No abstract

Humpback whales congregate annually in low-latitude winter breeding and calving grounds. While on these grounds, females with a dependent calf (‘maternal females’) are sometimes closely attended by one or more male escorts. Using data collected from a shore-based observation platform in the Hawaiian Islands, we tested the hypothesis that the spatial distribution of maternal females is driven primarily by avoidance of males. As predicted, we found that (1) pods containing a calf occurred in significantly shallower water than pods that did not contain a calf, (2) unescorted maternal females occurred in significantly shallower water than escorted maternal females, (3) the number of males escorting a female decreased significantly with decreasing water depth, and (4) the swimming speed of maternal females increased as a function of male presence, with escorted females travelling significantly more rapidly than unescorted females and a significant positive correlation between swimming speed and number of escorts. We suggest that maternal females incur increased energetic costs when escorted by males and consequently position themselves in shallow waters to reduce the likelihood of unwanted male attention.

Humpback whales (Megaptera novaeangliae) perform extensive seasonal migrations between high-latitude summer feeding grounds and low-latitude winter reproductive grounds. To investigate gender-related site fidelity to the Hawaiian Islands, an important wintering area for North Pacific humpback whales, photographically based resight histories of 224 females and 416 males observed in Hawai'i from 1976 through 1991 were compiled. Results indicated that the mean number of years in which they were seen was higher for males than for females, that the probability of an individual being resighted across years was higher for males than for females, and that more males than females were seen in consecutive years. Potential sources of bias favoring the photographic capture of males were considered, but none were judged to be significant contributors to the results obtained. It was proposed instead that not all females undertake or complete the migration to the winter grounds each year. Because of the high energy costs of migration and reproduction, some females may overwinter in high latitudes, while others may become pregnant en route to the area traditionally associated with breeding and return to the feeding grounds before arriving at the terminal winter destination.

Male humpback whales (Megaptera novaeangliae) outnumber females on the winter grounds and compete physically for proximity to females. Analyses of identification photographs collected in Hawai'i from 1976 through 1995 and scan samples collected in 1998 showed that (i) reproductive potential (calving rate) for the following winter was greater for females without a calf than females with a calf, (ii) females without a calf were less likely to be found alone and more likely to be found in large pods than females with a calf, (iii) individual females were found in larger pods when without a calf than when with a calf, (iv) the probability of females with a calf being escorted by one or more males increased as the reproductive season progressed, and (v) head lunges occurred more commonly in all-adult pods than in pods containing a calf. We concluded that male humpback whales associate preferentially with females with high reproductive potential, that the attractiveness of individual females varies with their status (with a calf versus without a calf), that males become progressively less choosy over the course of the reproductive season as females without a calf become increasingly rare on the winter grounds, and that males expend more energy in competition over females without a calf than females with a calf.

The main Hawaiian Islands are the primary winter reproductive area for the majority of North Pacific humpback whales Megaptera novaeangliae. Identification photographs of indiv~dual whales, including 63 females sighted in at least 2 bfferent years and with at least 1 calf, were collected from waters off the islands of Maui and Hawaii ('Big Island') between 1977 and 1994. Calves formed a significantly larger proportion of the populahon off Maui than off the Big Island. The overall proportion of calves to all whales identified (crude birth rate) was 0 099 off Maui and 0.061 off the Big Island. Also, considering only females seen in more than 1 year, the number of calves per female per year (calving rate) was 0.7 l off Maui and 0.52 off the Big Island. These rates may be inflated somewhat because of the greater likelihood of sighting and identifying females with calf than females without. Females sighted at both Maui and the Big Island in different years were with a calf sigruficantly more often in Maui waters than in Big Island waters. It was concluded that habitat utilisation by females varies between Maui and the Big Island, and appears to depend in part upon reproductive status.