Associations with multiple male groups increase the energy expenditure of humpback whale (Megaptera novaeangliae) female and calf pairs on the breeding grounds. Behaviour 146, 1573-1600.
Cartwright, R. and M. Sullivan. 2009.
Behavioral ontogeny of humpback whale (Megaptera novaeangliae) calves during their residence in Hawaiian waters. Marine Mammal Science, 25 (3), 659–680.
Cartwright, R., B. Gillespie, K. LaBonte, T. Mangold, A. Venema, K. Eden and M. Sullivan. 2012.
Between a Rock and a Hard Place: Habitat Selection in Female-Calf Humpback Whale (Megaptera novaeangliae) Pairs on the Hawaiian Breeding Grounds.
PLoS ONE: 10.1371/journal.pone.0038004
Our findings so far....
Anyone out on the water on a regular basis during the Hawaiian whale season will soon notice that calves come in a widely varying array of shapes and sizes, and widely contrasting patterns of behavior are also apparent. Our initial hypothesis was that many of these variations were related to the age of the calf, so in our first study we set out to test this hypothesis.
Our initial question was to determine if calf behavior varied with calf age, however birthing occurs across the winter seasons, calves within Maui waters are at various ages and stages of maturity, so timing alone can’t be used as an indicator of individual calf age. Big moms have big babies, so similarly size is also unreliable as an age indicator. It’s easy to tell young calves apart underwater, as they are very pale, and small compared to the mother. But getting in the water frequently disturbs the whales and we wanted to document natural, undisturbed patterns of behavior.
Estimating the relative age of humpback whale calves.
During our research study, we noticed distinct variations in the angle and form of the calf’s dorsal fin. Many new born cetaceans have furled flukes and fins, it’s due to their curled position in utero. Underwater pictures allowed us to determine which calves were still very young and we noticed that in these young calves the fin was still furled, while in older calves it looked to have straightened to an upright position. We collected photographs taken from the 6 o’clock position, directly behind the calf and measured the angle of each dorsal fin was using Adobe PhotoShop software.
We then compared the increase in the angle of the dorsal fin, as the fin unfurled, to the calf’s ability to breath hold, as this is a known, physiological limitation in young marine mammals. Our results showed that as the calf’s ability to breath hold increased, the angle of the dorsal fin increased proportionally as the dorsal straightening up. Resights of the same calves over periods of up to two weeks confirmed this in individual calves.
Putting this evidence together, we determined that we could therefore use the measurement of the angle of the calf’s dorsal fin as an indicator of the relative age of the calf. Furthermore, the image required could be obtained at surface, from the research boat and without disturbing the mother and calf pair.
Our Findings : Behavior varies with calf age.
We used the measurements of the dorsal fin to divide the calves up into three age groups, then compared the time budgets and patterns of behavior of calves in these three groups.
Our data indicated that the youngest calves with the most furled fins are the most active; they spend 85% of their time traveling at, or just below the surface. Average speeds of travel are around 4.5 km/hour, though we documented speeds as fast as 6.6 km/hour, and changes of direction during travel are frequent. These young calves surface frequently; average dives last less than 2 minutes and they have a different breathing pattern to adults, often surfacing alone for just a single intermittent breath between dives. The mother, though not seen at the surface, is always close by, generally directly below her calf. Young calves travel just about directly opposite the eye of the mom, and they are positively buoyant, often rising inadvertently, flank first to the surface during travel.
And the reason why such calves travel continually ? Well, as they are so small, travel requires more exertion than it does for adults. The young calves can be seen continually beating their flukes up and down, as the mother glides beside them. This constant activity exercises the locomotory muscles all along the peduncle region, and it’s possible that this muscular exertion leads to the production of myoglobin in these muscles. As myoglobin provides the young calf with the ability to store oxygen within the muscles, the calf’s breath holding capacity will increase as myoglobin levels rise. The calf will be able to make longer dives, and make fewer intermittent surfacings between dives. This means much more efficient swimming as the calf will be able to leave the surface between dives; surface drag makes swimming at the surface up to six times more costly than swimming at depth. Continual travel is therefore the first essential step in the young calf’s physiological development.
