterus leucas, observed from an aircraft

Abstract

Socio-sexual and mating behaviors, to our knowledge, have not been previously documented among free-ranging beluga whales, Delphinapterus leucas, but they have been described in detail for captive belugas. We report on the fi rst photodocumented interaction and display of socio-sexual and apparent mating behavior of noncaptive beluga whales in Cook Inlet, Alaska. This behavior was seen on two different days in the same river mouth in uncharacteristically clear waters of upper Cook Inlet. On 24 April 2014, social and possible mating behaviors were observed and photographed for approximately 12 min within a group of nine adult beluga whales in the mouth of Middle River on the west central side of Cook Inlet. A total of 136 photographs were taken at a radial distance Introduction Socio-sexual behavioral interactions among free-ranging beluga whales, Delphinapterus leucas, have not been previously documented, to our knowledge, and are thus poorly understood. Although mating and social behaviors have been described for captive beluga whales (Hill et al., 2015), it is logistically diffi cult to observe undisturbed behavior among free-ranging beluga. They inhabit remote, and in the case of the Cook Inlet beluga whale (CIBW), typically muddy waters. Observations of socio-sexual behaviors of freeranging CIBW are needed to address critical information gaps on the natural history, mating behavior, and potential behavioral habitat preference of this declining insular population. Little is known about the mating behavior or mating season of beluga whales in the wild. Reported age of sexual maturity varies from 4 to 10 years for females and 8 to 15 years for males (Nowak, 1991; Suydam et al.1). Gestation is 14.0–14.5 months, with a single calf born in late spring or early summer (Sergeant, 1973). This would suggest mating occurs in early spring. In autumn, beluga whale populations migrate toward a few common wintering grounds in Bering Sea offshore waters characterized by unconsolidated pack ice where mating is believed to occur during late winter or early spring (Brodie, 1971; Sergeant, 1973; Brown Gladden et al., 1997). Unlike the Arctic stocks, the Cook Inlet beluga population is not thought 1Suydam, R., J. J. Burns, and G. Carroll. 1999. Age, growth, and reproduction of beluga whales from the eastern Chukchi Sea, Alaska. Paper presented to the Alaska Beluga Whale Committee workshop, March 30–April 1, 1999. 5 p. to undertake seasonal migrations outside of Cook Inlet (Laidre et al., 2000; Rugh et al., 2000). Specifi c breeding areas are unknown or possibly nonexistent (i.e. mating might occur anywhere throughout their range). Similar to Arctic beluga populations, CIBW calving is believed to occur in early summer (Hobbs et al., 2015a) although Native hunters have observed newborn CIBW calves from April through August (Huntington, 2000). Alaska natives described CIBW calving areas as the northern side of Kachemak Bay in April and May, off the Beluga and Susitna River mouths in May, and in Chickaloon Bay and Turnagain Arm during summer (Huntington, 2000). McGuire et al.2, during vessel-based surveys of the upper reaches of Cook Inlet in 2005–07, did not document any specifi c calving locations or a defi nitive calving season and calves were encountered in all surveyed location and months (April– October). Thus CIBW are reported to continue to calve later in the season than the Arctic stocks, although their calving season is unclear. In part, some confusion on calving dates may be a function of sightings of calves well into the summer that were actually born weeks or months earlier. Global observations of both wild and captive beluga whales indicate that breeding is seasonal. Among captive beluga whales, Robeck et al. (2005) reported that both testosterone in males and progesterone in females were elevated during late winter/ear2McGuire, T. L., C. C. Kaplan, M. K. Blees, and M. R. Link. 2008. Photo-identifi cation of beluga whales in upper Cook Inlet, Alaska. 2007 Annu. Rep. Prep. LGL Alaska Res. Assoc., Inc., Anchorage, AK, for Chevron, Natl. Fish Wildl. Found., and ConocoPhillips Alaska, Inc., 52 p. > 500 m. On 7 May 2014, similar behaviors were observed among four adult beluga whales in the same location for about 7 min. The second group was not photo-documented due to fl ight limitations. In both circumstances, affi liative behavioral events such as echelon and contact swimming, and socio-sexual behaviors such as ventrum-toventrum contact, ventral presentations, pelvic thrusting, nodding, and rubbing were observed. These behaviors resemble those previously reported for captive beluga mating behaviors and copulation. Similarities between these observations with captive mating behaviors, and the timing of ovulation and peak calving periods from other wild beluga populations, provide strong evidence that mating occurs during early spring months in Cook Inlet.