The migration of animals is a complex phenomenon that requires a wide range of integrated adaptations in behavior, genetics, morphology and biomechanics. As a result, its study involves a variety of techniques and viewpoints.
Long-distance migration is a risky, energy-exhaustive behavior. It is likely that animals only undertake it when the benefits outweigh the costs.
South American river turtles
Amid the vast expanse of the Amazon and Orinoco basins, river turtles cling to life. Known as arrau or tartaruga in Brazil and Venezuela, or charapa in Colombia and Peru, they are one of the most widely consumed reptile species in Latin America, despite being listed as “near threatened” by the IUCN. Their populations are fragmented and localized, and their eggs are often targeted by poachers for sale to restaurants.
Unlike sea turtles, which have a higher survival rate, most freshwater turtles are vulnerable to human exploitation. The genus Podocnemis, including the giant South American river turtle (Podocnemis expansa), is especially at risk, with all members of this taxonomic group listed as near threatened or critically endangered. This is largely due to their high secondary productivity, which is the energy flow within and between ecosystems occupied by an organism or group of organisms.
The authors of the new paper urge governments and conservation organizations to develop standardized methods for estimating female reproductive output, and recommend creating an international network to monitor the population trends of this species. They also highlight the need for monitoring to include all stages of nesting, from initial egg laying to hatchling survival, and a longer-term perspective to allow conservation interventions to be evaluated over time. To achieve these goals, the team recommends prioritizing a set of sites that are surveyed over the entire breeding season every year.
Galapagos giant land tortoises
One of the best-known examples of reptile migration is that of the Galapagos giant land tortoises. These tortoises are essential ecosystem engineers in the Galapagos Islands, responsible for long-distance seed dispersal and a crucial part of their island’s survival. Unfortunately, this unique attribute became a curse when buccaneers and whalers used them for food and ship fuel. They were often stored upside down in the bilge, where they could withstand a year of starvation and dehydration. The species was nearly wiped out, with only around 15,000 tortoises left today.
Galapagos tortoises move up and down the mountainous islands to find a more consistent supply of vegetation, depending on weather conditions. They head down to the lower parts of the islands in November and December when rains come, to take advantage of more nutrient-rich vegetation that sticks around during the wet season. They also move back up to the highlands when the weather becomes dry again.
Whether they are heading down or up, the tortoises follow specific paths that can stretch from sea level to the summit of the islands. They use their legs and necks to walk on the rocky landscape, and they have glands that release water when needed.
In June 2012, Lonesome George, the world-famous tortoise from Pinta Island in the northern Galapagos Islands, died. He was thought to be 100 years old. Lonesome George’s death was a significant loss to conservationists, who were working to save his subspecies from extinction. The Kleberg Endowed Director of Conservation Genetics at San Diego Zoo Wildlife Alliance, Dr. Oliver Ryder, attended an international workshop in the Galapagos Islands in 2012 to help plan strategies for the next decade of tortoise conservation.
Snakes
Snakes are a fascinating group of reptiles: long, legless, with skin of supple living scales; staring eyes that never blink or close; a flickering forked tongue; and fangs that secrete lethal venom. They can live in a wide variety of habitats, including open oceans, vast deserts, and mountain regions at high altitudes.
Most snakes are oviparous, meaning that they lay eggs. However, some snakes–like sea snakes–are viviparous and give birth to live young. These females may carry their eggs within their body throughout the embryonic period, raising their own body temperature slightly to aid incubation. Regardless of their reproductive mode, most snakes do not show maternal care for their young.
As in other tetrapods, snakes have a backbone made of vertebrae. In some snakes, the neck and tail vertebrae are fused into a single bone known as the hyoid. This enables snakes to eat their prey whole, as is the case for boas and pythons.
Male snakes court their female counterparts by gathering in groups, called mating balls. The male will crawl along the female’s back to encourage her to lift her tail so he can insert one of his hemipenes into her reproductive opening (the cloaca).
Red foxes
A red fox can be heard more often than it is seen in the twilight hours, trotting about a brush pile where a rabbit may be hiding or an apple tree dropping ripe fruit. A fox’s calls can range from sharp, quick “yaps” and whines to long howls and screeches. Although they are cousins to wolves and dogs, red foxes tend to be more solitary.
The breeding season in New York lasts from December to April, with a peak between January and February. After a gestation period of 51 to 53 days, females give birth to litters averaging 4 or 5 pups in a grass-lined den. The young are born blind and helpless, but soon develop their senses and follow the adults on foraging trips.
In general, a fox uses about 36-70% of its home range each day, and the areas used most are those that border neighbouring territory. A fox’s activity area seems to shrink with the approach of the breeding season, and in some cases, it has been observed that it becomes what German biologist Ad Vos called a “floater” (local itinerant), moving between neighbouring territories.
It has been found that, even in these situations, a floater will visit its own territory borders each night. It also appears that a floater can recognise the scent of a neighbouring territory holder’s urine.