As the breath holding capacity increases, the young calf will also be able to nurse more efficiently. Though hard to record accurately we often observed nursing underwater. The calf approaches the mother’s genital slit and could be seen pushing against the slit. We observed many calves nursing on the fly, with both the mother and calf actively traveling. And nursing bouts were certainly brief, in young calves frequently lasting just a few seconds, before the young calf heads back to the surface to breathe.
The commonest surface behavior in calves of all age, is twirling, when the calf rolls, often through 360 degrees at the surface, with pectoral fin waves, tail slaps and tail swishes incorporated into the twirling sequences, especially by younger calves.These activities all utilize the large locomotory muscles, and while they no doubt allow the calf to develop co-ordination, the exercise involved will also further enhance the production of muscular myoglobin.
Time spent at rest increases as calves mature.
As calves mature they tend to travel above the mother’s pectoral fin, they are certainly more co-ordinated and now they surface alone, while the mother travels slowly or rests below, often some 6-8 meters down in the water column. These first independent forays to the surface increase in duration with the increasing relative age of the calf. Once at the surface they generally swim in a small circle at the surface - presumably this means they don’t roam or relocate out of range of mother’s watchful gaze. These circling periods are initially quite brief, but as the calf matures they will generally last 1.5 to 2.5 minutes; the calf breathes several times at the surface, then deeply inhales, and dives back down to the resting mom below.
Breaching, is a common behavior in maturing calves. The calf leaps out of the water and attempts to twirl in mid air, then lands with a large splash. It’s possible that and again the primary function may be exercise of the locomotory muscles, or perhaps the calf is gaining in co-ordination or some other skill that will be useful on the feeding grounds. Long sequences of breaching are also common; one calf breached 47 times in row, however we also observed mothers deliberately quietening active calves, so perhaps there can be too much a good thing.....
As the calves mature, behavior changes again and in the oldest and most mature calves resting takes over as the predominant activity. Calves may rest at the surface beside the mother, but more frequently they rest at depth, tucked underneath the mother, and drifting along with the prevailing currents. Mean dive times increase and now last between 3 and 4 minutes, and circling intervals at the surface may last over 2.5 minutes, the calf breathes several times, but may be very calm, almost placid when surfacing and circling within these resting periods. Travel is intermittent and their speed is slower, all in all the overall profile of the calf’s activities now requires much less energy- and this may in fact be the most critical time during the calf’s residency in Hawaiian waters.
During these resting periods, the calf’s metabolic rate will fall, and the calf will be able to direct a larger proportion of it’s available energy to the biochemical work of growth. Other researchers have determined that calves may grow in excess of 38 cm a month during this time. The larger the calf the more easily it can swim, the better it can hold it’s breath, it can stay submerged and nurse more efficiently, and avoid predators more effectively. Essentially a larger calf has a better chance of successfully completing the impending migration to Alaskan waters. In fact, this period of behavior, when growth rates can be maximized, may be one of the key reasons why mothers choose these waters as a nursery. The waters of the inter-island channels, where mothers and calves can rest undisturbed, provide the premium conditions for calf growth, and so moms persist, despite their own dwindling fat reserves, until their calves are chubby, girthy and ready for the challenges that await on their first natal migration
Multiple escort associations impact the behavior of mother and calf pairs.
We also compared the age andbehavior according to group composition, and found some clear differences.
Mother and calf pairs in multiple escort groups spent much more time traveling, in fact these groups spend over 95% of their time traveling . The risk of injury to the calf is higher in these groups, and there is also a tangible risk of the calf becoming separated from the mother. Mothers use various means to avoid these multiple male groups, such as heading inshore, into shallow waters and even hiding under whale watching boats.
UH researchers have shown that mother and calf pairs receive more male attention as the season progresses and females without calves leave. However these associations with multiple male groups will increase the energy expenditure of the calf and this could have a negative impact on the calf, especially if the calf would otherwise be resting; the fast and continual travel will raise energy expenditure and may reduce potential growth and gain in size.
