Seal voices

For the first four months of its life, an Antarctic fur seal pup depends on its lactating mother for sustenance. The mother seal spends the majority of her time in the ocean on foraging trips, returning to the land every four to seven days to feed her pup for a few days or less.

During the breeding season, Antarctic fur seals can congregate in dense colonies of over a thousand individuals – and, because mother seals will only nurse their own pups and can be aggressive toward pups that are not their own, a pup’s ability to recognize its mother is crucial.

Scientists know that seals in the family Otariidae, or eared seals (so named because they have external ears), including the Antarctic fur seal, use vocal cues to communicate and recognize one another. Mothers and pups may also use sight and smell to find each other, but auditory clues appear to be their most effective means of reunion. A new paper recently published in the journal PLoS ONE elucidates new details of how an Antarctic fur seal pup recognizes the voice of its mother, and some limits to that auditory recognition.

Researchers working on Courbet Peninsula in the Kerguelen Islands in the southern Indian Ocean studied a colony of 750 pairs of mothers and pups. They recorded the calls of mother seals and played them back to the pups, sometimes with modifications to amplitude and frequency, to determine which acoustic aspects of the calls the pups were using to recognize their mothers. The scientists recorded the number of calls the pups made in response to the recordings, as well as how long it took the pups to respond, and how long it took the pups to look at the loudspeaker standing in for the mother seal.

The researchers also noted that the pups often gathered in groups of about 10 individuals while waiting for their mothers to return to land; they took advantage of these gatherings, and played mother seal calls to them from about 25, 100, and 200 feet away to see how well the pups could discriminate mother seal calls at progressively longer distances.

The pups responded best to mother seal calls that were not modified in amplitude or frequency, suggesting that they use both of those signals in recognizing their mothers’ voice. They also appeared to be better at discriminating mother seal calls at shorter distances – when the scientists played the vocalizations of one of their mothers to the groups of about 10 seal pups, about four pups typically responded from 200 feet away, three from 100 feet away, and just one from 25 feet away. The researchers also note that “[f]or all tested distances, the filial pup of the female chosen for the playback always responded.” In other words, while some pups got it wrong, the pup whose mother they were actually listening to always got it right. 

Antarctic fur seal pups at Salisbury Plain, on South Georgia Island, in the southern Atlantic Ocean.

(Image by Liam Quinn via Flickr/Creative Commons license)

Ducks and seeds

Seeds from aquatic plants have been known to successfully germinate even after passing through the digestive tract of a bird – this is one method by which plants can spread from one body of water to another.

Sometimes, though, seeds don’t make the full, daunting journey through a bird’s bowls.

“Regurgitation, or vomiting, is a common behavior in the daily life of many bird species,” write the authors of a paper recently published in the journal Aquatic Botany. If a plant seed is thrown up before passing all the way through a bird’s body, thereby “circumvent[ing] many of the damaging digestive processes,” so much the better for the seed.

The scientists, interested in whether or not the seeds of aquatic plants might spread via bird vomit, fed ducks seeds collected from 10 species of wetland plants. The ducks regurgitated seeds in about half of the 64 feeding trials the researchers conducted, seemingly in response to one of two conditions: overfeeding, or eating large, indigestible seeds.

If the birds ate a lot of food in a short time (as they sometimes do in the wild, when they happen upon an abundant food source), they threw up some of the seeds they had recently eaten, regardless of seed size, within three hours of eating. Large (more than 10 millimeters, or a little bit less than half an inch), tough seeds were sometimes regurgitated early on in response to overeating, but also between 11 and 24 hours post feeding, apparently after the seeds were rejected from the birds’ gizzards due to their size.

“As regurgitation in birds requires a suffocate movement which is impossible during flight,” the scientists note, “regurgitation most likely occurs after landing in wetland habitat,” meaning that bird vomit may be an important way for aquatic plants to spread.

Seeds from the aquatic plant Iris pseudacorus were among the largest seeds fed to ducks (and later regurgitated by them) during feeding trials. 

(Original image by Paul van de Velde via Flickr/Creative Commons license)

Toxicity testing

“Environmental risk assessment of chemicals is essential but often relies on ethically controversial and expensive methods,” i.e., testing the effects of chemicals on the growth of live, juvenile fish. So begins a paper recently published in the journal Science Advances.

“Every year, more than a million fish are used for experimental and other scientific purposes in the European Union,” the authors of the paper write; they also note that three to six million fish are “used for whole effluent testing” every year in the United States.

Researchers have developed another method for testing the effects of chemicals on living tissue, one that uses cells for test subjects instead of whole fish. If this method is equally as effective as the old technique, then both money and the lives of a multitude of fish could be saved.

In order to evaluate the new, cells-only method, scientists cultured cells from fish gills, exposed the cells to two different pesticides, then monitored their survival and proliferation. Based on how the cells grew, they estimated how much an entire fish would grow if it were exposed to the relative levels of the two chemicals they tested.

The estimates of whole-fish growth were a good match for data collected in a previous experiment, during which fish themselves were exposed to the pesticides. This, the researchers write, “comprises a very promising step toward alternatives to whole-organism toxicity testing, especially taking into consideration the simplicity, rapidity, and low costs of this method.”

“We hope,” they write, “that our very encouraging results inspire further work on alternatives to animal testing,” a prospect that could benefit both fish and humans.

Currently, many toxicology studies rely on live fish as test subjects. Such studies often assess fish at different life stages, from embryos (such as the zebrafish embryo pictured here) to adults. 

(Image by ZEISS Microscopy via Flickr/Creative Commons license)

Polluted protected areas

Artificial light can have large consequences for stream communities – aquatic insects, for example, are less likely to drift downstream in the presence of streetlights. Because some aquatic organisms use natural variations in sunlight as cues telling them when and where to migrate, or eat, or gather up into a group, artificially altered light patterns can disrupt those behaviors.

Marine protected areas, despite their status as ocean zones that have been set aside and (to some extent) shielded from human impacts, are affected by light pollution, too.

A group of researchers from the University of Exeter, in the United Kingdom, recently published an analysis of just how much artificial light is reaching the global network of marine protected areas. By analyzing satellite images taken at night, the scientists were able to quantify the extent of light pollution within marine protected areas, and how that number changed over 20 years, between 1992 and 2012.

The researchers found that in 2012, 35 percent of the protected areas they analyzed were exposed to some degree of artificial light. Of those areas, more than half experienced “widespread” light pollution, indicated by visible illumination present in all of the image pixels within the protected area.

Furthermore, although the majority of protected areas did not experience any change in artificial light levels over the 20 years the scientists analyzed, light pollution increased in 14.7 percent of the protected areas. (The authors note, however, that a lack of change doesn’t indicate a lack of light pollution – it simply means that light levels were constant between 1992 and 2012.)

“Given the importance of light in guiding the behaviors of many marine species,” the authors write, “these results suggest that nighttime lighting may influence the ecology of many of the most valued regions of the ocean.”

The first step in mitigating the negative consequences of artificially illuminating marine protected areas is to determine the extent of the problem. Now that researchers are aware of which areas are particularly impacted by light pollution, resource managers can begin addressing the problem by “[s]witching off, dimming or shielding lights, preserving naturally dark landscapes, and limiting use of spectra known to cause ecological impacts,” among other possible solutions.

Marine protected areas provide shelter from human activities for marine organisms like the sea lion pictured here. 

(Image by NOAA's National Ocean Service via Flickr/Creative Commons license)