Why do village dogs eat endangered sea turtle eggs?

Feral and outdoor cats kill millions of birds each year, proving that when humans encroach on wildlife habitats, the risks to wildlife aren’t strictly human in origin. With humans come our domesticated pets. Cats get a bad rap, and perhaps they deserve it, but dogs aren’t entirely free from blame either.

Domestic dogs are perhaps the most numerous terrestrial carnivores in the world, numbering around 700 million individuals worldwide. And around 80% of those dogs are so-called “village dogs,” free-roaming canines who scavenge from our garbage but who are also often intentionally fed by humans. In some villages, individual dogs may be thought of as “owned” by a particular family, but they still get to wander around on their own. These are not the tiny pooches in purses of Hollywood fame, sleeping at the foot of lavishly decorated Beverly Hills beds.

Some studies have attempted to document village dogs’ hunting behaviors, and they’ve found that in some cases, village dogs are responsible for predation on smaller mammals, ungulates, reptiles, and some birds (though not nearly to the degree that domestic cats are, just saying). But just like their canid relatives the coyotes and foxes, village dogs don’t only impact wildlife through hunting. They can also affect wildlife populations indirectly, such as by scavenging on sea turtle nests. Village dogs around the world reportedly gobble up somewhere between five and fifty percent of a beach’s nests, depending on the location, but according to researcher Eliza Ruiz-Izaguirre from the Netherlands’ Wageningen University, nobody has really intensively studied dog scavenging behavior on sea turtle nesting beaches.

That’s why Ruiz-Izaguirre, together with her colleagues, set out for Mexico. Six of the seven species of sea turtles, all of which are included on the IUCN red list, nest in Mexico. There are more than thirty “important nesting beaches” along both the Pacific coast and the Gulf of Mexico, and most are in close proximity to rural villages. In those villages, some sixty percent of houses “keep” village dogs.

If humans are feeding the dogs, why would they be motivated to scavenge sea turtle nests? Perhaps they’re not getting enough nutrition that way and are forced to look for alternative sources of sustenance.

The researchers spent several months monitoring Colola Sanctuary and neighboring Colola village. The beach there is an important nesting site for eastern Pacific green turtles (Chelonia mydas). They create nearly three thousand nests each year, most of which are in Michoacán, Mexico. In addition, olive ridley turtles (Lepidochelys olivacea) and leatherback turtles (Dermochelys coriacea) also nest at Colola, though in smaller numbers.

Some Colola villagers collect turtle eggs themselves for consumption from the “village beach”, as is their cultural tradition, but the community also carries out nightly patrols of the “sanctuary beach” to guard against outright poaching.

For seven weeks the researchers kept their eyes on the Colola Sanctuary beach, ultimately choosing to keep track of 19 village dogs. These were dogs seen on the beach at least once without human company, and whose owners agreed to have the dogs radio-collared and to an interview. Some of them were categorized as nest scavengers (if a researcher ever saw them scavenge a nest) and others as non-scavengers.

When comparing non-scavengers with scavengers, Ruiz-Izaguirre discovered that the scavengers ate fewer tortillas fed to them by humans than did the non-scavengers. The main food provisioned to village dogs in Colola was tortillas, with other food scraps as available. In part, the researchers suspect that this reflects cultural attitudes. “The fact that 43% of owners consider that juvenile/adult dogs should search for food themselves was reflected in the limited amounts of tortillas fed to dogs in this study,” they write. So adults dogs are motivated to find food elsewhere, and for many of them, sea turtle nests are a widely available, easily accessible source of nutrition.

And that’s not all: the dogs may have initially developed a taste for turtle eggs by being fed scraps and shells by the humans, from the turtle eggs they are themselves. Indeed, nearly 40% of owners admitted to giving their dogs such scraps. And developing puppies acquire certain dietary preferences even in utero and based on their mothers’ milk; if pregnant and nursing females feed on turtle eggs, them their offspring are likely already predisposed to enjoy the taste of turtle eggs as well.

All of that makes sense, but it still doesn’t explain why a dog would walk more than one kilometer from its village and then spend all that time digging for eggs when, ostensibly, other scraps ought to be more available in the village itself. It seems inefficient.

Ruiz-Izaguirre calculated that a medium sized, moderately active dog (think: border collie) needs around 125 calories per kilogram, but even those that were provisioned with tortillas got much less nutrition than necessary from them. Tortillas themselves are also los in protein and amino acids, providing even further motivation for dogs to turn elsewhere for a well-rounded meal. Dogs are indeed capable of seeking out particular food sources to meet energy and nutrient requirements, and sea turtle eggs, with 28% protein and 67% fat, “may well balance out possible amino acid and energy deficiencies of human-given food in the studied dogs,” write the researchers.

It turns out that alternative sources of protein within the village are scarce. A few chickens and some fish are available, but unpredictably so. There are lots of dogs and competition over those scraps is high. Less competitive hungry dogs might therefore be compelled to travel farther and work harder to find enough food.

In some places, the culling of village dogs is the preferred method of control when the dogs are too destructive on native wildlife, but even in the most humane cases, it’s woefully insensitive to local cultures. “Although village dogs are widely tolerated to roam free, they are nonetheless important as household guardians, companions and protectors of farming fields,” notes Ruiz-Izaguirre.

As an alternative, the researchers argue that local villagers ought to be educated about the negative impacts of village dogs on nesting turtles. That they already help to patrol sanctuary beaches suggests that they understand the value of native biodiversity.

In addition, they recommend three actionable steps that dog owners can take to help. The first is to curtail the movements of village dogs between night (~9pm) and dawn (~6am), which is when the dogs were most likely to feed on sea turtle nests. The second is to provision the dogs with more tortillas so that they receive adequate nutrition and are therefore less motivated to seek out sea turtles as an alternative food source. Finally, they suggest that owners avoid giving sea turtle egg scraps to their dogs as a means of preventing dogs from acquiring a taste for the eggs in the first place. Burning the shells and scraps would be preferable, they say. – Jason G. Goldman | 22 April 2015

Source: Ruiz-Izaguirre E., A. van Woersem, K. C. H. A. M. Eilers, S. E. van Wieren, G. Bosch, A. J. van der Zijpp & I. J. M. de Boer (2014). Roaming characteristics and feeding practices of village dogs scavenging sea-turtle nests, Animal Conservation, 18(2) 146-156. DOI: 10.1111/acv.12143.

Header image: Green sea turtle via shutterstock.com