There’s a grim joke circulating among wildlife biologists who fought wild pigs and lost: “The average sow gives birth to eight piglets, and nine of them will survive.” Plug this fatalistic humor into a mathematical formula and you might find it is not far from statistical accuracy. Dr. Sarah Chinn recently earned her Ph.D. from the University of Georgia by measuring wild pig fertility, and the results are not funny.
Sarah and her co-authors were part of one of the largest, longest studies of wild pig reproduction ever. They captured and tracked pregnant sows – and then did the same with their newborn piglets – to gather data never before collected. I did a rundown of some of Sarah’s findings and compared them to productivity factors for deer. Consider the following.
- Wild pigs breed any time of year, all year long. Whitetails breed only once annually.
- A female wild piglet can reach sexual maturity by as little as 3 months of age given exceptional conditions and nutrition, though 5 to 6 months old is typical under good conditions. A female whitetail, about 7 to 9 months old if she hits a certain weight threshold her first fall, but most do not breed until their second fall at age 1½.
- Wild piglets develop from embryos in 115 days. Fawns gestate for about 200.
- Wild sows bear about six piglets per litter on average, with a range of one to 12 (wildlife photographer Tes Jolly of Alabama witnessed the litter of 11 shown above). The average adult doe produces one to two fawns per birth, averaging between two and three only at mature ages, in the best conditions, and in the most productive regions.
- After giving birth, a sow can go into estrus again and become pregnant in as little as three months but an average of five to six months – so the average sow can produce two full litters in a year-and-a-half. A whitetail doe cannot exceed one litter per year.
- If a sow loses her litter to predators, she will compensate by going into estrus again in about two weeks. Whitetails don’t possess this ability.
Now you can see why wild pigs have the highest reproductive potential of any wild, hoofed mammal in North America. Their productivity steamrolls whitetails and swamps most of our efforts to control them. They also compete aggressively with whitetails for many of the same foods, possibly reducing deer productivity by stealing nutrients. This is why NDA promotes wild pig control wherever they occur, and we support efforts to prevent this non-native, invasive animal from spreading to new areas.
Sarah’s research at the University of Georgia’s Savannah River Ecology Lab and Warnell School of Forestry and Natural Resources produced many alarming new details about wild pig multiplication. It also provided insights that can help deer hunters shut down the piglet factory.
How Young Can a Wild Pig Breed?
From 2017 to 2020, Sarah and her co-authors trapped and collected data from hundreds of wild pigs on the 310-square-mile Savannah River Site (SRS), a U.S. Department of Energy installation in South Carolina. They studied how genetic ancestry, hormone levels, weight, age and body condition related to fertility. They counted and measured viable fetuses to determine conception dates.
Ages were estimated for all sows using tooth replacement patterns, and 211 adults were aged to specific years by cementum annuli tooth rings. They collared and released 22 of the pregnant sows for tracking and eventually captured their piglets and tagged them as well. (The rest were culled as part of wild pig control efforts on the study site, which remove 1,100 to 1,900 hogs per year).
Out of 514 sows they captured for the study, 160 (31%) were pregnant at the time, and 93 (18%) were lactating, meaning they were currently nursing piglets. Out of the 27 juvenile sows (under 1 year of age), five of them (17%) were pregnant.
Simply to cut down on the potential sampling work, Sarah did not collect data from any sow under 44 pounds, since 66 pounds was the lower limit for most pregnant sows in previous studies on the site and elsewhere. But she still may have missed a few pregnant juveniles. One sow that was exactly 44 pounds and just made the cut-off was carrying fetuses that were already 37 days old.
“I would guess she was around 6 months old,” said Sarah. “We can’t say for sure since there are no cementum rings for months that a lab can count.”
That means this sow went into estrus and bred between 4 and 5 months of age.
“We are definitely underestimating reproduction among the youngest sows, because we didn’t necropsy those,” said Sarah. “We are probably missing the very earliest extremes of breeding.”
The youngest breeding Sarah documented was seen in two sows she captured as 2-day-old piglets. She fitted them with small VHF ear tags, recaptured them later when they were pregnant, and eventually documented the births of their first litters. They both got pregnant at 5 to 5½ months of age.
The mother of those two was also a prolific piglet popper. Sarah documented this sow giving birth to three litters in 1½ years. Her first litter of six all survived. She lost her second litter and quickly went back into estrus. Her third litter of five produced three survivors – that’s nine pigs recruited in 18 months from one sow.
Double Shifts of Piglet Production
Earlier I told you Sarah found 31% of her study sows were pregnant, and 18% were lactating. Interestingly, 1% (four sows) were pregnant and lactating.
“That means those four sows went into estrus while nursing dependent piglets,” said Sarah. “This is not common for mammals, most of which don’t have the energy to do that. She’s allocating all of these resources to lactation and then incurring the extra cost of pregnancy. It’s not unheard of, and it has to do with food availability. I would guess those females were in really good condition.”
It’s the “interbirth interval” – the period when piglets are dependent on the sow’s milk – that determines how fast most sows breed again. It’s usually five to six months. When resources are more abundant, piglets may wean faster, and the interval can be shorter.
No Pork for Coyotes
Coyote diet studies find almost every creature you can imagine going into coyote digestive tracts – except one. Wild pigs rarely show up, even in study areas where both coyotes and pigs are abundant. A separate study on the SRS by UGA’s Jordan Youngmann identified prey items in 146 coyote scats; 65% of them contained deer DNA, but Jordan said he found no evidence of wild pigs. I’ve always wondered why coyotes apparently aren’t a significant piglet predator, and I asked Sarah for her thoughts.