For late born calves the odds are stacked against them.
We determined that calves of all ages were all equally likely to be associated with multiple male groups. Early born calves, that are already mature by February will be at advantage, as they are less likely to be associated with multiple male groups during this time. This may explain why some females choose to mate before even reaching Hawaii. These early pregnant females will give birth earlier the following year, and therefore receive less male attention during their calf’s crucial resting period.
From late born calves, this whole scenario represents quite a threat. The increased activity associated with multiple males is more likely to occur as the season progresses, co-inciding with the resting period for these late born calves. Additionally, as the mother’s fat resources dwindle this can only increase the lure of the feeding grounds. In right whale calves, mothers with late born calves spend 50% less time in nursery areas than mothers with early born calves. For humpback whale calves shorter stays will minimize the time that these late born calves have to rest, mature and grow prior to migration, and their rest is more likely to be disturbed as the season progresses. Cumulatively, this may directly affect their chances of attaining full calf size prior to the migration, and this in turn may increase the likelihood of becoming the “1 in every 5” that doesn’t survive migration.
The threat of predation.
Many researchers believe that calves face few lethal predation threats while within the shallow waters of the Hawaiian islands.
We documented evidence of calf attacks by false killer whale (Pseudorca crassidens) on several calves over the course of this study. False killer whale teeth marks, as seen here, are quite distinctive and there is plenty of evidence to suggest that false killer whales predate humpback whale calves, however the types of wounds commonly seen suggest that this may be limited to opportunistic “snacking” on the energy rich calf blubber. Although lethal predation may well occur on occasion, over the course of our six year study, there were no reports of this, despite numerous sightings of false killer whales. False killer whale forays into the shallow inter-island water are infrequent; habitually they are a deepwater species, most frequently seen in water depths of 1000 feet or more.
The small, round scars seen here are inflicted by cookie-cutter sharks, (Isistus brasiliensis), and this too is a deep water species. These luminous bodied parasitic sharks are usually found below 1000 feet, and although very little is known about them, UH marine biologist, Dr. K. Holland, who studies these sharks, believes that the sharks may make a vertical migration at night and “snack” on any wayward calves that venture to the peripheries of the shallow waters shelves. Many calves bear these cookie cutter scars, so they are certainly vulnerable to this type of predation, even though these sharks have never been caught or even seen in shallow waters.
The tiger shark, (Galeocerdo cuvier), ranks as one of the most aggressive and ubiquitous predators in Hawaiian waters and there are quite numerous accounts of predation on humpback whale calves by tiger sharks. Five of the 10 calf mortalities that we chronicled during the course of our study ended in witnessed or presumed tiger shark predation. These attacks are swift, efficient and co-ordinated, with little to no visible trace of the calf within thirty minutes of the onset of the attack.
However, a close examination of these incidents reveals that shark predation is in fact a secondary cause of death and the calves in question were already sick, injured or abandoned. Additionally, a tiger shark tagging study, conducted by Dr. Holland, at UH produced data showing that the waters of the Penguin Banks, the most favored nursery region for mother and calf pairs, are also heavily utilized as a resting area by the tiger sharks that forage along Oahu shorelines. Biologically speaking, if healthy humpback whale calves were a feasible component of the tiger shark’s diet, then these sharks would surely just rise to the surface of the Penguin Banks to find lunch, rather than take an 150 mile round trip to feeding ranges in Oahu.
Our research highlights the importance of rest for young calves. It is during this time that they will be able to divert all their energy to growth, and there is no doubt, bigger is better , when it comes to calves...
The warm, calm waters of the inter-island channels of Hawaii offer a perfect nursery for maturing calves and the arrival of so many mothers with their young calves each year is testament to the importance of this region.
We have some recommendations for sustainable whale watching practices in these waters; with a little extra care there is no doubt that Hawaii will can remain a sanctuary for these Keiki Kohola for many years to come.