“Sows can be very good mothers, and I definitely think that’s a factor,” she said. “Even a very young mother, even a 30 kg sow [66 pounds], is bigger than a coyote. A coyote would have to find the nest very early, before the young are traveling but while the mother is out foraging. That’s not very long. The young are definitely mobile within a week, and they’re very good at hiding. I’ve walked into nests on day two [following birth] and I’ve seen some hunker down, and I’ve seen some run. I’ve had to go chase them.”
Sarah recalled only one litter of piglets she tracked that was entirely lost to predators. Examination of the nest site suggested, but did not confirm, a coyote was the culprit. It was the first litter for the sow being tracked, so lack of experience could have been a factor.
Wild pigs weigh about 2 pounds at birth, while newborn fawns are two to three times bigger. Fawns are larger packages of protein that lack dangerous mothers and are far more vulnerable to coyotes and other predators than piglets appear to be.
Piglets are not bullet-proof: Out of 50 piglets Sarah tagged in 13 litters, 44% of them survived to 6 weeks old (after this age, survival soars above 90%). But 44% is still more than double fawn survival rates reported at SRS (20%). The much lower frequency of coyote predation is yet another huge reproduction advantage for pigs over deer.
Could Wild Pig Production Be Even Higher?
At the same time Sarah was conducting her research, 57 pigs on the study area, including 26 sows, were tested for diseases. Almost all of them (70%) carried pseudorabies, and nearly half (46%) had brucellosis. Brucellosis is known to cause miscarriages in mammals. Sarah’s study did not look at the role of disease in pig reproduction, so she could not comment on this. But if it is affecting fertility in sows that carry it, that means the extremely high productivity rates Sarah documented are happening despite potential losses to brucellosis. “It’s a fair question,” said Sarah.
By the way, this is a good point to remind you to wear protective gloves when cleaning wild pigs and to cook pork thoroughly. Brucellosis can be transmitted to humans through blood-to-blood contact.
We Created This Monster
Like a comic book villain created accidentally by the superhero, wild pigs got their mutant multiplication power from us.
Native European wild boars are more like whitetails in fertility. They have a single annual breeding season. A female won’t breed the first time until she’s about a year old. They produce average litters of 3 or 4 piglets. They even have fewer teats on average than wild pigs.
Over centuries, we selectively bred these low-productivity European wild boars into domesticated meat machines. Imported to North America, the new domestic breeds escaped and took their artificially engineered productivity into the wild. They hooked up with European wild boars – which we also imported and released – and hybridized.
Past studies show the wild pig population on Sarah’s study site is a mix of European wild boar crossed with various “heritage breeds” of domestic pig, which are older breeds. Through DNA tests, Sarah found a range of wild boar ancestry from 6 to 61% in individual pigs. She did not find that litter size varied with ancestry.
However, it’s likely that in areas where newer improved varieties of domestic pigs have escaped and hybridized with wild boars, average litter sizes could be higher than what Sarah found, because newer breeds of domestic pigs have even greater fertility. The American Yorkshire, for example, which is the most common domestic breed in America, averages 13 piglets per litter.
5 Tips For Stopping The Monster
How can Sarah’s study help us control wild pigs more effectively? We can use their reproductive super powers against them.
- Trap When More Sows are Pregnant – Sarah’s sampling found pregnant sows every month of the year, but two periods were higher than others: February through April, which accounted for 46% of the pregnant sows, and September to December, when 31% were observed. Trap wild pigs during these times, when you are more likely to catch sows before they can give birth, and when pregnancy increases their need to find food.
- Trap When Food is Scarce – Sarah also emphasizes winter and spring as the best overall time for trapping effectiveness because corn is a more effective bait when it’s not competing against acorns. “It’s pretty hard to trap pigs when there’s an abundance of natural food on the ground,” she said.
- Trap in Bottomlands – Sarah published a separate study on how wild pig reproduction affects their use of the landscape. She found pigs spend most of their time in lower areas closer to water. “If I had a choice of trapping in upland pine versus bottomland hardwood, I’d put my traps in bottomland hardwoods.”
- Capture the Entire Sounder – Whether you employ a mechanical trap with a remote-triggered door like the JagerPro system or passive trap like a Pig Brig, follow “conditioning” guidelines to ensure the entire sounder, including all adult and young in the group, is comfortable feeding inside the trap before you set it to catch them. Given their powerful productivity, missing even one female in the group is a setback to control efforts – especially if that female is now trap-wary.
- Target the Big Sows – The average litter size of pigs in this study increased with both age and weight of the mother. Older, healthier females produce more piglets. Sometimes it is the biggest, oldest sows in a sounder that are most reluctant to feed inside traps. They may require more time to condition. It’s worth the wait. Do not set your traps until your camera shows every hog in the group is ready to be caught.
To control wild pigs, you must use trapping to remove whole sounders at a time. But meanwhile, if you’re deer hunting on the same site and see a sounder of pigs, look for the biggest sow in the group and target them first.
Wild Pig Control is Up To Us
Deer produce more offspring than are necessary for sustaining the population to compensate for losses to predation and other causes of death. Yet, this level of productivity can’t compete with the wild pig’s human-engineered output. And there are no predators in North America that specialize in wild pigs. They’re still focused on deer. We have to step in and be the control that Nature doesn’t possess. We created this monster. We have to be the ones to stop it, and we can.